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Eur J Mineral2008 20 233ndash240Published online February 2008
X and Q band EPR studies of paramagnetic centres in naturaland heated tourmaline
Joanna BABINSKA1 Krystyna DYREK2 Adam PIECZKA1 and Zbigniew SOJKA23
1 AGH University of Science and Technology Faculty of Geology Geophysics and Environmental ProtectionAl Mickiewicza 30 30-059 Krakow Poland
2 Jagiellonian University Faculty of Chemistry ul Ingardena 3 30-060 Krakow PolandCorresponding author e-mail dyrekchemiaujedupl
3 Regional Laboratory of Physicochemical Analyses and Structural Research ul Ingardena 3 30-060 Krakow Poland
Abstract Various members of the tourmaline group (schorl dravite and elbaite) as well as the products of their gradual oxidationwere investigated by EPR spectroscopy in X and Q bands at 293 K and 77 K In the EPR spectra of the schorl and dravite samplesthe signals at g asymp 2 and 43 were attributed to clustered and isolated Fe3+ ions respectively The EPR spectra of Fe-poor elbaiteare dominated by signals at g asymp 25 and 35 assigned to Mn2+ ions
In the schorl and dravite samples gradually annealed in air above 750 K the total intensity of the EPR spectrum increased withincreasing temperature due to the oxidation of Fe2+ (d6) to Fe3+ (d5) ions The Fe3+ ion being a product of thermal oxidationinitially occupies sites with g asymp 43 and after heating at temperatures above 1070 K forms clusters with g asymp 20
In the Fe-poor elbaite the total intensity of the spectrum gradually decreased with the increasing oxidation temperature up to1150 K due to the transformation of paramagnetic Mn2+ (d5) into Mn3+ (d4) ions Simultaneously the signal of Fe3+ at g asymp 43became more pronounced At still higher temperatures (T gt 1150 K) the intensity of the signal around g asymp 20 increased indicatingfurther oxidation of Mn3+ to Mn4+ (d3)
Key-words tourmaline EPR spectroscopy simulation thermal oxidation paramagnetic centres Fe2+ Fe3+ Mn2+ Mn3+ Mn4+V4+ ions
Introduction
Crystal structures of tourmaline one of the most com-mon cyclosilicates exhibit various substitutions at theirionic positions leading to complex chemical compositionsrepresented by a general formula XY3Z6T6O18(BO3)3V3W(Hawthorne amp Henry 1999) where [IX]X = Na+ K+ Ca2+ [VI]Y = Li+ Mg2+ Fe2+ Mn2+ Al3+ Fe3+ Cr3+ V3+Ti4+ (Zn2+ Cu2+) [VI]Z = Al3+ Mg2+ Fe3+ Cr3+ V3+[III]B = B3+ [IV]T = Si4+ Al3+ B3+ whereas [IV]V =OHminus O2minus [III]W = OHminus O2minus Fminus Roman numbers insquare brackets denote the coordination number while stands for the cation vacancy Major cationic substituentsare Al Si and Fe the latter occurring mainly as Fe2+ ionsat Y and Z-octahedral sites
Two main units may be distinguished in the structure oftourmaline (Fig 1) a ditrigonal ring of six silicon-oxygentetrahedra [Si6O18]12minus and an octahedral cluster built ofthe three larger Y-octahedra subsequently referred to astriad surrounded by three pairs of smaller Z-octahedraBoth units are linked spirally around 31 screw axes Itis commonly accepted that in the Y-octahedra larger di-valent cations eg Fe2+ Mg2+ andor Mn2+ are locatedwhereas smaller Z-octahedra are occupied mainly by triva-
lent cations such as Al3+ or more rarely Fe3+ V3+ Cr3+Very often the Z-octahedra contain also significant amountof Mg2+ as an effect of the CaMgNaAl substitution or dueto MgAl disordering over the Y and Z sites The type ofcations occupying Y and Z their distribution and orderinginto the octahedral clusters affect the strength of bondingof particular atoms the degree of deformation of the octa-hedra and the crystal lattice parameters as well Recentlyparticular attention has been focussed on unclear details ofthe tourmaline structure especially on distribution of Fe2+
and Fe3+ ions over the octahedral sitesThe identification of major components and their lo-
calization in the tourmaline structure was widely stud-ied by spectroscopic methods such as Moumlssbauer spec-troscopy (Burns 1972 Hermon et al 1973 Saegusaet al 1979 Korovushkin et al 1979 Ferrow et al 1988Pieczka et al 1998 Dyar et al 1998 Pieczka amp Kraczka2004 Eeckhout et al 2004 Castaneda et al 2006) andultraviolet-visible (UV-Vis) spectroscopy (Manning 1968Leckebusch 1978 De Camargo amp Isotani 1988 Ertl et al2003 Castaneda et al 2006) In contrast the electron para-magnetic resonance (EPR) technique so far has been usedscarcely to the study of tourmalines Most applicationsrefer to identification of radiation induced defects (RID
0935-1221080020-1790 $ 360DOI 1011270935-122120080020-1790 ccopy 2008 E Schweizerbartrsquosche Verlagsbuchhandlung D-70176 Stuttgart
234 J Babinska K Dyrek A Pieczka Z Sojka
Fig 1 Structure of tourmaline (Godovicov 1975)
Novozhilov et al 1968 Bershov et al 1968 Ja 1972Krambrock et al 2002 2004)
The aim of this work was to provide an insight into thestructure of tourmalines especially concerning the chem-ical status and speciation of paramagnetic ions Fe3+ V4+Mn2+ and their transformation upon heating under oxidiz-ing conditions EPR studies of selected tourmaline sampleswere performed and compared with published Moumlssbauerand X-ray diffraction (XRD) data particularly those refer-ring to localization of Fe3+ and Fe2+ ions in the crystalstructure and their behaviour during oxidation (Pieczka ampKraczka 2004) Due to the complexity of the tourmalinestructure and composition those topics deserve more atten-tion The data available are not univocal mainly due to thelimitations of particular methods used so far For instanceMoumlssbauer spectroscopy provides primarily information onFe3+ and Fe2+ ions while XRD can not describe preciselypartial substitution or change in the oxidation state of ametallic ion at its site especially if crystallographic posi-tions are occupied by more than one atomic species There-fore the ability of EPR spectroscopy to reveal the coordi-nation states of paramagnetic ions hosted in the tourmalinestructure is complementary to these methods
Experimental procedure
Four tourmaline samples representing Fe-poor and Fe-richmembers of schorl- dravite solid solutions from Poland(S-5 S-25 S-28 S-41) a pink coloured Mn-bearing andFe-poor elbaite from Morawy Bohemia (N-8) a pale-green(N-13) and a rose (N-20) elbaite from Madagascar havebeen selected for the investigations (Table 1) All of themwere recently studied using Moumlssbauer spectroscopy andXRD (Pieczka amp Kraczka 2004 Pieczka unpublished re-sults)
Intermittent thermal treatments of the samples S-25S-28 S-41 and N-20 in air were performed in the
temperature range 750ndash1310 K according to procedureused earlier by Pieczka amp Kraczka (2004) After mixing thepowdered samples with quartz beads in the ratio 31 theywere heated in air at temperatures increasing stepwise from750 K up to 1310 K (with 40 K steps) At each temperaturethe samples were kept for one hour then they were cooleddown to room temperature and the EPR spectra were mea-sured
EPR spectra were registered at 295 K and 77 K with aBruker ELEXSYS 500 spectrometer operating in X band(92 GHz) with the modulation frequency of 100 kHzThe measurements in Q band (S-28 N-8 N-20) were per-formed at temperatures of 285 K and 130 K with a BrukerESP 300E spectrometer in the Laboratoire de Mineacuteralo-gie et Crystallographie Universiteacute Paris VII Paris Thesimulation program EPR-NMR version 64 developed byMombourquette et al (2000) was used to determine thespin Hamiltonian parameters of Fe3+ V4+ and Mn2+ ions
The EPR experiment consists in measurements of theresponse of a paramagnet placed in linearly swept mag-netic field to microwave irradiation of constant frequencyTo understand the resultant spectrum we need to considerall the interactions which take place between the param-agnetic centers and the external magnetic field The elec-tronic Zeeman interaction is gauged by g-tensor with theprincipal values gxx gyy and gzz The anisotropic hyperfineinteraction of the unpaired electron with magnetic nucleipresent in the sample is described by A-tensor with theprincipal values Axx Ayy and Azz In the case of axial sym-metry gxx = gyy = gperp whereas gzz = g|| andAxx = Ayy = AperpAzz = A|| For systems with S gt 12 the zero field splitting(ZFS) term is described by the traceless fine structure ten-sor It is frequently expressed in terms of D and E parame-ters describing axial and rhombic components of ZFS
Results and discussion
EPR spectra of natural tourmaline
X band EPR
The X band spectra of the parent samples presented inFig 2 3 differ significantly for schorl dravite and elbaiteTypical spectra of a dravite type (S-25 S-28 S-41) and aschorl type (S-5) tourmaline are shown in Fig 2 Two sig-nals are characteristic a relatively sharp signal at g asymp 43with the peak-to-peak line width ΔBpp = 10ndash25 mT and abroad one at g asymp 20 (ΔBpp = 147ndash185 mT) With increas-ing content of Fe3+ (upon passing from S-25 to S-41) theintensity of the broad signal increased This signal domi-nated the spectrum of sample S-41 with the highest con-tent of Fe3+ whereas in the sample S-25 with the small-est content of Fe3+ the relative intensity of the signal withg asymp 43 was the highest The intensity of the signal withg asymp 43 increased 15ndash3 times at 77 K as compared tothat registered at room temperature whereas the width onlyslightly decreased or remained constant Similar increase(15ndash4 times) in the total intensity of the signal with g asymp 20was observed at 77 K but simultaneously it was broadened
EPR studies of tourmaline 235
Table 1 Chemical composition of the tourmalines studied in wt of corresponding oxides (after Pieczka amp Kraczka 2004 for S-5 S-25S-28 S-41) and unpublished results (N-8 N-13 N-20)
Component S-5 S-25 S-28 S-41 N-8 N-13 N-20Na2O 165 164 142 180 184 265 195K2O 008 013 024 013 043 017 037CaO 014 102 061 087 027 024 149MgO 089 932 550 783 0061 0084 006FeO 1423 2775 696 417 058 647 075Fe2O3 039 022 045 667 lt 001 lt 001 lt 001MnO 011 0017 006 009 077 105 324TiO2 039 037 076 064 005 030 0006ZnO 0064 0037 0058 0044 0015 0015 0001Li2O 0015 001 0002 0002 163 126 199Cr2O3 lt 0001 0001 0003 lt 0001 - - -V2O5 0003 0041 0029 0020 - - -Al2O3 3362 3294 3365 2795 4165 3760 3937B2O3 1006 107 1090 1038 1090 1040 1061SiO2 3501 3691 3604 3580 3790 3580 3790H2O 294 329 329 334 332 294 332F 079 073 096 022 088 138 088Total 10007 9982 10008 9987 10029 9978 10197Component Number of atoms on the basis of 31(O OH F)Na 0544 0516 0454 0584 0564 0844 0607K 0017 0027 0050 0028 0087 0036 0076Ca 0025 0177 0108 0156 0046 0042 0256 0414 0280 0388 0768 - 0078 -Mg 0226 2254 1351 1954 0014 0021 0014Fe2+ 1996 0360 0906 0584 0077 0888 0101Fe3+ 0077 0039 0109 0840 0000 lt 0001 0000Mn 0016 0002 0008 0013 0103 0146 0440Ti 0050 0045 0095 0081 0006 0037 0007Zn 0008 0004 0007 0005 0002 0002 0000Li 0010 0007 0001 0001 1037 0832 1284Cr lt 0001 lt 0001 lt 0001 lt 0001 0000 0000 0000V lt 0001 0005 0004 0003 0000 0000 0000Al 6736 6300 6546 5513 7764 7223 7445B 2952 2996 3019 2999 2976 2947 3019Si 5951 5989 5969 5992 5995 5878 5783O 27411 27291 27560 27390 27395 27230 27142OH 3164 3334 3299 3494 3165 3045 3371F 0425 0375 0141 0116 0440 0717 0487
The EPR spectra of schorl and dravite may be assigned toat least two kinds of paramagnetic centres The signal withg asymp 43 was attributed to isolated Fe3+ ions (d5 S = 52)in a strongly distorted octahedral surrounding (Menil et al1979) The broad signal with g about 20 was related withmagnetically coupled clusters Fe3+-O- Fe2+ andor Fe3+-O-Fe3+ (Friebele et al 1971 Narducci et al 1989) at theYminusoctahedra triad Broadening of the signal with g asymp 20with the decreasing temperature confirms that the iron cen-tres tend to be magnetically coupled This remark per-tains mainly to Fe3+ ions present in the Fe3+MgMg orFe3+MgFe2+ Y-octahedra triad respectively Rarely partic-ularly in Al-depleted varieties (eg S-41) they refer also toFe3+ and Fe2+ ions within the pair of adjacent Y and Z sites
It should be noted however that extra-framework ironpossibly present in the samples in the form of small XRDamorphous nanometric particles may also contribute to theobserved broad EPR signal
In sample S-25 containing the smallest amount of ironan additional sharp signal with gperp = 1956 g = 1952and hyperfine structure with Aperp = 53 mT A = 179 mTwas observed (Fig 7) The intensity of that signal increasedafter annealing the sample at 1130 K Such hyperfine struc-ture may be attributed to V4+ ions (d1 S = 12 I = 72)present as a trace in the natural tourmaline
EPR spectra of elbaite (N-8 N-13 N-20) are shown inFig 3 They are more complex than those of schorl anddravite Two dominating broad signals at g asymp 35 andg asymp 25 both with ΔBpp = 50ndash60 mT (N-8) or one dom-inating broad line only with g = 25 and ΔBpp = 60 mT(N-20) should be noted At 77 K the intensity of all signalsincreased while their widths did not change significantly
The content of Mn2+ ions influences intensity and widthof the signals For instance ΔBpp of the signal at g asymp 25was broadened from 50 mT for the sample N-8 with thesmallest content of Mn to 60 mT for N-20 with higher Mn
236 J Babinska K Dyrek A Pieczka Z Sojka
Fig 2 EPR spectra of Fe-poor (S-25) and Fe-rich (S-5 S-28 S-41)dravite and schorl samples registered in X band at 293 K (a) and77 K (b) Zoom of signal V4+ in tourmaline S-25 is shown in (c)
content Simultaneously the total intensity of this signalincreased about 40 times indicating that the spectra resultundoubtedly from the presence of Mn2+ ions (d5 S = 52I = 52) The concentration of Fe3+ ions in these sampleswas very low (lt 001 wt Fe2O3) thus it cannot be asso-ciated neither with the complexity nor with the high inten-sity of the observed EPR spectra This does however notpreclude that they may contribute to the spectrum to someextent
Q band EPR
The Q band spectra of the tourmalines may be divided intotwo groups simple spectra of the schorl and dravite typesamples and more complex spectra of the elbaite-type sam-ples (Fig 4 and 5) The resolution was much higher than inthe X band Moreover the spectra revealed some additionalfeatures not seen in the X band eg the hyperfine structureof isolated Mn2+ ions in the case of the samples N-8 andS-28 (Fig 4c d) In the spectrum of S-28 the signal withg sim 20 was narrower (ΔBpp asymp 100 mT) than in the X band(ΔBpp asymp 170 mT) and a new component with g asymp 218appeared On the other hand the signal at g asymp 43 be-came broadened that is probably related with microhetero-geneity (slightly varying geometry) of the Fe3+ sites withinthe triad of the distorted Y-octahedra which generally re-
Fig 3 EPR spectra of elbaite samples registered in X band at 293 K(a) and 77 K (b)
Fig 4 EPR spectra of dravite (S-28) and elbaite (N-8 N-20) typetourmaline registered in Q band at 285 K (a) and 130 K (b) withzoom of Mn2+ hyperfine structure in (c) and (d)
sults in a strong dependence of ΔBpp on the microwave fre-quency v according to the relation (Sperlich et al 1973)
ΔBpp(Θν) = 2hβe gminus2(Θ)δg(Θ)ν
where Θ is the angle between the magnetic field vector andprincipal axis of the g tensor h the Planck constant and βethe electron Bohr magneton
EPR studies of tourmaline 237
Fig 5 Experimental and simulated spectra of dravite S-28 and el-baite N-20 type tourmaline registered in Q band at 285 K
Simulation of the Fe3+ (S = 52) and Mn2+(S = 52)EPR spectra was performed by diagonalisation of the spinHamiltonian Hsp for S ge 1 (Abraham amp Bleaney 1986)
