American Mineralogist, Volume 76, pages 1380-1388, 1991

Crystal chemistry of Fe- and Cr-rich warwickite

SrNroN.l Brcr. Mlml FRANcI Bntcltl:r, Srr,vro C.lpr'onrIstituto di Mineralogia e Petrografia, Universiti di Modena, Via S. Eufemia 19, 41100 Modena, Italy

AssrRAcr

Warwickite is an orthorhombic oxyborate mineral with the general formula t6lM2Ot3lBOj

(M : Mg2+, Mn2+, Fe3+, Ti4+, Al3+). The studied warwickite samples are from lamproiticrocks (umillites) and associated carbonatite-like veins (Jumilla, Murcia province, south-east Spain). Their chemical compositions are quite variable and different from those ofwarwickite described in the literature: in particular, CrrO, varies between 0.0 and 13.5wto/0, total wto/o Fe as FerO, is as high as 50.6 Mo/o and total wto/o TiO, as high as 10.2wto/0. Two main substitutions occur in Jumilla warwickite: Mg'* + Tia+ = 2Fe3+ and Fe3*+ Cr3+.

The structures of seven crystals of different compositions were refined in space group

Pnma, giving R : 0.018-0.038. The structural results show that: (l) The Ml octahedronis smaller and more distorted than the M2 octahedron. (2) Distortion of the octahedrallayer is greater in Fe-rich warwickite. (3) Fe preferentially enters the Ml site. (4) The M2-

04 distance decreases with increasing Ti, suggesting preferential ordering at the M2 site.TEM studies show planar (100) defects. These defects cause variations in the octahedralchain length.

INrnooucrroN

The name warwickite is applied to an unusual familyof orthorhombic oxyborate minerals (space group Pnma;a = 9.20, b = 3.09, c = 9.36 A) of iOeal compositiont6lM2Ot3lBO3 (M : Mg,+, Mn2+, Fe3+, Tia*, Al3*). Thestructure was first investigated by Tak6uchi et al. (1950)for warwickite of composition (Mg,Fe)' ,Tio 5OBO3.Compositions of other warwickite structures refined sub-sequently include Mg, ,rAlo,Feo 12TL 3,OBO3 (Moore andAraki, 1974) and the synthetic compounds FeCoOBO,(Venkatakrishnan and Buerger, 1972) and MgScOBO, andMgo ruMn,.roOBO, (Norrestam, I 989).

According to Hawthorne (1986), warwickite belongs toa subgroup of the infinite framework t6lM,l3lTyoz miner-als, characterized by chains of edge-sharing octahedra(rurMr) with a repeat distance that is an integral multipleof3 A. The chains, canted at 60" to each other, are crosslinked by (BOr;'- triangles in the plane perpendicular tothe length ofthe chain and by sharing edges to form rib-bons four octahedra wide (Fig. l). These geometrical fea-tures are common to other mineral groups; thus, Merlinoand Perchiazzi (1988) proposed structural analogies be-tween oxyborates and the minerals of the cuspidine fam-ily, in which two parallel (BOr)'- g.roups are replaced byone (SirO,)6- group. Moore and Araki (1974) considerwarwickite a member of the structural group denoted as"3 A wallpaper structures" because of the two-dimen-sional character and the short length ofthe c-axis (ifthemineral is referred to the nonstandard space group Pnam).According to Norrestam (1989), the ratio of divalent totrivalent cations is l:3 and 3:l at the Ml and M2 sites.

respectively, for MgScOBO, (Pnam) and M80,.-

Mn,roOBO. (P2'/a).Natural occrurences of warwickite are rare; they have

been described in marbles from Edenville (Tak6uchi etal., 1950) and from Amity (Moore and Araki, 1974), bothin the town of Warwick (Orange County, New York), andfrom Taigs (Southern Yakutia, U.S.S.R.) (Malinko et al.,l 986).

Warwickite samples from the lamproites of Jumilla(Murcia province, southeast Spain) are compositionallyunlike those previously described; they are rich in Fe(FerO, up to 50.6 wto/o) and contain significant Cr (CrrO,up to 13.5 wto/o). As the composition of Jumilla samplesvaries widely, they are suitable for examination of thestructural changes associated with variation in composi-tion.