Hsp = βe middot B middot g middot S + D[S 2z minus 13 S (S + 1)] + E(S 2
x minus S 2y)
where B is the magnetic field vector S the spin operatorg the g tensor S = 12 D and E are the zero field splittingparameters
Simulation of the signal of sample S-28 results in a satis-factory agreement with the experimental spectrum for theparameters g = 200 ΔBpp = 30 mT D = minus80 mT and E =10 mT corresponding to the orthorhombic symmetry withλ = 18 (Fig 5)
The spectra of elbaite samples N-20 and N-8 (Fig 4and 5) were more complex than those of schorl and draviteThe signal at g asymp 20 was surrounded by the componentsof the fine structure of Mn2+ (S = 52) with a splittingof about 150 mT Additionally hyperfine structure due toMn2+ (I = 52) was also present (Fig 4)
Simulation of the spectrum of elbaite N-20 (Fig 5) gavea good fit with experiment for the parameters g = 199ΔBpp = 50 mT D = minus90 mT E = 30 mT λ = 13 Thezero field splitting parameter E indicating the degree oforthorhombic distortion of the octahedra is greater for el-baite (E = 30 mT) than for schorl and dravite (E = 10 mT)Similarly the D parameter gauging an axial distortion isalso greater in elbaite than in S-28 The results indicate thatMn2+ ions occupy lower symmetry sites than those popu-lated by Fe3+ in schorl and dravite probably due to thecoexistence of the Mn2+ (cation radius 083 Aring Shannon1976) and Al3+ (radius 0535 Aring Shannon 1976) withinthe Y-octahedra triad
EPR spectra of tourmaline heated in air
To obtain additional information about the tourmalinestructure and the nature of thermal dehydration and oxida-
Fig 6 EPR spectra of dravite type tourmaline S-25 registered inX band at 293 K after heating in the temperature range 770ndash1130 K
tion processes selected samples were heated in the temper-ature range of 750ndash1310 K in air The concomitant gradualoxidation of metal ions such as Fe2+ Mn2+ and V3+ wasfollowed by EPR spectroscopy
Thermal decomposition of tourmalines in air was investi-gated using XRD and DTA by Bogdanova et al (1981) andrecently using jointly XRD and Moumlssbauer spectroscopyby Pieczka amp Kraczka (2004) Effect of heat treatment wasalso studied by Castaneda et al (2006) with respect to colortransformations It was found that at temperatures above820 K deformation of the crystal lattice of tourmalinestakes place resulting in a progressive decrease of the latticeparameter ao and simultaneously slight increase of the pa-rameter co This effect was explained as resulting from theoxidation of Fe2+ with the ionic radius of 078 Aring presentin the Y sites to the smaller Fe3+ ions with the radius of0645 Aring (Shannon 1976) At the final stage of the thermaltreatment above 1050ndash1110 K the intensity of the X-raydiffraction peaks declines indicating deterioration of longrange order and finally a total destruction of the tourmalinestructure occurring above 1120ndash1170 K
Four samples of tourmaline (S-25 S-28 S-41 and N-20) were selected for thermal experiments Changes inthe spectra occurring upon heating of schorl and draviteare shown in Fig 6ndash8 In sample S-25 with the smallestamount of iron (Table 1) the total intensity of the EPRspectrum increased with increasing temperature (Fig 6)Up to 1050 K this increase was mainly due to the signalwith g asymp 43 but at higher temperatures the signal withg asymp 20 increased dramatically as well In the low fieldpart of the spectrum a small peak at g asymp 90 was observed
238 J Babinska K Dyrek A Pieczka Z Sojka
Fig 7 EPR spectrum of dravite type tourmaline S-25 registered inX band at 77 K after heating at 1130 K in air complete spectrum (a)and the signal of V4+ (b)
whose intensity was not affected significantly by the tem-perature The observed changes may be attributed to the ox-idation of Fe2+ to Fe3+ which at lower temperatures takesplace preferentially at the sites with g asymp 43 ie within theoctahedral Y triad At higher temperatures formation ofthe coupled pairs Fe3+-O- Fe2+ andor Fe3+-O- Fe3+ withg asymp 20 is mainly responsible for further increase of thesignal
Simultaneously it was found that at temperatures above890 K an additional sharp signal with gperp = 1956 and g =1952 attributed to V4+ ions became more intense withincreasing temperature The highest increase in signal in-tensity was observed between 1050 and 1130 K At thistemperature range sharp decrease in bond length 〈 Y-O 〉connected with oxidation of primary Fe2+ at Y sites wasfound by XRD (Pieczka amp Kraczka 2004) Both obser-vations may be explained by oxidation of V3+ (ionic ra-dius 064 Aring) to V4+ (ionic radius 054 Aring) The hyperfinestructure with two sets of eight lines (Aperp = 53 mT A =179 mT Fig 7) is typical of tetravalent vanadium (S = 12I = 72 100) which indicates that the vanadium centreswere well dispersed within the hosting sites of the studiedtourmalines
In sample S-28 the intensity of the EPR signals increasedup to 1070 K mainly in the region of g asymp 43 whereas thesignal with g asymp 20 was dominating at higher temperaturesduring decomposition of the tourmaline structure (Fig 8)Above 1190 K both signals became narrower which maybe assigned to increasing exchange interactions betweenFe3+ ions andor a more statistical distribution of Fe3+ ionsamong adjacent sites The latter case is less probable be-cause above 1090 K pronounced deformation of the crystallattice of tourmaline occurs due to structural collapse andformation of new phases probably silicates andor oxidesas shown by Pieczka amp Kraczka (2004) After one hour of
Fig 8 EPR spectra of dravite type tourmaline S-28 registered inX band at 293 K after heating in the temperature range 750ndash1310 K
heating at 1310 K the signal at g asymp 20 disappeared be-cause of oxidation while that at g asymp 43 persisted
For sample S-41 (not shown in a figure) the signals werevery broad due to the high content of iron (Table 1) Withincreasing temperature gradual increase in the intensity ofthe signal with g asymp 43 (up to 970 K) and the signal withg asymp 20 (up to 1130 K) was observed Similarly as in thecase of other dravite samples (S-25 S-28) after one hourof heating at 1310 K the total intensity of the spectrumconsiderably decreased due to oxidation
Different changes in the spectra were observed in the caseof oxidation of the elbaite samples In sample N-20 the totalintensity of the spectrum steadily decreased with increasingtemperature up to 1150 K whereas the signal at g asymp 43 be-came more pronounced (Fig 9) In contrast to schorl anddravite the region of g asymp 20 was dominated particularlyby an initial decrease of the Mn2+ signal (d5) probably dueto oxidation to Mn3+ (d4) ions not seen in EPR As a re-sult the signal of Fe3+ at g asymp 43 became dominant in thespectrum Its intensity increased upon oxidation of Fe2+
(d6) to Fe3+(d5) Above 1150 K after total collaps of the
EPR studies of tourmaline 239
Fig 9 EPR spectra of elbaite tourmaline N-20 registered in X bandat 293 K after heating in the temperature range 750ndash1310 K
tourmaline structure the spectrum became much simplerOnly a single relatively symmetric broad (50 mT) signal atg asymp 20 was observed with the intensity increasing up to1310 K The signal is most probably related with clusteredMn4+ (d3) ions formed upon oxidation of Mn3+(d4) Theinitial decrease in the EPR signal intensity indicated thatthe oxidation of Mn2+ (d5) up to 1150 K led to Mn3+(d4)The formation of Mn4+ (d3) begins above this temperatureafter the structure collapse
Conclusions
The EPR spectra of natural and oxidized schorl and dravitediffer strongly from those of elbaite In schorl and draviteisolated Fe3+ (d5 S = 52) ions with g asymp 43 located atthe octahedral Y triad or at Z sites are present Addition-ally broad signals with g asymp 20 were observed which maybe attributed to magnetically coupled clusters of the typeFe2+
Y -O-Fe3+Y andor Fe3+
Y -O-Fe3+Z Upon heating in air at
temperatures up to about 1000 K newly-oxidized Fe3+ ionsare isolated and mainly localized at positions with g asymp 43whereas those produced at still higher temperatures aremagnetically coupled and occupy sites with g asymp 20 Sig-nals attributed to paramagnetic V4+ (d1 S = 12) and Mn2+
(d5 S = 52) ions were also found in natural samples ofschorl and dravite Presence of V3+ (d2) was inferred bynoting the increase of the EPR signal of V4+ ions in thecourse of thermal treatment
In elbaite isolated and coupled Mn2+ ions were foundwhich occupy lower symmetry sites than those populatedby Fe3+ in schorl and dravite This effect revealed by spec-tral simulation may be due to the simultaneous presenceof Mn2+ and Al3+ ions of very different radii in the Y-octahedra Upon heating in air Mn2+ (d5) were graduallyoxidized to Mn3+ (d4) Finally after collapse of the tour-maline structure oxidation of Mn3+ (d4) to Mn4+ (d3) wasobserved
The unique selectivity as well as sensitivity of EPR toparamagnetic ions allowed us to obtain information aboutthe valence states of iron vanadium and manganese in thetourmaline structure and their transformations upon ther-mal treatment The results concerning the iron corroboratewell previous Moumlssbauer data
Acknowledgements The authors wish to dedicate thiswork to the memory of Professor Dr Witold Zabinski anoutstanding Polish mineralogist who passed away in Jan-uary 2007
The studies were supported by the AGH University ofScience and Technology Grant No 1111140158
References
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Bershov LV Martirosyan VO Marfunin AS Platonov ANTarashchan AN (1968) Color centres in lithium tourmaline(elbaite) Soviet Phys Cryst 13 629-630
Bogdanova LA Afonina GG Glyuk DS Makagon VM(1981) X-ray study of the thermal transformations of tourma-lines Izw AN SSSR ser Geol 8 136-142 (in Russian)
Burns RG (1972) Mixed valencies and site occupancies of iron insilicate minerals from Moumlssbauer spectroscopy Can J Spectr17 51-59
Castaneda C Eeckhout SG Magela da Costa G Botelho NFDe Grave E (2006) Effect of heat treatment on tourmalinefrom Brazil Phys Chem Minerals 33 207-216
De Camargo MB amp Isotani S (1988) Optical absorptionspectroscopy of natural and irradiated pink tourmaline AmMineral 73 172-180
Dyar MD Taylor EM Lutz TM Francis CA Guidotti CVWise M (1998) Inclusive chemical characterization of tour-maline Moumlssbauer study of Fe valence and site occupancy AmMineral 83 848-864
Eeckhout SG Corteel C Van Coster E De Grave E De PaepeP (2004) Crystal-chemical characterization of tourmalinesfrom the English Lake District Electron-microprobe analysesand Moumlssbauer spectroscopy Am Mineral 89 1743-1751
Ertl A Hughes JM Prowatke S Rossman GR London DFritz EA (2003) Mn-rich tourmaline from Austria structurechemistry optical spectra and relations to synthetic solid solu-tions Am Mineral 88 1379-1376
Ferrow EA Annersten A Gunawardane RP (1988) Moumlssbauereffect study on the mixed valence state of iron in tourmalineMineral Mag 52 221-228
240 J Babinska K Dyrek A Pieczka Z Sojka
Friebele EJ Wilson LK Dozier AW Kinser DL (1971)Antiferromagnetism in an oxide semiconducting glass PhysStat Sol 45 323-331
Godovikov AA (1975) Mineralogija ndash Izdatielstvo NedraMoskwa (in Russian)
Hawthorne FC amp Henry DJ (1999) Classification of the mineralsof the tourmaline group Eur J Mineral 11 201-215
Hermon E Simkin DJ Donnay G Muir WB (1973) Thedistribution of Fe2+ and Fe3+ in iron-bearing tourmalines aMoumlssbauer Study Tschermaks Min Petr Mitt 19 124-132
Ja YH (1972) g asymp 43 Isotropic EPR Line in Tourmaline J ChemPhys 57 3020-3022
Korovushkin VV Kuzmin VI Belov VF (1979) Moumlssbauerstudies of structural features in tourmaline of various genesisPhys Chem Minerals 4 209-220
Krambrock K Pinheiro MVB Medeiros SM Guedes KJSchweizer S Spaeth J-M (2002) Investigation of radiation-induced yellow colour in tourmaline by magnetic resonanceNucl Instr Methods in Phys Research Sect B 191 1-4 241-245
Krambrock K Pinheiro MVB Guedes KJ Medeiros SMSchweizer S Spaeth J-M (2004) Correlation of irradiation-induced yellow color with the Ominus hole centre in tourmalinePhys Chem Minerals 31 168-175
Leckebusch R (1978) Chemical composition and colour of tour-malines from Darre Pech (Nuristan Afghanistan) N Jb MinerAbh 133 53-70
Manning PG (1968) An optical absorption study of the origin ofcolour and pleochroism in pink and brown tourmalines CanMineral 9 678-690
Menil F Fournes L Dance JM Videau JJ (1979) Sodium ironfluorophosphate glasses Part 2 EPR and Moumlssbauer resonancestudy J Non Cryst Solids 34 209-265
Mombourquette MJ Weil JA McGavin DG (2000) Computerprogram EPR-NMR Version 64 University of SaskatchewanSaskatoon Sask
Narducci D Lucca M Morazzoni F Scotti R (1989) Electronspin resonance investigation of the electronic structure of hop-ping centres and the polaronic conduction in iron containingphosphate glasses J Chem Soc Faraday Trans I 85 124099-4110
Novozhilov AI Woskresenskaja IE Samojlovich MI (1968)Electron paramagnetic resonance study of tourmalines SovietPhys 14 416-418
Pieczka A amp Kraczka J (2004) Oxidized tourmalines ndash a com-bined chemical XRD and Motildessbauer study Eur J Mineral16 309-321
Pieczka A Kraczka J Zabinski W (1998) Moumlssbauer spectra ofFe3+ poor schoumlrls reinterpretation on the basis of the orderedstructure model J Czech Geol Soc 43 1-2 69-74
Saegusa N Price DC Smith G (1979) Analysis of theMoumlssbauer spectra of several iron-rich tourmalines (schoumlrls) JPhys (Paris) 40 C2 456-459
Shannon RD (1976) Revised effective ionic radii and systematicsstudies of interatomic distances in halides and chalcogenidesActa Cryst A32 751-767
Sperlich G Urban P Frank G (1973) d1 Electrons in amorphoussemiconducting V2O5 and MoO3 compounds (ESR measure-ments) Z Phys 263 315-323
Received 20 July 2006Modified version received 31 January 2007Accepted 29 November 2007
234 J Babinska K Dyrek A Pieczka Z Sojka
Fig 1 Structure of tourmaline (Godovicov 1975)
Novozhilov et al 1968 Bershov et al 1968 Ja 1972Krambrock et al 2002 2004)
The aim of this work was to provide an insight into thestructure of tourmalines especially concerning the chem-ical status and speciation of paramagnetic ions Fe3+ V4+Mn2+ and their transformation upon heating under oxidiz-ing conditions EPR studies of selected tourmaline sampleswere performed and compared with published Moumlssbauerand X-ray diffraction (XRD) data particularly those refer-ring to localization of Fe3+ and Fe2+ ions in the crystalstructure and their behaviour during oxidation (Pieczka ampKraczka 2004) Due to the complexity of the tourmalinestructure and composition those topics deserve more atten-tion The data available are not univocal mainly due to thelimitations of particular methods used so far For instanceMoumlssbauer spectroscopy provides primarily information onFe3+ and Fe2+ ions while XRD can not describe preciselypartial substitution or change in the oxidation state of ametallic ion at its site especially if crystallographic posi-tions are occupied by more than one atomic species There-fore the ability of EPR spectroscopy to reveal the coordi-nation states of paramagnetic ions hosted in the tourmalinestructure is complementary to these methods
Experimental procedure
Four tourmaline samples representing Fe-poor and Fe-richmembers of schorl- dravite solid solutions from Poland(S-5 S-25 S-28 S-41) a pink coloured Mn-bearing andFe-poor elbaite from Morawy Bohemia (N-8) a pale-green(N-13) and a rose (N-20) elbaite from Madagascar havebeen selected for the investigations (Table 1) All of themwere recently studied using Moumlssbauer spectroscopy andXRD (Pieczka amp Kraczka 2004 Pieczka unpublished re-sults)
Intermittent thermal treatments of the samples S-25S-28 S-41 and N-20 in air were performed in the
temperature range 750ndash1310 K according to procedureused earlier by Pieczka amp Kraczka (2004) After mixing thepowdered samples with quartz beads in the ratio 31 theywere heated in air at temperatures increasing stepwise from750 K up to 1310 K (with 40 K steps) At each temperaturethe samples were kept for one hour then they were cooleddown to room temperature and the EPR spectra were mea-sured
EPR spectra were registered at 295 K and 77 K with aBruker ELEXSYS 500 spectrometer operating in X band(92 GHz) with the modulation frequency of 100 kHzThe measurements in Q band (S-28 N-8 N-20) were per-formed at temperatures of 285 K and 130 K with a BrukerESP 300E spectrometer in the Laboratoire de Mineacuteralo-gie et Crystallographie Universiteacute Paris VII Paris Thesimulation program EPR-NMR version 64 developed byMombourquette et al (2000) was used to determine thespin Hamiltonian parameters of Fe3+ V4+ and Mn2+ ions
The EPR experiment consists in measurements of theresponse of a paramagnet placed in linearly swept mag-netic field to microwave irradiation of constant frequencyTo understand the resultant spectrum we need to considerall the interactions which take place between the param-agnetic centers and the external magnetic field The elec-tronic Zeeman interaction is gauged by g-tensor with theprincipal values gxx gyy and gzz The anisotropic hyperfineinteraction of the unpaired electron with magnetic nucleipresent in the sample is described by A-tensor with theprincipal values Axx Ayy and Azz In the case of axial sym-metry gxx = gyy = gperp whereas gzz = g|| andAxx = Ayy = AperpAzz = A|| For systems with S gt 12 the zero field splitting(ZFS) term is described by the traceless fine structure ten-sor It is frequently expressed in terms of D and E parame-ters describing axial and rhombic components of ZFS
Results and discussion
EPR spectra of natural tourmaline
X band EPR
The X band spectra of the parent samples presented inFig 2 3 differ significantly for schorl dravite and elbaiteTypical spectra of a dravite type (S-25 S-28 S-41) and aschorl type (S-5) tourmaline are shown in Fig 2 Two sig-nals are characteristic a relatively sharp signal at g asymp 43with the peak-to-peak line width ΔBpp = 10ndash25 mT and abroad one at g asymp 20 (ΔBpp = 147ndash185 mT) With increas-ing content of Fe3+ (upon passing from S-25 to S-41) theintensity of the broad signal increased This signal domi-nated the spectrum of sample S-41 with the highest con-tent of Fe3+ whereas in the sample S-25 with the small-est content of Fe3+ the relative intensity of the signal withg asymp 43 was the highest The intensity of the signal withg asymp 43 increased 15ndash3 times at 77 K as compared tothat registered at room temperature whereas the width onlyslightly decreased or remained constant Similar increase(15ndash4 times) in the total intensity of the signal with g asymp 20was observed at 77 K but simultaneously it was broadened
EPR studies of tourmaline 235
Table 1 Chemical composition of the tourmalines studied in wt of corresponding oxides (after Pieczka amp Kraczka 2004 for S-5 S-25S-28 S-41) and unpublished results (N-8 N-13 N-20)
Component S-5 S-25 S-28 S-41 N-8 N-13 N-20Na2O 165 164 142 180 184 265 195K2O 008 013 024 013 043 017 037CaO 014 102 061 087 027 024 149MgO 089 932 550 783 0061 0084 006FeO 1423 2775 696 417 058 647 075Fe2O3 039 022 045 667 lt 001 lt 001 lt 001MnO 011 0017 006 009 077 105 324TiO2 039 037 076 064 005 030 0006ZnO 0064 0037 0058 0044 0015 0015 0001Li2O 0015 001 0002 0002 163 126 199Cr2O3 lt 0001 0001 0003 lt 0001 - - -V2O5 0003 0041 0029 0020 - - -Al2O3 3362 3294 3365 2795 4165 3760 3937B2O3 1006 107 1090 1038 1090 1040 1061SiO2 3501 3691 3604 3580 3790 3580 3790H2O 294 329 329 334 332 294 332F 079 073 096 022 088 138 088Total 10007 9982 10008 9987 10029 9978 10197Component Number of atoms on the basis of 31(O OH F)Na 0544 0516 0454 0584 0564 0844 0607K 0017 0027 0050 0028 0087 0036 0076Ca 0025 0177 0108 0156 0046 0042 0256 0414 0280 0388 0768 - 0078 -Mg 0226 2254 1351 1954 0014 0021 0014Fe2+ 1996 0360 0906 0584 0077 0888 0101Fe3+ 0077 0039 0109 0840 0000 lt 0001 0000Mn 0016 0002 0008 0013 0103 0146 0440Ti 0050 0045 0095 0081 0006 0037 0007Zn 0008 0004 0007 0005 0002 0002 0000Li 0010 0007 0001 0001 1037 0832 1284Cr lt 0001 lt 0001 lt 0001 lt 0001 0000 0000 0000V lt 0001 0005 0004 0003 0000 0000 0000Al 6736 6300 6546 5513 7764 7223 7445B 2952 2996 3019 2999 2976 2947 3019Si 5951 5989 5969 5992 5995 5878 5783O 27411 27291 27560 27390 27395 27230 27142OH 3164 3334 3299 3494 3165 3045 3371F 0425 0375 0141 0116 0440 0717 0487
The EPR spectra of schorl and dravite may be assigned toat least two kinds of paramagnetic centres The signal withg asymp 43 was attributed to isolated Fe3+ ions (d5 S = 52)in a strongly distorted octahedral surrounding (Menil et al1979) The broad signal with g about 20 was related withmagnetically coupled clusters Fe3+-O- Fe2+ andor Fe3+-O-Fe3+ (Friebele et al 1971 Narducci et al 1989) at theYminusoctahedra triad Broadening of the signal with g asymp 20with the decreasing temperature confirms that the iron cen-tres tend to be magnetically coupled This remark per-tains mainly to Fe3+ ions present in the Fe3+MgMg orFe3+MgFe2+ Y-octahedra triad respectively Rarely partic-ularly in Al-depleted varieties (eg S-41) they refer also toFe3+ and Fe2+ ions within the pair of adjacent Y and Z sites
It should be noted however that extra-framework ironpossibly present in the samples in the form of small XRDamorphous nanometric particles may also contribute to theobserved broad EPR signal
In sample S-25 containing the smallest amount of ironan additional sharp signal with gperp = 1956 g = 1952and hyperfine structure with Aperp = 53 mT A = 179 mTwas observed (Fig 7) The intensity of that signal increasedafter annealing the sample at 1130 K Such hyperfine struc-ture may be attributed to V4+ ions (d1 S = 12 I = 72)present as a trace in the natural tourmaline
EPR spectra of elbaite (N-8 N-13 N-20) are shown inFig 3 They are more complex than those of schorl anddravite Two dominating broad signals at g asymp 35 andg asymp 25 both with ΔBpp = 50ndash60 mT (N-8) or one dom-inating broad line only with g = 25 and ΔBpp = 60 mT(N-20) should be noted At 77 K the intensity of all signalsincreased while their widths did not change significantly
The content of Mn2+ ions influences intensity and widthof the signals For instance ΔBpp of the signal at g asymp 25was broadened from 50 mT for the sample N-8 with thesmallest content of Mn to 60 mT for N-20 with higher Mn
236 J Babinska K Dyrek A Pieczka Z Sojka
Fig 2 EPR spectra of Fe-poor (S-25) and Fe-rich (S-5 S-28 S-41)dravite and schorl samples registered in X band at 293 K (a) and77 K (b) Zoom of signal V4+ in tourmaline S-25 is shown in (c)
content Simultaneously the total intensity of this signalincreased about 40 times indicating that the spectra resultundoubtedly from the presence of Mn2+ ions (d5 S = 52I = 52) The concentration of Fe3+ ions in these sampleswas very low (lt 001 wt Fe2O3) thus it cannot be asso-ciated neither with the complexity nor with the high inten-sity of the observed EPR spectra This does however notpreclude that they may contribute to the spectrum to someextent
Q band EPR
The Q band spectra of the tourmalines may be divided intotwo groups simple spectra of the schorl and dravite typesamples and more complex spectra of the elbaite-type sam-ples (Fig 4 and 5) The resolution was much higher than inthe X band Moreover the spectra revealed some additionalfeatures not seen in the X band eg the hyperfine structureof isolated Mn2+ ions in the case of the samples N-8 andS-28 (Fig 4c d) In the spectrum of S-28 the signal withg sim 20 was narrower (ΔBpp asymp 100 mT) than in the X band(ΔBpp asymp 170 mT) and a new component with g asymp 218appeared On the other hand the signal at g asymp 43 be-came broadened that is probably related with microhetero-geneity (slightly varying geometry) of the Fe3+ sites withinthe triad of the distorted Y-octahedra which generally re-
Fig 3 EPR spectra of elbaite samples registered in X band at 293 K(a) and 77 K (b)
Fig 4 EPR spectra of dravite (S-28) and elbaite (N-8 N-20) typetourmaline registered in Q band at 285 K (a) and 130 K (b) withzoom of Mn2+ hyperfine structure in (c) and (d)
sults in a strong dependence of ΔBpp on the microwave fre-quency v according to the relation (Sperlich et al 1973)
ΔBpp(Θν) = 2hβe gminus2(Θ)δg(Θ)ν
where Θ is the angle between the magnetic field vector andprincipal axis of the g tensor h the Planck constant and βethe electron Bohr magneton
EPR studies of tourmaline 237
Fig 5 Experimental and simulated spectra of dravite S-28 and el-baite N-20 type tourmaline registered in Q band at 285 K
Simulation of the Fe3+ (S = 52) and Mn2+(S = 52)EPR spectra was performed by diagonalisation of the spinHamiltonian Hsp for S ge 1 (Abraham amp Bleaney 1986)
Hsp = βe middot B middot g middot S + D[S 2z minus 13 S (S + 1)] + E(S 2
x minus S 2y)
where B is the magnetic field vector S the spin operatorg the g tensor S = 12 D and E are the zero field splittingparameters
Simulation of the signal of sample S-28 results in a satis-factory agreement with the experimental spectrum for theparameters g = 200 ΔBpp = 30 mT D = minus80 mT and E =10 mT corresponding to the orthorhombic symmetry withλ = 18 (Fig 5)
The spectra of elbaite samples N-20 and N-8 (Fig 4and 5) were more complex than those of schorl and draviteThe signal at g asymp 20 was surrounded by the componentsof the fine structure of Mn2+ (S = 52) with a splittingof about 150 mT Additionally hyperfine structure due toMn2+ (I = 52) was also present (Fig 4)
Simulation of the spectrum of elbaite N-20 (Fig 5) gavea good fit with experiment for the parameters g = 199ΔBpp = 50 mT D = minus90 mT E = 30 mT λ = 13 Thezero field splitting parameter E indicating the degree oforthorhombic distortion of the octahedra is greater for el-baite (E = 30 mT) than for schorl and dravite (E = 10 mT)Similarly the D parameter gauging an axial distortion isalso greater in elbaite than in S-28 The results indicate thatMn2+ ions occupy lower symmetry sites than those popu-lated by Fe3+ in schorl and dravite probably due to thecoexistence of the Mn2+ (cation radius 083 Aring Shannon1976) and Al3+ (radius 0535 Aring Shannon 1976) withinthe Y-octahedra triad
EPR spectra of tourmaline heated in air
To obtain additional information about the tourmalinestructure and the nature of thermal dehydration and oxida-
Fig 6 EPR spectra of dravite type tourmaline S-25 registered inX band at 293 K after heating in the temperature range 770ndash1130 K
tion processes selected samples were heated in the temper-ature range of 750ndash1310 K in air The concomitant gradualoxidation of metal ions such as Fe2+ Mn2+ and V3+ wasfollowed by EPR spectroscopy
Thermal decomposition of tourmalines in air was investi-gated using XRD and DTA by Bogdanova et al (1981) andrecently using jointly XRD and Moumlssbauer spectroscopyby Pieczka amp Kraczka (2004) Effect of heat treatment wasalso studied by Castaneda et al (2006) with respect to colortransformations It was found that at temperatures above820 K deformation of the crystal lattice of tourmalinestakes place resulting in a progressive decrease of the latticeparameter ao and simultaneously slight increase of the pa-rameter co This effect was explained as resulting from theoxidation of Fe2+ with the ionic radius of 078 Aring presentin the Y sites to the smaller Fe3+ ions with the radius of0645 Aring (Shannon 1976) At the final stage of the thermaltreatment above 1050ndash1110 K the intensity of the X-raydiffraction peaks declines indicating deterioration of longrange order and finally a total destruction of the tourmalinestructure occurring above 1120ndash1170 K
Four samples of tourmaline (S-25 S-28 S-41 and N-20) were selected for thermal experiments Changes inthe spectra occurring upon heating of schorl and draviteare shown in Fig 6ndash8 In sample S-25 with the smallestamount of iron (Table 1) the total intensity of the EPRspectrum increased with increasing temperature (Fig 6)Up to 1050 K this increase was mainly due to the signalwith g asymp 43 but at higher temperatures the signal withg asymp 20 increased dramatically as well In the low fieldpart of the spectrum a small peak at g asymp 90 was observed
238 J Babinska K Dyrek A Pieczka Z Sojka
Fig 7 EPR spectrum of dravite type tourmaline S-25 registered inX band at 77 K after heating at 1130 K in air complete spectrum (a)and the signal of V4+ (b)
whose intensity was not affected significantly by the tem-perature The observed changes may be attributed to the ox-idation of Fe2+ to Fe3+ which at lower temperatures takesplace preferentially at the sites with g asymp 43 ie within theoctahedral Y triad At higher temperatures formation ofthe coupled pairs Fe3+-O- Fe2+ andor Fe3+-O- Fe3+ withg asymp 20 is mainly responsible for further increase of thesignal
Simultaneously it was found that at temperatures above890 K an additional sharp signal with gperp = 1956 and g =1952 attributed to V4+ ions became more intense withincreasing temperature The highest increase in signal in-tensity was observed between 1050 and 1130 K At thistemperature range sharp decrease in bond length 〈 Y-O 〉connected with oxidation of primary Fe2+ at Y sites wasfound by XRD (Pieczka amp Kraczka 2004) Both obser-vations may be explained by oxidation of V3+ (ionic ra-dius 064 Aring) to V4+ (ionic radius 054 Aring) The hyperfinestructure with two sets of eight lines (Aperp = 53 mT A =179 mT Fig 7) is typical of tetravalent vanadium (S = 12I = 72 100) which indicates that the vanadium centreswere well dispersed within the hosting sites of the studiedtourmalines
In sample S-28 the intensity of the EPR signals increasedup to 1070 K mainly in the region of g asymp 43 whereas thesignal with g asymp 20 was dominating at higher temperaturesduring decomposition of the tourmaline structure (Fig 8)Above 1190 K both signals became narrower which maybe assigned to increasing exchange interactions betweenFe3+ ions andor a more statistical distribution of Fe3+ ionsamong adjacent sites The latter case is less probable be-cause above 1090 K pronounced deformation of the crystallattice of tourmaline occurs due to structural collapse andformation of new phases probably silicates andor oxidesas shown by Pieczka amp Kraczka (2004) After one hour of
Fig 8 EPR spectra of dravite type tourmaline S-28 registered inX band at 293 K after heating in the temperature range 750ndash1310 K
heating at 1310 K the signal at g asymp 20 disappeared be-cause of oxidation while that at g asymp 43 persisted
For sample S-41 (not shown in a figure) the signals werevery broad due to the high content of iron (Table 1) Withincreasing temperature gradual increase in the intensity ofthe signal with g asymp 43 (up to 970 K) and the signal withg asymp 20 (up to 1130 K) was observed Similarly as in thecase of other dravite samples (S-25 S-28) after one hourof heating at 1310 K the total intensity of the spectrumconsiderably decreased due to oxidation