OccunnsxcE oF wARwTcKITE AND PETROGRAPHTCFEATURES OF THE HOST ROCKS

Lamproitic rocks (7.2-7.6 Ma; Nobel et al., 198l) con-stitute two outcrops near La Celia, about 13 km west ofJumilla (Fig. 2). The smaller outcrop is composed of por-phyritic lavas only, whereas the larger outcrop containsporphyritic and coarser-grained subvolcanic rocks as wellas genetically related carbonatite-like rocks (Venturelli etal., l99l). The lamproitic rocks (umillites as defined byOsann, 1906) have been extensively studied (e.g., Fusteret al., 1967 Venturelli et al., 1984). They are composedmainly of phlogopite, olivine, apatite, and occasional cli-nopyroxene as phenocrysts, whereas the groundmass iscomposed of the same phases together with sanidine or

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BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE l 38 l

a

Fig. l. polyhedral ;;;"rwickite strucrureviewed down b. Ruled octahedra : M I ; dashed octahedra : M2;stippled triangles : B; O atoms at y : r/r are black circlesl Oatoms at y :3/c, -Va are open circles.

analcime, K-rich richterite, subordinate carbonates, andaccessory pseudobrookite, ilmenite, magnetite, and war-wickite. Brown glass is occasionally present in thegroundmass. In the subvolcanic rocks, the groundmass iscomposed mainly of sanidine oikocrysts (up to 5 mm)containing phlogopite, olivine, clinopyroxene, analcime,pseudobrookite, and opaques. The interstices defined bythe large sanidine crystals are filled either with richterite,analcime, phlogopite, sanidine, oxides, and glass or withmagmatic carbonates and Na-rich clinopyroxene, phlog-opite, sanidine, potassium amphibole, apatite, pseudo-brookite, hematite, and warwickite.

Carbonatitelike veins permeate lamproites in a smallarea (150 m long, 40 m wide) in the larger outcrop. ThecarbonatiteJike rocks probably crystallized in the range700-600 oC and under high -fo, (=HM buffer). They arecomposed mainly of granular calcite (with subordinateglobular dolomite that replaces calcite) with fluorapatite,hematite-ferrian ilmenite crystals, subordinate Na-richclinopyroxene, phlogopite, sanidine, pseudobrookite, veryrare zircon, and quartz. Warwickite is frequently an ac-cessory phase and occasionally a main phase. This is thecase for a carbonatite-like veinlet (l cm across) whichmay be seen along the pathway cut into the lamproitesleading to the southern entrance of the old hematite quar-ry, the location of most of the warwickites reported here.Warwickite constitutes about l0o/o by volume and is con-centrated along the walls ofthe veinlet in sheaflike aggre-gates of black shiny prismatic crystals (0.6 mm long withsquare cross section) extending toward the center of thevein. The luster is pearly to adamantine. The crvstals are

Fig.2. Sketch map ofthe geological relations in the area ofthe Jumilla lamproite. I : Quaternary continental sediments; 2: Miocenic continental sediments; 3 : Triassic evaporites; 4 :jumillites. The dotted area refers to the carbonatiteJike outcropwhere the warwickite occurs (Venturelli, personal communica-tion).

brittle and show good {010} cleavage. In thin section,they are deep yellow to light brown and show slight ple-ochroism with maximum absorption parallel to the length;they are uniaxial positive.