Different changes in the spectra were observed in the caseof oxidation of the elbaite samples In sample N-20 the totalintensity of the spectrum steadily decreased with increasingtemperature up to 1150 K whereas the signal at g asymp 43 be-came more pronounced (Fig 9) In contrast to schorl anddravite the region of g asymp 20 was dominated particularlyby an initial decrease of the Mn2+ signal (d5) probably dueto oxidation to Mn3+ (d4) ions not seen in EPR As a re-sult the signal of Fe3+ at g asymp 43 became dominant in thespectrum Its intensity increased upon oxidation of Fe2+
(d6) to Fe3+(d5) Above 1150 K after total collaps of the
EPR studies of tourmaline 239
Fig 9 EPR spectra of elbaite tourmaline N-20 registered in X bandat 293 K after heating in the temperature range 750ndash1310 K
tourmaline structure the spectrum became much simplerOnly a single relatively symmetric broad (50 mT) signal atg asymp 20 was observed with the intensity increasing up to1310 K The signal is most probably related with clusteredMn4+ (d3) ions formed upon oxidation of Mn3+(d4) Theinitial decrease in the EPR signal intensity indicated thatthe oxidation of Mn2+ (d5) up to 1150 K led to Mn3+(d4)The formation of Mn4+ (d3) begins above this temperatureafter the structure collapse
Conclusions
The EPR spectra of natural and oxidized schorl and dravitediffer strongly from those of elbaite In schorl and draviteisolated Fe3+ (d5 S = 52) ions with g asymp 43 located atthe octahedral Y triad or at Z sites are present Addition-ally broad signals with g asymp 20 were observed which maybe attributed to magnetically coupled clusters of the typeFe2+
Y -O-Fe3+Y andor Fe3+
Y -O-Fe3+Z Upon heating in air at
temperatures up to about 1000 K newly-oxidized Fe3+ ionsare isolated and mainly localized at positions with g asymp 43whereas those produced at still higher temperatures aremagnetically coupled and occupy sites with g asymp 20 Sig-nals attributed to paramagnetic V4+ (d1 S = 12) and Mn2+
(d5 S = 52) ions were also found in natural samples ofschorl and dravite Presence of V3+ (d2) was inferred bynoting the increase of the EPR signal of V4+ ions in thecourse of thermal treatment
In elbaite isolated and coupled Mn2+ ions were foundwhich occupy lower symmetry sites than those populatedby Fe3+ in schorl and dravite This effect revealed by spec-tral simulation may be due to the simultaneous presenceof Mn2+ and Al3+ ions of very different radii in the Y-octahedra Upon heating in air Mn2+ (d5) were graduallyoxidized to Mn3+ (d4) Finally after collapse of the tour-maline structure oxidation of Mn3+ (d4) to Mn4+ (d3) wasobserved
The unique selectivity as well as sensitivity of EPR toparamagnetic ions allowed us to obtain information aboutthe valence states of iron vanadium and manganese in thetourmaline structure and their transformations upon ther-mal treatment The results concerning the iron corroboratewell previous Moumlssbauer data
Acknowledgements The authors wish to dedicate thiswork to the memory of Professor Dr Witold Zabinski anoutstanding Polish mineralogist who passed away in Jan-uary 2007
The studies were supported by the AGH University ofScience and Technology Grant No 1111140158
References
Abraham A amp Bleaney B (1986) Paramagnetic Resonance ofTransition Metal Ions Ed Dover Publications N York 1986
Bershov LV Martirosyan VO Marfunin AS Platonov ANTarashchan AN (1968) Color centres in lithium tourmaline(elbaite) Soviet Phys Cryst 13 629-630
Bogdanova LA Afonina GG Glyuk DS Makagon VM(1981) X-ray study of the thermal transformations of tourma-lines Izw AN SSSR ser Geol 8 136-142 (in Russian)
Burns RG (1972) Mixed valencies and site occupancies of iron insilicate minerals from Moumlssbauer spectroscopy Can J Spectr17 51-59
Castaneda C Eeckhout SG Magela da Costa G Botelho NFDe Grave E (2006) Effect of heat treatment on tourmalinefrom Brazil Phys Chem Minerals 33 207-216
De Camargo MB amp Isotani S (1988) Optical absorptionspectroscopy of natural and irradiated pink tourmaline AmMineral 73 172-180
Dyar MD Taylor EM Lutz TM Francis CA Guidotti CVWise M (1998) Inclusive chemical characterization of tour-maline Moumlssbauer study of Fe valence and site occupancy AmMineral 83 848-864
Eeckhout SG Corteel C Van Coster E De Grave E De PaepeP (2004) Crystal-chemical characterization of tourmalinesfrom the English Lake District Electron-microprobe analysesand Moumlssbauer spectroscopy Am Mineral 89 1743-1751
Ertl A Hughes JM Prowatke S Rossman GR London DFritz EA (2003) Mn-rich tourmaline from Austria structurechemistry optical spectra and relations to synthetic solid solu-tions Am Mineral 88 1379-1376
Ferrow EA Annersten A Gunawardane RP (1988) Moumlssbauereffect study on the mixed valence state of iron in tourmalineMineral Mag 52 221-228
240 J Babinska K Dyrek A Pieczka Z Sojka
Friebele EJ Wilson LK Dozier AW Kinser DL (1971)Antiferromagnetism in an oxide semiconducting glass PhysStat Sol 45 323-331
Godovikov AA (1975) Mineralogija ndash Izdatielstvo NedraMoskwa (in Russian)
Hawthorne FC amp Henry DJ (1999) Classification of the mineralsof the tourmaline group Eur J Mineral 11 201-215
Hermon E Simkin DJ Donnay G Muir WB (1973) Thedistribution of Fe2+ and Fe3+ in iron-bearing tourmalines aMoumlssbauer Study Tschermaks Min Petr Mitt 19 124-132
Ja YH (1972) g asymp 43 Isotropic EPR Line in Tourmaline J ChemPhys 57 3020-3022
Korovushkin VV Kuzmin VI Belov VF (1979) Moumlssbauerstudies of structural features in tourmaline of various genesisPhys Chem Minerals 4 209-220
Krambrock K Pinheiro MVB Medeiros SM Guedes KJSchweizer S Spaeth J-M (2002) Investigation of radiation-induced yellow colour in tourmaline by magnetic resonanceNucl Instr Methods in Phys Research Sect B 191 1-4 241-245
Krambrock K Pinheiro MVB Guedes KJ Medeiros SMSchweizer S Spaeth J-M (2004) Correlation of irradiation-induced yellow color with the Ominus hole centre in tourmalinePhys Chem Minerals 31 168-175
Leckebusch R (1978) Chemical composition and colour of tour-malines from Darre Pech (Nuristan Afghanistan) N Jb MinerAbh 133 53-70
Manning PG (1968) An optical absorption study of the origin ofcolour and pleochroism in pink and brown tourmalines CanMineral 9 678-690
Menil F Fournes L Dance JM Videau JJ (1979) Sodium ironfluorophosphate glasses Part 2 EPR and Moumlssbauer resonancestudy J Non Cryst Solids 34 209-265
Mombourquette MJ Weil JA McGavin DG (2000) Computerprogram EPR-NMR Version 64 University of SaskatchewanSaskatoon Sask
Narducci D Lucca M Morazzoni F Scotti R (1989) Electronspin resonance investigation of the electronic structure of hop-ping centres and the polaronic conduction in iron containingphosphate glasses J Chem Soc Faraday Trans I 85 124099-4110
Novozhilov AI Woskresenskaja IE Samojlovich MI (1968)Electron paramagnetic resonance study of tourmalines SovietPhys 14 416-418
Pieczka A amp Kraczka J (2004) Oxidized tourmalines ndash a com-bined chemical XRD and Motildessbauer study Eur J Mineral16 309-321
Pieczka A Kraczka J Zabinski W (1998) Moumlssbauer spectra ofFe3+ poor schoumlrls reinterpretation on the basis of the orderedstructure model J Czech Geol Soc 43 1-2 69-74
Saegusa N Price DC Smith G (1979) Analysis of theMoumlssbauer spectra of several iron-rich tourmalines (schoumlrls) JPhys (Paris) 40 C2 456-459
Shannon RD (1976) Revised effective ionic radii and systematicsstudies of interatomic distances in halides and chalcogenidesActa Cryst A32 751-767
Sperlich G Urban P Frank G (1973) d1 Electrons in amorphoussemiconducting V2O5 and MoO3 compounds (ESR measure-ments) Z Phys 263 315-323
Received 20 July 2006Modified version received 31 January 2007Accepted 29 November 2007
EPR studies of tourmaline 235
Table 1 Chemical composition of the tourmalines studied in wt of corresponding oxides (after Pieczka amp Kraczka 2004 for S-5 S-25S-28 S-41) and unpublished results (N-8 N-13 N-20)
Component S-5 S-25 S-28 S-41 N-8 N-13 N-20Na2O 165 164 142 180 184 265 195K2O 008 013 024 013 043 017 037CaO 014 102 061 087 027 024 149MgO 089 932 550 783 0061 0084 006FeO 1423 2775 696 417 058 647 075Fe2O3 039 022 045 667 lt 001 lt 001 lt 001MnO 011 0017 006 009 077 105 324TiO2 039 037 076 064 005 030 0006ZnO 0064 0037 0058 0044 0015 0015 0001Li2O 0015 001 0002 0002 163 126 199Cr2O3 lt 0001 0001 0003 lt 0001 - - -V2O5 0003 0041 0029 0020 - - -Al2O3 3362 3294 3365 2795 4165 3760 3937B2O3 1006 107 1090 1038 1090 1040 1061SiO2 3501 3691 3604 3580 3790 3580 3790H2O 294 329 329 334 332 294 332F 079 073 096 022 088 138 088Total 10007 9982 10008 9987 10029 9978 10197Component Number of atoms on the basis of 31(O OH F)Na 0544 0516 0454 0584 0564 0844 0607K 0017 0027 0050 0028 0087 0036 0076Ca 0025 0177 0108 0156 0046 0042 0256 0414 0280 0388 0768 - 0078 -Mg 0226 2254 1351 1954 0014 0021 0014Fe2+ 1996 0360 0906 0584 0077 0888 0101Fe3+ 0077 0039 0109 0840 0000 lt 0001 0000Mn 0016 0002 0008 0013 0103 0146 0440Ti 0050 0045 0095 0081 0006 0037 0007Zn 0008 0004 0007 0005 0002 0002 0000Li 0010 0007 0001 0001 1037 0832 1284Cr lt 0001 lt 0001 lt 0001 lt 0001 0000 0000 0000V lt 0001 0005 0004 0003 0000 0000 0000Al 6736 6300 6546 5513 7764 7223 7445B 2952 2996 3019 2999 2976 2947 3019Si 5951 5989 5969 5992 5995 5878 5783O 27411 27291 27560 27390 27395 27230 27142OH 3164 3334 3299 3494 3165 3045 3371F 0425 0375 0141 0116 0440 0717 0487
The EPR spectra of schorl and dravite may be assigned toat least two kinds of paramagnetic centres The signal withg asymp 43 was attributed to isolated Fe3+ ions (d5 S = 52)in a strongly distorted octahedral surrounding (Menil et al1979) The broad signal with g about 20 was related withmagnetically coupled clusters Fe3+-O- Fe2+ andor Fe3+-O-Fe3+ (Friebele et al 1971 Narducci et al 1989) at theYminusoctahedra triad Broadening of the signal with g asymp 20with the decreasing temperature confirms that the iron cen-tres tend to be magnetically coupled This remark per-tains mainly to Fe3+ ions present in the Fe3+MgMg orFe3+MgFe2+ Y-octahedra triad respectively Rarely partic-ularly in Al-depleted varieties (eg S-41) they refer also toFe3+ and Fe2+ ions within the pair of adjacent Y and Z sites
It should be noted however that extra-framework ironpossibly present in the samples in the form of small XRDamorphous nanometric particles may also contribute to theobserved broad EPR signal
In sample S-25 containing the smallest amount of ironan additional sharp signal with gperp = 1956 g = 1952and hyperfine structure with Aperp = 53 mT A = 179 mTwas observed (Fig 7) The intensity of that signal increasedafter annealing the sample at 1130 K Such hyperfine struc-ture may be attributed to V4+ ions (d1 S = 12 I = 72)present as a trace in the natural tourmaline
EPR spectra of elbaite (N-8 N-13 N-20) are shown inFig 3 They are more complex than those of schorl anddravite Two dominating broad signals at g asymp 35 andg asymp 25 both with ΔBpp = 50ndash60 mT (N-8) or one dom-inating broad line only with g = 25 and ΔBpp = 60 mT(N-20) should be noted At 77 K the intensity of all signalsincreased while their widths did not change significantly
The content of Mn2+ ions influences intensity and widthof the signals For instance ΔBpp of the signal at g asymp 25was broadened from 50 mT for the sample N-8 with thesmallest content of Mn to 60 mT for N-20 with higher Mn
236 J Babinska K Dyrek A Pieczka Z Sojka
Fig 2 EPR spectra of Fe-poor (S-25) and Fe-rich (S-5 S-28 S-41)dravite and schorl samples registered in X band at 293 K (a) and77 K (b) Zoom of signal V4+ in tourmaline S-25 is shown in (c)
content Simultaneously the total intensity of this signalincreased about 40 times indicating that the spectra resultundoubtedly from the presence of Mn2+ ions (d5 S = 52I = 52) The concentration of Fe3+ ions in these sampleswas very low (lt 001 wt Fe2O3) thus it cannot be asso-ciated neither with the complexity nor with the high inten-sity of the observed EPR spectra This does however notpreclude that they may contribute to the spectrum to someextent
Q band EPR
The Q band spectra of the tourmalines may be divided intotwo groups simple spectra of the schorl and dravite typesamples and more complex spectra of the elbaite-type sam-ples (Fig 4 and 5) The resolution was much higher than inthe X band Moreover the spectra revealed some additionalfeatures not seen in the X band eg the hyperfine structureof isolated Mn2+ ions in the case of the samples N-8 andS-28 (Fig 4c d) In the spectrum of S-28 the signal withg sim 20 was narrower (ΔBpp asymp 100 mT) than in the X band(ΔBpp asymp 170 mT) and a new component with g asymp 218appeared On the other hand the signal at g asymp 43 be-came broadened that is probably related with microhetero-geneity (slightly varying geometry) of the Fe3+ sites withinthe triad of the distorted Y-octahedra which generally re-
Fig 3 EPR spectra of elbaite samples registered in X band at 293 K(a) and 77 K (b)
Fig 4 EPR spectra of dravite (S-28) and elbaite (N-8 N-20) typetourmaline registered in Q band at 285 K (a) and 130 K (b) withzoom of Mn2+ hyperfine structure in (c) and (d)
sults in a strong dependence of ΔBpp on the microwave fre-quency v according to the relation (Sperlich et al 1973)
ΔBpp(Θν) = 2hβe gminus2(Θ)δg(Θ)ν
where Θ is the angle between the magnetic field vector andprincipal axis of the g tensor h the Planck constant and βethe electron Bohr magneton
EPR studies of tourmaline 237
Fig 5 Experimental and simulated spectra of dravite S-28 and el-baite N-20 type tourmaline registered in Q band at 285 K
Simulation of the Fe3+ (S = 52) and Mn2+(S = 52)EPR spectra was performed by diagonalisation of the spinHamiltonian Hsp for S ge 1 (Abraham amp Bleaney 1986)
Hsp = βe middot B middot g middot S + D[S 2z minus 13 S (S + 1)] + E(S 2
x minus S 2y)
where B is the magnetic field vector S the spin operatorg the g tensor S = 12 D and E are the zero field splittingparameters
Simulation of the signal of sample S-28 results in a satis-factory agreement with the experimental spectrum for theparameters g = 200 ΔBpp = 30 mT D = minus80 mT and E =10 mT corresponding to the orthorhombic symmetry withλ = 18 (Fig 5)
The spectra of elbaite samples N-20 and N-8 (Fig 4and 5) were more complex than those of schorl and draviteThe signal at g asymp 20 was surrounded by the componentsof the fine structure of Mn2+ (S = 52) with a splittingof about 150 mT Additionally hyperfine structure due toMn2+ (I = 52) was also present (Fig 4)
Simulation of the spectrum of elbaite N-20 (Fig 5) gavea good fit with experiment for the parameters g = 199ΔBpp = 50 mT D = minus90 mT E = 30 mT λ = 13 Thezero field splitting parameter E indicating the degree oforthorhombic distortion of the octahedra is greater for el-baite (E = 30 mT) than for schorl and dravite (E = 10 mT)Similarly the D parameter gauging an axial distortion isalso greater in elbaite than in S-28 The results indicate thatMn2+ ions occupy lower symmetry sites than those popu-lated by Fe3+ in schorl and dravite probably due to thecoexistence of the Mn2+ (cation radius 083 Aring Shannon1976) and Al3+ (radius 0535 Aring Shannon 1976) withinthe Y-octahedra triad
EPR spectra of tourmaline heated in air
To obtain additional information about the tourmalinestructure and the nature of thermal dehydration and oxida-
Fig 6 EPR spectra of dravite type tourmaline S-25 registered inX band at 293 K after heating in the temperature range 770ndash1130 K
tion processes selected samples were heated in the temper-ature range of 750ndash1310 K in air The concomitant gradualoxidation of metal ions such as Fe2+ Mn2+ and V3+ wasfollowed by EPR spectroscopy
Thermal decomposition of tourmalines in air was investi-gated using XRD and DTA by Bogdanova et al (1981) andrecently using jointly XRD and Moumlssbauer spectroscopyby Pieczka amp Kraczka (2004) Effect of heat treatment wasalso studied by Castaneda et al (2006) with respect to colortransformations It was found that at temperatures above820 K deformation of the crystal lattice of tourmalinestakes place resulting in a progressive decrease of the latticeparameter ao and simultaneously slight increase of the pa-rameter co This effect was explained as resulting from theoxidation of Fe2+ with the ionic radius of 078 Aring presentin the Y sites to the smaller Fe3+ ions with the radius of0645 Aring (Shannon 1976) At the final stage of the thermaltreatment above 1050ndash1110 K the intensity of the X-raydiffraction peaks declines indicating deterioration of longrange order and finally a total destruction of the tourmalinestructure occurring above 1120ndash1170 K
Four samples of tourmaline (S-25 S-28 S-41 and N-20) were selected for thermal experiments Changes inthe spectra occurring upon heating of schorl and draviteare shown in Fig 6ndash8 In sample S-25 with the smallestamount of iron (Table 1) the total intensity of the EPRspectrum increased with increasing temperature (Fig 6)Up to 1050 K this increase was mainly due to the signalwith g asymp 43 but at higher temperatures the signal withg asymp 20 increased dramatically as well In the low fieldpart of the spectrum a small peak at g asymp 90 was observed
238 J Babinska K Dyrek A Pieczka Z Sojka
Fig 7 EPR spectrum of dravite type tourmaline S-25 registered inX band at 77 K after heating at 1130 K in air complete spectrum (a)and the signal of V4+ (b)
whose intensity was not affected significantly by the tem-perature The observed changes may be attributed to the ox-idation of Fe2+ to Fe3+ which at lower temperatures takesplace preferentially at the sites with g asymp 43 ie within theoctahedral Y triad At higher temperatures formation ofthe coupled pairs Fe3+-O- Fe2+ andor Fe3+-O- Fe3+ withg asymp 20 is mainly responsible for further increase of thesignal
Simultaneously it was found that at temperatures above890 K an additional sharp signal with gperp = 1956 and g =1952 attributed to V4+ ions became more intense withincreasing temperature The highest increase in signal in-tensity was observed between 1050 and 1130 K At thistemperature range sharp decrease in bond length 〈 Y-O 〉connected with oxidation of primary Fe2+ at Y sites wasfound by XRD (Pieczka amp Kraczka 2004) Both obser-vations may be explained by oxidation of V3+ (ionic ra-dius 064 Aring) to V4+ (ionic radius 054 Aring) The hyperfinestructure with two sets of eight lines (Aperp = 53 mT A =179 mT Fig 7) is typical of tetravalent vanadium (S = 12I = 72 100) which indicates that the vanadium centreswere well dispersed within the hosting sites of the studiedtourmalines
In sample S-28 the intensity of the EPR signals increasedup to 1070 K mainly in the region of g asymp 43 whereas thesignal with g asymp 20 was dominating at higher temperaturesduring decomposition of the tourmaline structure (Fig 8)Above 1190 K both signals became narrower which maybe assigned to increasing exchange interactions betweenFe3+ ions andor a more statistical distribution of Fe3+ ionsamong adjacent sites The latter case is less probable be-cause above 1090 K pronounced deformation of the crystallattice of tourmaline occurs due to structural collapse andformation of new phases probably silicates andor oxidesas shown by Pieczka amp Kraczka (2004) After one hour of
Fig 8 EPR spectra of dravite type tourmaline S-28 registered inX band at 293 K after heating in the temperature range 750ndash1310 K
heating at 1310 K the signal at g asymp 20 disappeared be-cause of oxidation while that at g asymp 43 persisted
For sample S-41 (not shown in a figure) the signals werevery broad due to the high content of iron (Table 1) Withincreasing temperature gradual increase in the intensity ofthe signal with g asymp 43 (up to 970 K) and the signal withg asymp 20 (up to 1130 K) was observed Similarly as in thecase of other dravite samples (S-25 S-28) after one hourof heating at 1310 K the total intensity of the spectrumconsiderably decreased due to oxidation
Different changes in the spectra were observed in the caseof oxidation of the elbaite samples In sample N-20 the totalintensity of the spectrum steadily decreased with increasingtemperature up to 1150 K whereas the signal at g asymp 43 be-came more pronounced (Fig 9) In contrast to schorl anddravite the region of g asymp 20 was dominated particularlyby an initial decrease of the Mn2+ signal (d5) probably dueto oxidation to Mn3+ (d4) ions not seen in EPR As a re-sult the signal of Fe3+ at g asymp 43 became dominant in thespectrum Its intensity increased upon oxidation of Fe2+
(d6) to Fe3+(d5) Above 1150 K after total collaps of the
EPR studies of tourmaline 239
Fig 9 EPR spectra of elbaite tourmaline N-20 registered in X bandat 293 K after heating in the temperature range 750ndash1310 K
tourmaline structure the spectrum became much simplerOnly a single relatively symmetric broad (50 mT) signal atg asymp 20 was observed with the intensity increasing up to1310 K The signal is most probably related with clusteredMn4+ (d3) ions formed upon oxidation of Mn3+(d4) Theinitial decrease in the EPR signal intensity indicated thatthe oxidation of Mn2+ (d5) up to 1150 K led to Mn3+(d4)The formation of Mn4+ (d3) begins above this temperatureafter the structure collapse
Conclusions
The EPR spectra of natural and oxidized schorl and dravitediffer strongly from those of elbaite In schorl and draviteisolated Fe3+ (d5 S = 52) ions with g asymp 43 located atthe octahedral Y triad or at Z sites are present Addition-ally broad signals with g asymp 20 were observed which maybe attributed to magnetically coupled clusters of the typeFe2+
Y -O-Fe3+Y andor Fe3+
Y -O-Fe3+Z Upon heating in air at
temperatures up to about 1000 K newly-oxidized Fe3+ ionsare isolated and mainly localized at positions with g asymp 43whereas those produced at still higher temperatures aremagnetically coupled and occupy sites with g asymp 20 Sig-nals attributed to paramagnetic V4+ (d1 S = 12) and Mn2+
(d5 S = 52) ions were also found in natural samples ofschorl and dravite Presence of V3+ (d2) was inferred bynoting the increase of the EPR signal of V4+ ions in thecourse of thermal treatment
In elbaite isolated and coupled Mn2+ ions were foundwhich occupy lower symmetry sites than those populatedby Fe3+ in schorl and dravite This effect revealed by spec-tral simulation may be due to the simultaneous presenceof Mn2+ and Al3+ ions of very different radii in the Y-octahedra Upon heating in air Mn2+ (d5) were graduallyoxidized to Mn3+ (d4) Finally after collapse of the tour-maline structure oxidation of Mn3+ (d4) to Mn4+ (d3) wasobserved
The unique selectivity as well as sensitivity of EPR toparamagnetic ions allowed us to obtain information aboutthe valence states of iron vanadium and manganese in thetourmaline structure and their transformations upon ther-mal treatment The results concerning the iron corroboratewell previous Moumlssbauer data
Acknowledgements The authors wish to dedicate thiswork to the memory of Professor Dr Witold Zabinski anoutstanding Polish mineralogist who passed away in Jan-uary 2007
The studies were supported by the AGH University ofScience and Technology Grant No 1111140158
References
Abraham A amp Bleaney B (1986) Paramagnetic Resonance ofTransition Metal Ions Ed Dover Publications N York 1986
Bershov LV Martirosyan VO Marfunin AS Platonov ANTarashchan AN (1968) Color centres in lithium tourmaline(elbaite) Soviet Phys Cryst 13 629-630
Bogdanova LA Afonina GG Glyuk DS Makagon VM(1981) X-ray study of the thermal transformations of tourma-lines Izw AN SSSR ser Geol 8 136-142 (in Russian)
Burns RG (1972) Mixed valencies and site occupancies of iron insilicate minerals from Moumlssbauer spectroscopy Can J Spectr17 51-59
Castaneda C Eeckhout SG Magela da Costa G Botelho NFDe Grave E (2006) Effect of heat treatment on tourmalinefrom Brazil Phys Chem Minerals 33 207-216
De Camargo MB amp Isotani S (1988) Optical absorptionspectroscopy of natural and irradiated pink tourmaline AmMineral 73 172-180
Dyar MD Taylor EM Lutz TM Francis CA Guidotti CVWise M (1998) Inclusive chemical characterization of tour-maline Moumlssbauer study of Fe valence and site occupancy AmMineral 83 848-864
Eeckhout SG Corteel C Van Coster E De Grave E De PaepeP (2004) Crystal-chemical characterization of tourmalinesfrom the English Lake District Electron-microprobe analysesand Moumlssbauer spectroscopy Am Mineral 89 1743-1751
Ertl A Hughes JM Prowatke S Rossman GR London DFritz EA (2003) Mn-rich tourmaline from Austria structurechemistry optical spectra and relations to synthetic solid solu-tions Am Mineral 88 1379-1376
Ferrow EA Annersten A Gunawardane RP (1988) Moumlssbauereffect study on the mixed valence state of iron in tourmalineMineral Mag 52 221-228
240 J Babinska K Dyrek A Pieczka Z Sojka
Friebele EJ Wilson LK Dozier AW Kinser DL (1971)Antiferromagnetism in an oxide semiconducting glass PhysStat Sol 45 323-331
Godovikov AA (1975) Mineralogija ndash Izdatielstvo NedraMoskwa (in Russian)
Hawthorne FC amp Henry DJ (1999) Classification of the mineralsof the tourmaline group Eur J Mineral 11 201-215
Hermon E Simkin DJ Donnay G Muir WB (1973) Thedistribution of Fe2+ and Fe3+ in iron-bearing tourmalines aMoumlssbauer Study Tschermaks Min Petr Mitt 19 124-132
Ja YH (1972) g asymp 43 Isotropic EPR Line in Tourmaline J ChemPhys 57 3020-3022
Korovushkin VV Kuzmin VI Belov VF (1979) Moumlssbauerstudies of structural features in tourmaline of various genesisPhys Chem Minerals 4 209-220
Krambrock K Pinheiro MVB Medeiros SM Guedes KJSchweizer S Spaeth J-M (2002) Investigation of radiation-induced yellow colour in tourmaline by magnetic resonanceNucl Instr Methods in Phys Research Sect B 191 1-4 241-245
Krambrock K Pinheiro MVB Guedes KJ Medeiros SMSchweizer S Spaeth J-M (2004) Correlation of irradiation-induced yellow color with the Ominus hole centre in tourmalinePhys Chem Minerals 31 168-175
Leckebusch R (1978) Chemical composition and colour of tour-malines from Darre Pech (Nuristan Afghanistan) N Jb MinerAbh 133 53-70
Manning PG (1968) An optical absorption study of the origin ofcolour and pleochroism in pink and brown tourmalines CanMineral 9 678-690
Menil F Fournes L Dance JM Videau JJ (1979) Sodium ironfluorophosphate glasses Part 2 EPR and Moumlssbauer resonancestudy J Non Cryst Solids 34 209-265
Mombourquette MJ Weil JA McGavin DG (2000) Computerprogram EPR-NMR Version 64 University of SaskatchewanSaskatoon Sask
Narducci D Lucca M Morazzoni F Scotti R (1989) Electronspin resonance investigation of the electronic structure of hop-ping centres and the polaronic conduction in iron containingphosphate glasses J Chem Soc Faraday Trans I 85 124099-4110
Novozhilov AI Woskresenskaja IE Samojlovich MI (1968)Electron paramagnetic resonance study of tourmalines SovietPhys 14 416-418
Pieczka A amp Kraczka J (2004) Oxidized tourmalines ndash a com-bined chemical XRD and Motildessbauer study Eur J Mineral16 309-321
Pieczka A Kraczka J Zabinski W (1998) Moumlssbauer spectra ofFe3+ poor schoumlrls reinterpretation on the basis of the orderedstructure model J Czech Geol Soc 43 1-2 69-74
Saegusa N Price DC Smith G (1979) Analysis of theMoumlssbauer spectra of several iron-rich tourmalines (schoumlrls) JPhys (Paris) 40 C2 456-459
Shannon RD (1976) Revised effective ionic radii and systematicsstudies of interatomic distances in halides and chalcogenidesActa Cryst A32 751-767
Sperlich G Urban P Frank G (1973) d1 Electrons in amorphoussemiconducting V2O5 and MoO3 compounds (ESR measure-ments) Z Phys 263 315-323
Received 20 July 2006Modified version received 31 January 2007Accepted 29 November 2007
236 J Babinska K Dyrek A Pieczka Z Sojka
Fig 2 EPR spectra of Fe-poor (S-25) and Fe-rich (S-5 S-28 S-41)dravite and schorl samples registered in X band at 293 K (a) and77 K (b) Zoom of signal V4+ in tourmaline S-25 is shown in (c)
content Simultaneously the total intensity of this signalincreased about 40 times indicating that the spectra resultundoubtedly from the presence of Mn2+ ions (d5 S = 52I = 52) The concentration of Fe3+ ions in these sampleswas very low (lt 001 wt Fe2O3) thus it cannot be asso-ciated neither with the complexity nor with the high inten-sity of the observed EPR spectra This does however notpreclude that they may contribute to the spectrum to someextent
Q band EPR
The Q band spectra of the tourmalines may be divided intotwo groups simple spectra of the schorl and dravite typesamples and more complex spectra of the elbaite-type sam-ples (Fig 4 and 5) The resolution was much higher than inthe X band Moreover the spectra revealed some additionalfeatures not seen in the X band eg the hyperfine structureof isolated Mn2+ ions in the case of the samples N-8 andS-28 (Fig 4c d) In the spectrum of S-28 the signal withg sim 20 was narrower (ΔBpp asymp 100 mT) than in the X band(ΔBpp asymp 170 mT) and a new component with g asymp 218appeared On the other hand the signal at g asymp 43 be-came broadened that is probably related with microhetero-geneity (slightly varying geometry) of the Fe3+ sites withinthe triad of the distorted Y-octahedra which generally re-
Fig 3 EPR spectra of elbaite samples registered in X band at 293 K(a) and 77 K (b)
Fig 4 EPR spectra of dravite (S-28) and elbaite (N-8 N-20) typetourmaline registered in Q band at 285 K (a) and 130 K (b) withzoom of Mn2+ hyperfine structure in (c) and (d)
sults in a strong dependence of ΔBpp on the microwave fre-quency v according to the relation (Sperlich et al 1973)
ΔBpp(Θν) = 2hβe gminus2(Θ)δg(Θ)ν
where Θ is the angle between the magnetic field vector andprincipal axis of the g tensor h the Planck constant and βethe electron Bohr magneton
EPR studies of tourmaline 237
Fig 5 Experimental and simulated spectra of dravite S-28 and el-baite N-20 type tourmaline registered in Q band at 285 K
Simulation of the Fe3+ (S = 52) and Mn2+(S = 52)EPR spectra was performed by diagonalisation of the spinHamiltonian Hsp for S ge 1 (Abraham amp Bleaney 1986)
Hsp = βe middot B middot g middot S + D[S 2z minus 13 S (S + 1)] + E(S 2
x minus S 2y)
where B is the magnetic field vector S the spin operatorg the g tensor S = 12 D and E are the zero field splittingparameters
Simulation of the signal of sample S-28 results in a satis-factory agreement with the experimental spectrum for theparameters g = 200 ΔBpp = 30 mT D = minus80 mT and E =10 mT corresponding to the orthorhombic symmetry withλ = 18 (Fig 5)
The spectra of elbaite samples N-20 and N-8 (Fig 4and 5) were more complex than those of schorl and draviteThe signal at g asymp 20 was surrounded by the componentsof the fine structure of Mn2+ (S = 52) with a splittingof about 150 mT Additionally hyperfine structure due toMn2+ (I = 52) was also present (Fig 4)
Simulation of the spectrum of elbaite N-20 (Fig 5) gavea good fit with experiment for the parameters g = 199ΔBpp = 50 mT D = minus90 mT E = 30 mT λ = 13 Thezero field splitting parameter E indicating the degree oforthorhombic distortion of the octahedra is greater for el-baite (E = 30 mT) than for schorl and dravite (E = 10 mT)Similarly the D parameter gauging an axial distortion isalso greater in elbaite than in S-28 The results indicate thatMn2+ ions occupy lower symmetry sites than those popu-lated by Fe3+ in schorl and dravite probably due to thecoexistence of the Mn2+ (cation radius 083 Aring Shannon1976) and Al3+ (radius 0535 Aring Shannon 1976) withinthe Y-octahedra triad
EPR spectra of tourmaline heated in air
To obtain additional information about the tourmalinestructure and the nature of thermal dehydration and oxida-
Fig 6 EPR spectra of dravite type tourmaline S-25 registered inX band at 293 K after heating in the temperature range 770ndash1130 K
tion processes selected samples were heated in the temper-ature range of 750ndash1310 K in air The concomitant gradualoxidation of metal ions such as Fe2+ Mn2+ and V3+ wasfollowed by EPR spectroscopy
Thermal decomposition of tourmalines in air was investi-gated using XRD and DTA by Bogdanova et al (1981) andrecently using jointly XRD and Moumlssbauer spectroscopyby Pieczka amp Kraczka (2004) Effect of heat treatment wasalso studied by Castaneda et al (2006) with respect to colortransformations It was found that at temperatures above820 K deformation of the crystal lattice of tourmalinestakes place resulting in a progressive decrease of the latticeparameter ao and simultaneously slight increase of the pa-rameter co This effect was explained as resulting from theoxidation of Fe2+ with the ionic radius of 078 Aring presentin the Y sites to the smaller Fe3+ ions with the radius of0645 Aring (Shannon 1976) At the final stage of the thermaltreatment above 1050ndash1110 K the intensity of the X-raydiffraction peaks declines indicating deterioration of longrange order and finally a total destruction of the tourmalinestructure occurring above 1120ndash1170 K
Four samples of tourmaline (S-25 S-28 S-41 and N-20) were selected for thermal experiments Changes inthe spectra occurring upon heating of schorl and draviteare shown in Fig 6ndash8 In sample S-25 with the smallestamount of iron (Table 1) the total intensity of the EPRspectrum increased with increasing temperature (Fig 6)Up to 1050 K this increase was mainly due to the signalwith g asymp 43 but at higher temperatures the signal withg asymp 20 increased dramatically as well In the low fieldpart of the spectrum a small peak at g asymp 90 was observed
238 J Babinska K Dyrek A Pieczka Z Sojka
Fig 7 EPR spectrum of dravite type tourmaline S-25 registered inX band at 77 K after heating at 1130 K in air complete spectrum (a)and the signal of V4+ (b)
whose intensity was not affected significantly by the tem-perature The observed changes may be attributed to the ox-idation of Fe2+ to Fe3+ which at lower temperatures takesplace preferentially at the sites with g asymp 43 ie within theoctahedral Y triad At higher temperatures formation ofthe coupled pairs Fe3+-O- Fe2+ andor Fe3+-O- Fe3+ withg asymp 20 is mainly responsible for further increase of thesignal
Simultaneously it was found that at temperatures above890 K an additional sharp signal with gperp = 1956 and g =1952 attributed to V4+ ions became more intense withincreasing temperature The highest increase in signal in-tensity was observed between 1050 and 1130 K At thistemperature range sharp decrease in bond length 〈 Y-O 〉connected with oxidation of primary Fe2+ at Y sites wasfound by XRD (Pieczka amp Kraczka 2004) Both obser-vations may be explained by oxidation of V3+ (ionic ra-dius 064 Aring) to V4+ (ionic radius 054 Aring) The hyperfinestructure with two sets of eight lines (Aperp = 53 mT A =179 mT Fig 7) is typical of tetravalent vanadium (S = 12I = 72 100) which indicates that the vanadium centreswere well dispersed within the hosting sites of the studiedtourmalines
In sample S-28 the intensity of the EPR signals increasedup to 1070 K mainly in the region of g asymp 43 whereas thesignal with g asymp 20 was dominating at higher temperaturesduring decomposition of the tourmaline structure (Fig 8)Above 1190 K both signals became narrower which maybe assigned to increasing exchange interactions betweenFe3+ ions andor a more statistical distribution of Fe3+ ionsamong adjacent sites The latter case is less probable be-cause above 1090 K pronounced deformation of the crystallattice of tourmaline occurs due to structural collapse andformation of new phases probably silicates andor oxidesas shown by Pieczka amp Kraczka (2004) After one hour of
Fig 8 EPR spectra of dravite type tourmaline S-28 registered inX band at 293 K after heating in the temperature range 750ndash1310 K
heating at 1310 K the signal at g asymp 20 disappeared be-cause of oxidation while that at g asymp 43 persisted
For sample S-41 (not shown in a figure) the signals werevery broad due to the high content of iron (Table 1) Withincreasing temperature gradual increase in the intensity ofthe signal with g asymp 43 (up to 970 K) and the signal withg asymp 20 (up to 1130 K) was observed Similarly as in thecase of other dravite samples (S-25 S-28) after one hourof heating at 1310 K the total intensity of the spectrumconsiderably decreased due to oxidation
Different changes in the spectra were observed in the caseof oxidation of the elbaite samples In sample N-20 the totalintensity of the spectrum steadily decreased with increasingtemperature up to 1150 K whereas the signal at g asymp 43 be-came more pronounced (Fig 9) In contrast to schorl anddravite the region of g asymp 20 was dominated particularlyby an initial decrease of the Mn2+ signal (d5) probably dueto oxidation to Mn3+ (d4) ions not seen in EPR As a re-sult the signal of Fe3+ at g asymp 43 became dominant in thespectrum Its intensity increased upon oxidation of Fe2+
(d6) to Fe3+(d5) Above 1150 K after total collaps of the
EPR studies of tourmaline 239
Fig 9 EPR spectra of elbaite tourmaline N-20 registered in X bandat 293 K after heating in the temperature range 750ndash1310 K
tourmaline structure the spectrum became much simplerOnly a single relatively symmetric broad (50 mT) signal atg asymp 20 was observed with the intensity increasing up to1310 K The signal is most probably related with clusteredMn4+ (d3) ions formed upon oxidation of Mn3+(d4) Theinitial decrease in the EPR signal intensity indicated thatthe oxidation of Mn2+ (d5) up to 1150 K led to Mn3+(d4)The formation of Mn4+ (d3) begins above this temperatureafter the structure collapse
Conclusions
The EPR spectra of natural and oxidized schorl and dravitediffer strongly from those of elbaite In schorl and draviteisolated Fe3+ (d5 S = 52) ions with g asymp 43 located atthe octahedral Y triad or at Z sites are present Addition-ally broad signals with g asymp 20 were observed which maybe attributed to magnetically coupled clusters of the typeFe2+
Y -O-Fe3+Y andor Fe3+
Y -O-Fe3+Z Upon heating in air at
temperatures up to about 1000 K newly-oxidized Fe3+ ionsare isolated and mainly localized at positions with g asymp 43whereas those produced at still higher temperatures aremagnetically coupled and occupy sites with g asymp 20 Sig-nals attributed to paramagnetic V4+ (d1 S = 12) and Mn2+
(d5 S = 52) ions were also found in natural samples ofschorl and dravite Presence of V3+ (d2) was inferred bynoting the increase of the EPR signal of V4+ ions in thecourse of thermal treatment
In elbaite isolated and coupled Mn2+ ions were foundwhich occupy lower symmetry sites than those populatedby Fe3+ in schorl and dravite This effect revealed by spec-tral simulation may be due to the simultaneous presenceof Mn2+ and Al3+ ions of very different radii in the Y-octahedra Upon heating in air Mn2+ (d5) were graduallyoxidized to Mn3+ (d4) Finally after collapse of the tour-maline structure oxidation of Mn3+ (d4) to Mn4+ (d3) wasobserved
The unique selectivity as well as sensitivity of EPR toparamagnetic ions allowed us to obtain information aboutthe valence states of iron vanadium and manganese in thetourmaline structure and their transformations upon ther-mal treatment The results concerning the iron corroboratewell previous Moumlssbauer data
Acknowledgements The authors wish to dedicate thiswork to the memory of Professor Dr Witold Zabinski anoutstanding Polish mineralogist who passed away in Jan-uary 2007
The studies were supported by the AGH University ofScience and Technology Grant No 1111140158
References
Abraham A amp Bleaney B (1986) Paramagnetic Resonance ofTransition Metal Ions Ed Dover Publications N York 1986
Bershov LV Martirosyan VO Marfunin AS Platonov ANTarashchan AN (1968) Color centres in lithium tourmaline(elbaite) Soviet Phys Cryst 13 629-630
Bogdanova LA Afonina GG Glyuk DS Makagon VM(1981) X-ray study of the thermal transformations of tourma-lines Izw AN SSSR ser Geol 8 136-142 (in Russian)
Burns RG (1972) Mixed valencies and site occupancies of iron insilicate minerals from Moumlssbauer spectroscopy Can J Spectr17 51-59
Castaneda C Eeckhout SG Magela da Costa G Botelho NFDe Grave E (2006) Effect of heat treatment on tourmalinefrom Brazil Phys Chem Minerals 33 207-216
De Camargo MB amp Isotani S (1988) Optical absorptionspectroscopy of natural and irradiated pink tourmaline AmMineral 73 172-180
Dyar MD Taylor EM Lutz TM Francis CA Guidotti CVWise M (1998) Inclusive chemical characterization of tour-maline Moumlssbauer study of Fe valence and site occupancy AmMineral 83 848-864
Eeckhout SG Corteel C Van Coster E De Grave E De PaepeP (2004) Crystal-chemical characterization of tourmalinesfrom the English Lake District Electron-microprobe analysesand Moumlssbauer spectroscopy Am Mineral 89 1743-1751
Ertl A Hughes JM Prowatke S Rossman GR London DFritz EA (2003) Mn-rich tourmaline from Austria structurechemistry optical spectra and relations to synthetic solid solu-tions Am Mineral 88 1379-1376
Ferrow EA Annersten A Gunawardane RP (1988) Moumlssbauereffect study on the mixed valence state of iron in tourmalineMineral Mag 52 221-228
240 J Babinska K Dyrek A Pieczka Z Sojka
Friebele EJ Wilson LK Dozier AW Kinser DL (1971)Antiferromagnetism in an oxide semiconducting glass PhysStat Sol 45 323-331
Godovikov AA (1975) Mineralogija ndash Izdatielstvo NedraMoskwa (in Russian)
Hawthorne FC amp Henry DJ (1999) Classification of the mineralsof the tourmaline group Eur J Mineral 11 201-215
Hermon E Simkin DJ Donnay G Muir WB (1973) Thedistribution of Fe2+ and Fe3+ in iron-bearing tourmalines aMoumlssbauer Study Tschermaks Min Petr Mitt 19 124-132
Ja YH (1972) g asymp 43 Isotropic EPR Line in Tourmaline J ChemPhys 57 3020-3022
Korovushkin VV Kuzmin VI Belov VF (1979) Moumlssbauerstudies of structural features in tourmaline of various genesisPhys Chem Minerals 4 209-220
Krambrock K Pinheiro MVB Medeiros SM Guedes KJSchweizer S Spaeth J-M (2002) Investigation of radiation-induced yellow colour in tourmaline by magnetic resonanceNucl Instr Methods in Phys Research Sect B 191 1-4 241-245
Krambrock K Pinheiro MVB Guedes KJ Medeiros SMSchweizer S Spaeth J-M (2004) Correlation of irradiation-induced yellow color with the Ominus hole centre in tourmalinePhys Chem Minerals 31 168-175
Leckebusch R (1978) Chemical composition and colour of tour-malines from Darre Pech (Nuristan Afghanistan) N Jb MinerAbh 133 53-70
Manning PG (1968) An optical absorption study of the origin ofcolour and pleochroism in pink and brown tourmalines CanMineral 9 678-690
Menil F Fournes L Dance JM Videau JJ (1979) Sodium ironfluorophosphate glasses Part 2 EPR and Moumlssbauer resonancestudy J Non Cryst Solids 34 209-265
Mombourquette MJ Weil JA McGavin DG (2000) Computerprogram EPR-NMR Version 64 University of SaskatchewanSaskatoon Sask
Narducci D Lucca M Morazzoni F Scotti R (1989) Electronspin resonance investigation of the electronic structure of hop-ping centres and the polaronic conduction in iron containingphosphate glasses J Chem Soc Faraday Trans I 85 124099-4110
Novozhilov AI Woskresenskaja IE Samojlovich MI (1968)Electron paramagnetic resonance study of tourmalines SovietPhys 14 416-418
Pieczka A amp Kraczka J (2004) Oxidized tourmalines ndash a com-bined chemical XRD and Motildessbauer study Eur J Mineral16 309-321
Pieczka A Kraczka J Zabinski W (1998) Moumlssbauer spectra ofFe3+ poor schoumlrls reinterpretation on the basis of the orderedstructure model J Czech Geol Soc 43 1-2 69-74
Saegusa N Price DC Smith G (1979) Analysis of theMoumlssbauer spectra of several iron-rich tourmalines (schoumlrls) JPhys (Paris) 40 C2 456-459
Shannon RD (1976) Revised effective ionic radii and systematicsstudies of interatomic distances in halides and chalcogenidesActa Cryst A32 751-767
Sperlich G Urban P Frank G (1973) d1 Electrons in amorphoussemiconducting V2O5 and MoO3 compounds (ESR measure-ments) Z Phys 263 315-323
Received 20 July 2006Modified version received 31 January 2007Accepted 29 November 2007
EPR studies of tourmaline 237
Fig 5 Experimental and simulated spectra of dravite S-28 and el-baite N-20 type tourmaline registered in Q band at 285 K
Simulation of the Fe3+ (S = 52) and Mn2+(S = 52)EPR spectra was performed by diagonalisation of the spinHamiltonian Hsp for S ge 1 (Abraham amp Bleaney 1986)
Hsp = βe middot B middot g middot S + D[S 2z minus 13 S (S + 1)] + E(S 2
x minus S 2y)
where B is the magnetic field vector S the spin operatorg the g tensor S = 12 D and E are the zero field splittingparameters
Simulation of the signal of sample S-28 results in a satis-factory agreement with the experimental spectrum for theparameters g = 200 ΔBpp = 30 mT D = minus80 mT and E =10 mT corresponding to the orthorhombic symmetry withλ = 18 (Fig 5)
The spectra of elbaite samples N-20 and N-8 (Fig 4and 5) were more complex than those of schorl and draviteThe signal at g asymp 20 was surrounded by the componentsof the fine structure of Mn2+ (S = 52) with a splittingof about 150 mT Additionally hyperfine structure due toMn2+ (I = 52) was also present (Fig 4)
Simulation of the spectrum of elbaite N-20 (Fig 5) gavea good fit with experiment for the parameters g = 199ΔBpp = 50 mT D = minus90 mT E = 30 mT λ = 13 Thezero field splitting parameter E indicating the degree oforthorhombic distortion of the octahedra is greater for el-baite (E = 30 mT) than for schorl and dravite (E = 10 mT)Similarly the D parameter gauging an axial distortion isalso greater in elbaite than in S-28 The results indicate thatMn2+ ions occupy lower symmetry sites than those popu-lated by Fe3+ in schorl and dravite probably due to thecoexistence of the Mn2+ (cation radius 083 Aring Shannon1976) and Al3+ (radius 0535 Aring Shannon 1976) withinthe Y-octahedra triad
EPR spectra of tourmaline heated in air
To obtain additional information about the tourmalinestructure and the nature of thermal dehydration and oxida-
Fig 6 EPR spectra of dravite type tourmaline S-25 registered inX band at 293 K after heating in the temperature range 770ndash1130 K
tion processes selected samples were heated in the temper-ature range of 750ndash1310 K in air The concomitant gradualoxidation of metal ions such as Fe2+ Mn2+ and V3+ wasfollowed by EPR spectroscopy
Thermal decomposition of tourmalines in air was investi-gated using XRD and DTA by Bogdanova et al (1981) andrecently using jointly XRD and Moumlssbauer spectroscopyby Pieczka amp Kraczka (2004) Effect of heat treatment wasalso studied by Castaneda et al (2006) with respect to colortransformations It was found that at temperatures above820 K deformation of the crystal lattice of tourmalinestakes place resulting in a progressive decrease of the latticeparameter ao and simultaneously slight increase of the pa-rameter co This effect was explained as resulting from theoxidation of Fe2+ with the ionic radius of 078 Aring presentin the Y sites to the smaller Fe3+ ions with the radius of0645 Aring (Shannon 1976) At the final stage of the thermaltreatment above 1050ndash1110 K the intensity of the X-raydiffraction peaks declines indicating deterioration of longrange order and finally a total destruction of the tourmalinestructure occurring above 1120ndash1170 K
Four samples of tourmaline (S-25 S-28 S-41 and N-20) were selected for thermal experiments Changes inthe spectra occurring upon heating of schorl and draviteare shown in Fig 6ndash8 In sample S-25 with the smallestamount of iron (Table 1) the total intensity of the EPRspectrum increased with increasing temperature (Fig 6)Up to 1050 K this increase was mainly due to the signalwith g asymp 43 but at higher temperatures the signal withg asymp 20 increased dramatically as well In the low fieldpart of the spectrum a small peak at g asymp 90 was observed
238 J Babinska K Dyrek A Pieczka Z Sojka
Fig 7 EPR spectrum of dravite type tourmaline S-25 registered inX band at 77 K after heating at 1130 K in air complete spectrum (a)and the signal of V4+ (b)
whose intensity was not affected significantly by the tem-perature The observed changes may be attributed to the ox-idation of Fe2+ to Fe3+ which at lower temperatures takesplace preferentially at the sites with g asymp 43 ie within theoctahedral Y triad At higher temperatures formation ofthe coupled pairs Fe3+-O- Fe2+ andor Fe3+-O- Fe3+ withg asymp 20 is mainly responsible for further increase of thesignal
Simultaneously it was found that at temperatures above890 K an additional sharp signal with gperp = 1956 and g =1952 attributed to V4+ ions became more intense withincreasing temperature The highest increase in signal in-tensity was observed between 1050 and 1130 K At thistemperature range sharp decrease in bond length 〈 Y-O 〉connected with oxidation of primary Fe2+ at Y sites wasfound by XRD (Pieczka amp Kraczka 2004) Both obser-vations may be explained by oxidation of V3+ (ionic ra-dius 064 Aring) to V4+ (ionic radius 054 Aring) The hyperfinestructure with two sets of eight lines (Aperp = 53 mT A =179 mT Fig 7) is typical of tetravalent vanadium (S = 12I = 72 100) which indicates that the vanadium centreswere well dispersed within the hosting sites of the studiedtourmalines
In sample S-28 the intensity of the EPR signals increasedup to 1070 K mainly in the region of g asymp 43 whereas thesignal with g asymp 20 was dominating at higher temperaturesduring decomposition of the tourmaline structure (Fig 8)Above 1190 K both signals became narrower which maybe assigned to increasing exchange interactions betweenFe3+ ions andor a more statistical distribution of Fe3+ ionsamong adjacent sites The latter case is less probable be-cause above 1090 K pronounced deformation of the crystallattice of tourmaline occurs due to structural collapse andformation of new phases probably silicates andor oxidesas shown by Pieczka amp Kraczka (2004) After one hour of
Fig 8 EPR spectra of dravite type tourmaline S-28 registered inX band at 293 K after heating in the temperature range 750ndash1310 K
heating at 1310 K the signal at g asymp 20 disappeared be-cause of oxidation while that at g asymp 43 persisted
For sample S-41 (not shown in a figure) the signals werevery broad due to the high content of iron (Table 1) Withincreasing temperature gradual increase in the intensity ofthe signal with g asymp 43 (up to 970 K) and the signal withg asymp 20 (up to 1130 K) was observed Similarly as in thecase of other dravite samples (S-25 S-28) after one hourof heating at 1310 K the total intensity of the spectrumconsiderably decreased due to oxidation
Different changes in the spectra were observed in the caseof oxidation of the elbaite samples In sample N-20 the totalintensity of the spectrum steadily decreased with increasingtemperature up to 1150 K whereas the signal at g asymp 43 be-came more pronounced (Fig 9) In contrast to schorl anddravite the region of g asymp 20 was dominated particularlyby an initial decrease of the Mn2+ signal (d5) probably dueto oxidation to Mn3+ (d4) ions not seen in EPR As a re-sult the signal of Fe3+ at g asymp 43 became dominant in thespectrum Its intensity increased upon oxidation of Fe2+
(d6) to Fe3+(d5) Above 1150 K after total collaps of the
EPR studies of tourmaline 239
Fig 9 EPR spectra of elbaite tourmaline N-20 registered in X bandat 293 K after heating in the temperature range 750ndash1310 K
tourmaline structure the spectrum became much simplerOnly a single relatively symmetric broad (50 mT) signal atg asymp 20 was observed with the intensity increasing up to1310 K The signal is most probably related with clusteredMn4+ (d3) ions formed upon oxidation of Mn3+(d4) Theinitial decrease in the EPR signal intensity indicated thatthe oxidation of Mn2+ (d5) up to 1150 K led to Mn3+(d4)The formation of Mn4+ (d3) begins above this temperatureafter the structure collapse
Conclusions
The EPR spectra of natural and oxidized schorl and dravitediffer strongly from those of elbaite In schorl and draviteisolated Fe3+ (d5 S = 52) ions with g asymp 43 located atthe octahedral Y triad or at Z sites are present Addition-ally broad signals with g asymp 20 were observed which maybe attributed to magnetically coupled clusters of the typeFe2+
Y -O-Fe3+Y andor Fe3+
Y -O-Fe3+Z Upon heating in air at
temperatures up to about 1000 K newly-oxidized Fe3+ ionsare isolated and mainly localized at positions with g asymp 43whereas those produced at still higher temperatures aremagnetically coupled and occupy sites with g asymp 20 Sig-nals attributed to paramagnetic V4+ (d1 S = 12) and Mn2+
(d5 S = 52) ions were also found in natural samples ofschorl and dravite Presence of V3+ (d2) was inferred bynoting the increase of the EPR signal of V4+ ions in thecourse of thermal treatment
In elbaite isolated and coupled Mn2+ ions were foundwhich occupy lower symmetry sites than those populatedby Fe3+ in schorl and dravite This effect revealed by spec-tral simulation may be due to the simultaneous presenceof Mn2+ and Al3+ ions of very different radii in the Y-octahedra Upon heating in air Mn2+ (d5) were graduallyoxidized to Mn3+ (d4) Finally after collapse of the tour-maline structure oxidation of Mn3+ (d4) to Mn4+ (d3) wasobserved
The unique selectivity as well as sensitivity of EPR toparamagnetic ions allowed us to obtain information aboutthe valence states of iron vanadium and manganese in thetourmaline structure and their transformations upon ther-mal treatment The results concerning the iron corroboratewell previous Moumlssbauer data
Acknowledgements The authors wish to dedicate thiswork to the memory of Professor Dr Witold Zabinski anoutstanding Polish mineralogist who passed away in Jan-uary 2007
The studies were supported by the AGH University ofScience and Technology Grant No 1111140158
References
Abraham A amp Bleaney B (1986) Paramagnetic Resonance ofTransition Metal Ions Ed Dover Publications N York 1986
Bershov LV Martirosyan VO Marfunin AS Platonov ANTarashchan AN (1968) Color centres in lithium tourmaline(elbaite) Soviet Phys Cryst 13 629-630
Bogdanova LA Afonina GG Glyuk DS Makagon VM(1981) X-ray study of the thermal transformations of tourma-lines Izw AN SSSR ser Geol 8 136-142 (in Russian)
Burns RG (1972) Mixed valencies and site occupancies of iron insilicate minerals from Moumlssbauer spectroscopy Can J Spectr17 51-59
Castaneda C Eeckhout SG Magela da Costa G Botelho NFDe Grave E (2006) Effect of heat treatment on tourmalinefrom Brazil Phys Chem Minerals 33 207-216
De Camargo MB amp Isotani S (1988) Optical absorptionspectroscopy of natural and irradiated pink tourmaline AmMineral 73 172-180
Dyar MD Taylor EM Lutz TM Francis CA Guidotti CVWise M (1998) Inclusive chemical characterization of tour-maline Moumlssbauer study of Fe valence and site occupancy AmMineral 83 848-864
Eeckhout SG Corteel C Van Coster E De Grave E De PaepeP (2004) Crystal-chemical characterization of tourmalinesfrom the English Lake District Electron-microprobe analysesand Moumlssbauer spectroscopy Am Mineral 89 1743-1751
Ertl A Hughes JM Prowatke S Rossman GR London DFritz EA (2003) Mn-rich tourmaline from Austria structurechemistry optical spectra and relations to synthetic solid solu-tions Am Mineral 88 1379-1376
Ferrow EA Annersten A Gunawardane RP (1988) Moumlssbauereffect study on the mixed valence state of iron in tourmalineMineral Mag 52 221-228
240 J Babinska K Dyrek A Pieczka Z Sojka
Friebele EJ Wilson LK Dozier AW Kinser DL (1971)Antiferromagnetism in an oxide semiconducting glass PhysStat Sol 45 323-331
Godovikov AA (1975) Mineralogija ndash Izdatielstvo NedraMoskwa (in Russian)
Hawthorne FC amp Henry DJ (1999) Classification of the mineralsof the tourmaline group Eur J Mineral 11 201-215
Hermon E Simkin DJ Donnay G Muir WB (1973) Thedistribution of Fe2+ and Fe3+ in iron-bearing tourmalines aMoumlssbauer Study Tschermaks Min Petr Mitt 19 124-132
Ja YH (1972) g asymp 43 Isotropic EPR Line in Tourmaline J ChemPhys 57 3020-3022
Korovushkin VV Kuzmin VI Belov VF (1979) Moumlssbauerstudies of structural features in tourmaline of various genesisPhys Chem Minerals 4 209-220
Krambrock K Pinheiro MVB Medeiros SM Guedes KJSchweizer S Spaeth J-M (2002) Investigation of radiation-induced yellow colour in tourmaline by magnetic resonanceNucl Instr Methods in Phys Research Sect B 191 1-4 241-245
Krambrock K Pinheiro MVB Guedes KJ Medeiros SMSchweizer S Spaeth J-M (2004) Correlation of irradiation-induced yellow color with the Ominus hole centre in tourmalinePhys Chem Minerals 31 168-175
Leckebusch R (1978) Chemical composition and colour of tour-malines from Darre Pech (Nuristan Afghanistan) N Jb MinerAbh 133 53-70
Manning PG (1968) An optical absorption study of the origin ofcolour and pleochroism in pink and brown tourmalines CanMineral 9 678-690
Menil F Fournes L Dance JM Videau JJ (1979) Sodium ironfluorophosphate glasses Part 2 EPR and Moumlssbauer resonancestudy J Non Cryst Solids 34 209-265
Mombourquette MJ Weil JA McGavin DG (2000) Computerprogram EPR-NMR Version 64 University of SaskatchewanSaskatoon Sask
Narducci D Lucca M Morazzoni F Scotti R (1989) Electronspin resonance investigation of the electronic structure of hop-ping centres and the polaronic conduction in iron containingphosphate glasses J Chem Soc Faraday Trans I 85 124099-4110
Novozhilov AI Woskresenskaja IE Samojlovich MI (1968)Electron paramagnetic resonance study of tourmalines SovietPhys 14 416-418
Pieczka A amp Kraczka J (2004) Oxidized tourmalines ndash a com-bined chemical XRD and Motildessbauer study Eur J Mineral16 309-321
Pieczka A Kraczka J Zabinski W (1998) Moumlssbauer spectra ofFe3+ poor schoumlrls reinterpretation on the basis of the orderedstructure model J Czech Geol Soc 43 1-2 69-74
Saegusa N Price DC Smith G (1979) Analysis of theMoumlssbauer spectra of several iron-rich tourmalines (schoumlrls) JPhys (Paris) 40 C2 456-459
Shannon RD (1976) Revised effective ionic radii and systematicsstudies of interatomic distances in halides and chalcogenidesActa Cryst A32 751-767
Sperlich G Urban P Frank G (1973) d1 Electrons in amorphoussemiconducting V2O5 and MoO3 compounds (ESR measure-ments) Z Phys 263 315-323
Received 20 July 2006Modified version received 31 January 2007Accepted 29 November 2007
238 J Babinska K Dyrek A Pieczka Z Sojka
Fig 7 EPR spectrum of dravite type tourmaline S-25 registered inX band at 77 K after heating at 1130 K in air complete spectrum (a)and the signal of V4+ (b)
whose intensity was not affected significantly by the tem-perature The observed changes may be attributed to the ox-idation of Fe2+ to Fe3+ which at lower temperatures takesplace preferentially at the sites with g asymp 43 ie within theoctahedral Y triad At higher temperatures formation ofthe coupled pairs Fe3+-O- Fe2+ andor Fe3+-O- Fe3+ withg asymp 20 is mainly responsible for further increase of thesignal
Simultaneously it was found that at temperatures above890 K an additional sharp signal with gperp = 1956 and g =1952 attributed to V4+ ions became more intense withincreasing temperature The highest increase in signal in-tensity was observed between 1050 and 1130 K At thistemperature range sharp decrease in bond length 〈 Y-O 〉connected with oxidation of primary Fe2+ at Y sites wasfound by XRD (Pieczka amp Kraczka 2004) Both obser-vations may be explained by oxidation of V3+ (ionic ra-dius 064 Aring) to V4+ (ionic radius 054 Aring) The hyperfinestructure with two sets of eight lines (Aperp = 53 mT A =179 mT Fig 7) is typical of tetravalent vanadium (S = 12I = 72 100) which indicates that the vanadium centreswere well dispersed within the hosting sites of the studiedtourmalines
In sample S-28 the intensity of the EPR signals increasedup to 1070 K mainly in the region of g asymp 43 whereas thesignal with g asymp 20 was dominating at higher temperaturesduring decomposition of the tourmaline structure (Fig 8)Above 1190 K both signals became narrower which maybe assigned to increasing exchange interactions betweenFe3+ ions andor a more statistical distribution of Fe3+ ionsamong adjacent sites The latter case is less probable be-cause above 1090 K pronounced deformation of the crystallattice of tourmaline occurs due to structural collapse andformation of new phases probably silicates andor oxidesas shown by Pieczka amp Kraczka (2004) After one hour of
Fig 8 EPR spectra of dravite type tourmaline S-28 registered inX band at 293 K after heating in the temperature range 750ndash1310 K
heating at 1310 K the signal at g asymp 20 disappeared be-cause of oxidation while that at g asymp 43 persisted
For sample S-41 (not shown in a figure) the signals werevery broad due to the high content of iron (Table 1) Withincreasing temperature gradual increase in the intensity ofthe signal with g asymp 43 (up to 970 K) and the signal withg asymp 20 (up to 1130 K) was observed Similarly as in thecase of other dravite samples (S-25 S-28) after one hourof heating at 1310 K the total intensity of the spectrumconsiderably decreased due to oxidation
Different changes in the spectra were observed in the caseof oxidation of the elbaite samples In sample N-20 the totalintensity of the spectrum steadily decreased with increasingtemperature up to 1150 K whereas the signal at g asymp 43 be-came more pronounced (Fig 9) In contrast to schorl anddravite the region of g asymp 20 was dominated particularlyby an initial decrease of the Mn2+ signal (d5) probably dueto oxidation to Mn3+ (d4) ions not seen in EPR As a re-sult the signal of Fe3+ at g asymp 43 became dominant in thespectrum Its intensity increased upon oxidation of Fe2+
(d6) to Fe3+(d5) Above 1150 K after total collaps of the
EPR studies of tourmaline 239
Fig 9 EPR spectra of elbaite tourmaline N-20 registered in X bandat 293 K after heating in the temperature range 750ndash1310 K
tourmaline structure the spectrum became much simplerOnly a single relatively symmetric broad (50 mT) signal atg asymp 20 was observed with the intensity increasing up to1310 K The signal is most probably related with clusteredMn4+ (d3) ions formed upon oxidation of Mn3+(d4) Theinitial decrease in the EPR signal intensity indicated thatthe oxidation of Mn2+ (d5) up to 1150 K led to Mn3+(d4)The formation of Mn4+ (d3) begins above this temperatureafter the structure collapse
Conclusions
The EPR spectra of natural and oxidized schorl and dravitediffer strongly from those of elbaite In schorl and draviteisolated Fe3+ (d5 S = 52) ions with g asymp 43 located atthe octahedral Y triad or at Z sites are present Addition-ally broad signals with g asymp 20 were observed which maybe attributed to magnetically coupled clusters of the typeFe2+
Y -O-Fe3+Y andor Fe3+
Y -O-Fe3+Z Upon heating in air at
temperatures up to about 1000 K newly-oxidized Fe3+ ionsare isolated and mainly localized at positions with g asymp 43whereas those produced at still higher temperatures aremagnetically coupled and occupy sites with g asymp 20 Sig-nals attributed to paramagnetic V4+ (d1 S = 12) and Mn2+
(d5 S = 52) ions were also found in natural samples ofschorl and dravite Presence of V3+ (d2) was inferred bynoting the increase of the EPR signal of V4+ ions in thecourse of thermal treatment
In elbaite isolated and coupled Mn2+ ions were foundwhich occupy lower symmetry sites than those populatedby Fe3+ in schorl and dravite This effect revealed by spec-tral simulation may be due to the simultaneous presenceof Mn2+ and Al3+ ions of very different radii in the Y-octahedra Upon heating in air Mn2+ (d5) were graduallyoxidized to Mn3+ (d4) Finally after collapse of the tour-maline structure oxidation of Mn3+ (d4) to Mn4+ (d3) wasobserved
The unique selectivity as well as sensitivity of EPR toparamagnetic ions allowed us to obtain information aboutthe valence states of iron vanadium and manganese in thetourmaline structure and their transformations upon ther-mal treatment The results concerning the iron corroboratewell previous Moumlssbauer data
Acknowledgements The authors wish to dedicate thiswork to the memory of Professor Dr Witold Zabinski anoutstanding Polish mineralogist who passed away in Jan-uary 2007
The studies were supported by the AGH University ofScience and Technology Grant No 1111140158
References
Abraham A amp Bleaney B (1986) Paramagnetic Resonance ofTransition Metal Ions Ed Dover Publications N York 1986
Bershov LV Martirosyan VO Marfunin AS Platonov ANTarashchan AN (1968) Color centres in lithium tourmaline(elbaite) Soviet Phys Cryst 13 629-630
Bogdanova LA Afonina GG Glyuk DS Makagon VM(1981) X-ray study of the thermal transformations of tourma-lines Izw AN SSSR ser Geol 8 136-142 (in Russian)
Burns RG (1972) Mixed valencies and site occupancies of iron insilicate minerals from Moumlssbauer spectroscopy Can J Spectr17 51-59
Castaneda C Eeckhout SG Magela da Costa G Botelho NFDe Grave E (2006) Effect of heat treatment on tourmalinefrom Brazil Phys Chem Minerals 33 207-216
De Camargo MB amp Isotani S (1988) Optical absorptionspectroscopy of natural and irradiated pink tourmaline AmMineral 73 172-180
Dyar MD Taylor EM Lutz TM Francis CA Guidotti CVWise M (1998) Inclusive chemical characterization of tour-maline Moumlssbauer study of Fe valence and site occupancy AmMineral 83 848-864
Eeckhout SG Corteel C Van Coster E De Grave E De PaepeP (2004) Crystal-chemical characterization of tourmalinesfrom the English Lake District Electron-microprobe analysesand Moumlssbauer spectroscopy Am Mineral 89 1743-1751
Ertl A Hughes JM Prowatke S Rossman GR London DFritz EA (2003) Mn-rich tourmaline from Austria structurechemistry optical spectra and relations to synthetic solid solu-tions Am Mineral 88 1379-1376
Ferrow EA Annersten A Gunawardane RP (1988) Moumlssbauereffect study on the mixed valence state of iron in tourmalineMineral Mag 52 221-228
240 J Babinska K Dyrek A Pieczka Z Sojka
Friebele EJ Wilson LK Dozier AW Kinser DL (1971)Antiferromagnetism in an oxide semiconducting glass PhysStat Sol 45 323-331
Godovikov AA (1975) Mineralogija ndash Izdatielstvo NedraMoskwa (in Russian)
Hawthorne FC amp Henry DJ (1999) Classification of the mineralsof the tourmaline group Eur J Mineral 11 201-215
Hermon E Simkin DJ Donnay G Muir WB (1973) Thedistribution of Fe2+ and Fe3+ in iron-bearing tourmalines aMoumlssbauer Study Tschermaks Min Petr Mitt 19 124-132
Ja YH (1972) g asymp 43 Isotropic EPR Line in Tourmaline J ChemPhys 57 3020-3022
Korovushkin VV Kuzmin VI Belov VF (1979) Moumlssbauerstudies of structural features in tourmaline of various genesisPhys Chem Minerals 4 209-220
Krambrock K Pinheiro MVB Medeiros SM Guedes KJSchweizer S Spaeth J-M (2002) Investigation of radiation-induced yellow colour in tourmaline by magnetic resonanceNucl Instr Methods in Phys Research Sect B 191 1-4 241-245
Krambrock K Pinheiro MVB Guedes KJ Medeiros SMSchweizer S Spaeth J-M (2004) Correlation of irradiation-induced yellow color with the Ominus hole centre in tourmalinePhys Chem Minerals 31 168-175
Leckebusch R (1978) Chemical composition and colour of tour-malines from Darre Pech (Nuristan Afghanistan) N Jb MinerAbh 133 53-70
Manning PG (1968) An optical absorption study of the origin ofcolour and pleochroism in pink and brown tourmalines CanMineral 9 678-690
Menil F Fournes L Dance JM Videau JJ (1979) Sodium ironfluorophosphate glasses Part 2 EPR and Moumlssbauer resonancestudy J Non Cryst Solids 34 209-265
Mombourquette MJ Weil JA McGavin DG (2000) Computerprogram EPR-NMR Version 64 University of SaskatchewanSaskatoon Sask
Narducci D Lucca M Morazzoni F Scotti R (1989) Electronspin resonance investigation of the electronic structure of hop-ping centres and the polaronic conduction in iron containingphosphate glasses J Chem Soc Faraday Trans I 85 124099-4110
Novozhilov AI Woskresenskaja IE Samojlovich MI (1968)Electron paramagnetic resonance study of tourmalines SovietPhys 14 416-418
Pieczka A amp Kraczka J (2004) Oxidized tourmalines ndash a com-bined chemical XRD and Motildessbauer study Eur J Mineral16 309-321
Pieczka A Kraczka J Zabinski W (1998) Moumlssbauer spectra ofFe3+ poor schoumlrls reinterpretation on the basis of the orderedstructure model J Czech Geol Soc 43 1-2 69-74
Saegusa N Price DC Smith G (1979) Analysis of theMoumlssbauer spectra of several iron-rich tourmalines (schoumlrls) JPhys (Paris) 40 C2 456-459
Shannon RD (1976) Revised effective ionic radii and systematicsstudies of interatomic distances in halides and chalcogenidesActa Cryst A32 751-767
Sperlich G Urban P Frank G (1973) d1 Electrons in amorphoussemiconducting V2O5 and MoO3 compounds (ESR measure-ments) Z Phys 263 315-323
Received 20 July 2006Modified version received 31 January 2007Accepted 29 November 2007
EPR studies of tourmaline 239
Fig 9 EPR spectra of elbaite tourmaline N-20 registered in X bandat 293 K after heating in the temperature range 750ndash1310 K
tourmaline structure the spectrum became much simplerOnly a single relatively symmetric broad (50 mT) signal atg asymp 20 was observed with the intensity increasing up to1310 K The signal is most probably related with clusteredMn4+ (d3) ions formed upon oxidation of Mn3+(d4) Theinitial decrease in the EPR signal intensity indicated thatthe oxidation of Mn2+ (d5) up to 1150 K led to Mn3+(d4)The formation of Mn4+ (d3) begins above this temperatureafter the structure collapse
Conclusions
The EPR spectra of natural and oxidized schorl and dravitediffer strongly from those of elbaite In schorl and draviteisolated Fe3+ (d5 S = 52) ions with g asymp 43 located atthe octahedral Y triad or at Z sites are present Addition-ally broad signals with g asymp 20 were observed which maybe attributed to magnetically coupled clusters of the typeFe2+
Y -O-Fe3+Y andor Fe3+
Y -O-Fe3+Z Upon heating in air at
temperatures up to about 1000 K newly-oxidized Fe3+ ionsare isolated and mainly localized at positions with g asymp 43whereas those produced at still higher temperatures aremagnetically coupled and occupy sites with g asymp 20 Sig-nals attributed to paramagnetic V4+ (d1 S = 12) and Mn2+
(d5 S = 52) ions were also found in natural samples ofschorl and dravite Presence of V3+ (d2) was inferred bynoting the increase of the EPR signal of V4+ ions in thecourse of thermal treatment
In elbaite isolated and coupled Mn2+ ions were foundwhich occupy lower symmetry sites than those populatedby Fe3+ in schorl and dravite This effect revealed by spec-tral simulation may be due to the simultaneous presenceof Mn2+ and Al3+ ions of very different radii in the Y-octahedra Upon heating in air Mn2+ (d5) were graduallyoxidized to Mn3+ (d4) Finally after collapse of the tour-maline structure oxidation of Mn3+ (d4) to Mn4+ (d3) wasobserved
The unique selectivity as well as sensitivity of EPR toparamagnetic ions allowed us to obtain information aboutthe valence states of iron vanadium and manganese in thetourmaline structure and their transformations upon ther-mal treatment The results concerning the iron corroboratewell previous Moumlssbauer data
Acknowledgements The authors wish to dedicate thiswork to the memory of Professor Dr Witold Zabinski anoutstanding Polish mineralogist who passed away in Jan-uary 2007
The studies were supported by the AGH University ofScience and Technology Grant No 1111140158
References
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Castaneda C Eeckhout SG Magela da Costa G Botelho NFDe Grave E (2006) Effect of heat treatment on tourmalinefrom Brazil Phys Chem Minerals 33 207-216
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Eeckhout SG Corteel C Van Coster E De Grave E De PaepeP (2004) Crystal-chemical characterization of tourmalinesfrom the English Lake District Electron-microprobe analysesand Moumlssbauer spectroscopy Am Mineral 89 1743-1751
Ertl A Hughes JM Prowatke S Rossman GR London DFritz EA (2003) Mn-rich tourmaline from Austria structurechemistry optical spectra and relations to synthetic solid solu-tions Am Mineral 88 1379-1376
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240 J Babinska K Dyrek A Pieczka Z Sojka
Friebele EJ Wilson LK Dozier AW Kinser DL (1971)Antiferromagnetism in an oxide semiconducting glass PhysStat Sol 45 323-331
Godovikov AA (1975) Mineralogija ndash Izdatielstvo NedraMoskwa (in Russian)
Hawthorne FC amp Henry DJ (1999) Classification of the mineralsof the tourmaline group Eur J Mineral 11 201-215
Hermon E Simkin DJ Donnay G Muir WB (1973) Thedistribution of Fe2+ and Fe3+ in iron-bearing tourmalines aMoumlssbauer Study Tschermaks Min Petr Mitt 19 124-132
Ja YH (1972) g asymp 43 Isotropic EPR Line in Tourmaline J ChemPhys 57 3020-3022
Korovushkin VV Kuzmin VI Belov VF (1979) Moumlssbauerstudies of structural features in tourmaline of various genesisPhys Chem Minerals 4 209-220
Krambrock K Pinheiro MVB Medeiros SM Guedes KJSchweizer S Spaeth J-M (2002) Investigation of radiation-induced yellow colour in tourmaline by magnetic resonanceNucl Instr Methods in Phys Research Sect B 191 1-4 241-245
Krambrock K Pinheiro MVB Guedes KJ Medeiros SMSchweizer S Spaeth J-M (2004) Correlation of irradiation-induced yellow color with the Ominus hole centre in tourmalinePhys Chem Minerals 31 168-175
Leckebusch R (1978) Chemical composition and colour of tour-malines from Darre Pech (Nuristan Afghanistan) N Jb MinerAbh 133 53-70
Manning PG (1968) An optical absorption study of the origin ofcolour and pleochroism in pink and brown tourmalines CanMineral 9 678-690
Menil F Fournes L Dance JM Videau JJ (1979) Sodium ironfluorophosphate glasses Part 2 EPR and Moumlssbauer resonancestudy J Non Cryst Solids 34 209-265
Mombourquette MJ Weil JA McGavin DG (2000) Computerprogram EPR-NMR Version 64 University of SaskatchewanSaskatoon Sask
Narducci D Lucca M Morazzoni F Scotti R (1989) Electronspin resonance investigation of the electronic structure of hop-ping centres and the polaronic conduction in iron containingphosphate glasses J Chem Soc Faraday Trans I 85 124099-4110
Novozhilov AI Woskresenskaja IE Samojlovich MI (1968)Electron paramagnetic resonance study of tourmalines SovietPhys 14 416-418
Pieczka A amp Kraczka J (2004) Oxidized tourmalines ndash a com-bined chemical XRD and Motildessbauer study Eur J Mineral16 309-321
Pieczka A Kraczka J Zabinski W (1998) Moumlssbauer spectra ofFe3+ poor schoumlrls reinterpretation on the basis of the orderedstructure model J Czech Geol Soc 43 1-2 69-74
Saegusa N Price DC Smith G (1979) Analysis of theMoumlssbauer spectra of several iron-rich tourmalines (schoumlrls) JPhys (Paris) 40 C2 456-459
Shannon RD (1976) Revised effective ionic radii and systematicsstudies of interatomic distances in halides and chalcogenidesActa Cryst A32 751-767
Sperlich G Urban P Frank G (1973) d1 Electrons in amorphoussemiconducting V2O5 and MoO3 compounds (ESR measure-ments) Z Phys 263 315-323
Received 20 July 2006Modified version received 31 January 2007Accepted 29 November 2007
240 J Babinska K Dyrek A Pieczka Z Sojka
Friebele EJ Wilson LK Dozier AW Kinser DL (1971)Antiferromagnetism in an oxide semiconducting glass PhysStat Sol 45 323-331
Godovikov AA (1975) Mineralogija ndash Izdatielstvo NedraMoskwa (in Russian)
Hawthorne FC amp Henry DJ (1999) Classification of the mineralsof the tourmaline group Eur J Mineral 11 201-215
Hermon E Simkin DJ Donnay G Muir WB (1973) Thedistribution of Fe2+ and Fe3+ in iron-bearing tourmalines aMoumlssbauer Study Tschermaks Min Petr Mitt 19 124-132
Ja YH (1972) g asymp 43 Isotropic EPR Line in Tourmaline J ChemPhys 57 3020-3022
Korovushkin VV Kuzmin VI Belov VF (1979) Moumlssbauerstudies of structural features in tourmaline of various genesisPhys Chem Minerals 4 209-220
Krambrock K Pinheiro MVB Medeiros SM Guedes KJSchweizer S Spaeth J-M (2002) Investigation of radiation-induced yellow colour in tourmaline by magnetic resonanceNucl Instr Methods in Phys Research Sect B 191 1-4 241-245
Krambrock K Pinheiro MVB Guedes KJ Medeiros SMSchweizer S Spaeth J-M (2004) Correlation of irradiation-induced yellow color with the Ominus hole centre in tourmalinePhys Chem Minerals 31 168-175
Leckebusch R (1978) Chemical composition and colour of tour-malines from Darre Pech (Nuristan Afghanistan) N Jb MinerAbh 133 53-70
Manning PG (1968) An optical absorption study of the origin ofcolour and pleochroism in pink and brown tourmalines CanMineral 9 678-690
Menil F Fournes L Dance JM Videau JJ (1979) Sodium ironfluorophosphate glasses Part 2 EPR and Moumlssbauer resonancestudy J Non Cryst Solids 34 209-265
Mombourquette MJ Weil JA McGavin DG (2000) Computerprogram EPR-NMR Version 64 University of SaskatchewanSaskatoon Sask
Narducci D Lucca M Morazzoni F Scotti R (1989) Electronspin resonance investigation of the electronic structure of hop-ping centres and the polaronic conduction in iron containingphosphate glasses J Chem Soc Faraday Trans I 85 124099-4110
Novozhilov AI Woskresenskaja IE Samojlovich MI (1968)Electron paramagnetic resonance study of tourmalines SovietPhys 14 416-418
Pieczka A amp Kraczka J (2004) Oxidized tourmalines ndash a com-bined chemical XRD and Motildessbauer study Eur J Mineral16 309-321
Pieczka A Kraczka J Zabinski W (1998) Moumlssbauer spectra ofFe3+ poor schoumlrls reinterpretation on the basis of the orderedstructure model J Czech Geol Soc 43 1-2 69-74
Saegusa N Price DC Smith G (1979) Analysis of theMoumlssbauer spectra of several iron-rich tourmalines (schoumlrls) JPhys (Paris) 40 C2 456-459
Shannon RD (1976) Revised effective ionic radii and systematicsstudies of interatomic distances in halides and chalcogenidesActa Cryst A32 751-767
Sperlich G Urban P Frank G (1973) d1 Electrons in amorphoussemiconducting V2O5 and MoO3 compounds (ESR measure-ments) Z Phys 263 315-323
Received 20 July 2006Modified version received 31 January 2007Accepted 29 November 2007