ExpnnrpmNTAL TEcHNTeuES

Chernical analyses

Warwickite samples were analyzed (Tables I and 2)using an ARL-SEMQ electron microprobe at the Istitutodi Mineralogia e Petrografia, IJniversiti di Modena. Thewavelength-dispersive method was used, operating at 15kV accelerating voltage, 20 nA sample current, 3 pm beamdiameter, and 20 s counting time on peak and 2 s onbackgrounds. Natural minerals were used as standards;the data were reduced by the method ofZiebold and Ogil-vie (1964) using the correction factors of Albee and Ray(1970). B could not be determined quantitatively on sin-gle crystals; it was qualitatively analyzed by atomic ab-sorption on concentrates ofwarwickite. Warwickite, bothin polished thin sections and as crystals used for structurerefinement, was analyzed. Average chemical composi-tions ofhomogeneous grains and selected representativeanalyses obtained on zoned crystals are reported in Tablel. The crystals used for structural refinement are chemi-cally homogeneous, and their compositions (averagedfrom 6 to l0 analyses per grain) are reported in Table 2.

The chemical formulas reported in Tables I and 2 havebeen obtained, starting from the general formulat61M8t3lT4Or6 (Hawthorne, 1986). Only those analyses forwhich the total octahedral content (tulZ) was in the range7.99-8.01 apfu (atoms per formula unit) were used forfurther investigation. All Fe was calculated as Fe3+ assr ggested by the high oxidizing conditions ofthe carbon-

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BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE

o N n F C r ) F F F ( o N O Oo l r g q q a q r q q o l C C Ct I O O r N O l O ( ! a D O r t ( O

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\ r g q q \ q r q q o ? q q q$ I O O O F O r O @ ( i ) O t i @

O F F + O N N F F ( " r N O Oa ? q q q q n q I q C o l q q qr t O O O O 6 l O I O c O ( D O * @

O F F C \ I @ F - 4 1 F N t - O ) Oo { c q q q u ? r | | q o l q o ? qt o o o o ( 9 0 | | ( o c ) o ( ' ) @

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o F F s F @ N F F F C ) N O ON O O O t s F ( t O O O O O O O+ ct ci ci o ; ct ct ci <tt (.i ct + @

S p t B = 8 R , N R 3 3 8 5 3 R $ E , 5 E E 8 a t: < i c i 6 c i ' i i F | < i c i c i + o o d d J c i I d F . c i o + dO F N N

( O b ( o O r ( \ t r O ( 9 c ) ! i N O , o ) F N ( o A I I ( ) * F O t ( 0 o OA q N l C e 4 q n t \ u ? o ? e c | q q u ? d ? | q q o ? q q od o o o r . - t r i + o o o i o + o o o o a i d l o @ F i o + @N ( 9 N

, E g g , , S g S 5 5 , 9 3 9 , , t E 8 5 tl o i c t d I l c j o + o d l - - < j I l c r t c i o + < t

F N A l

+ F * \ t o , o N r o 6 l ( D r r o l o Q Qr Q l o ! | r t a \ r q r \ \ \ l r g o l q q ql o o o I l o o * l o l o F o I l @ c ) o * @

N 6 l

O + + 6 N O O N F . + ( ' , N F t A l ( o $N O - ( " , c ) < O r O F t ( O 6 t O O N ( O F . Ia i d d c t d 6 i c i o o c i c t + l d c t d ; c i I( 7 ) N F A l

6 O N O O * O O O r F b F F : t * F Nc o F F d ) o r @ o ) o - q \ A q q q o l o ? u ? |d d d d = R d d d 5 i c i

+ o c t d o ; d I

@ N @ N o o ( ' ) N O @ ( 9 F F A I * @ F ( O ! E T O s l Q o; b - o N u t o i ; ; ( 9 \ i N o o O F - @ ( o o I o o ) o o o6 i c t d 6 d + d d d c i o + d c t c t o J d d l d d o + ( oO N N

* @ F N 6 O ( O s < f o F F F F ) O @ O , r E F q - { q , od 6 - a \ u t 6 6 ' ; o N ( o o o o ( D a o o I o o o o o r o; d c i d - r ; r : I d c i d { o o d o J c i I c i a ; d o d i t o(7 ) -61 N

o F F O F * O F O C ) F F ON o O O O N b I O O O O O O

+ o o c t d - c i I o c i d c t + dO O O N O \ t e F F N @r q r q q q g q q o l \N O O O F C T @ O O ( D o( O r N N

F N @ ( t F N A l r t r r r t ( " ,r o ? d ? d ? o q I q l I a rF O O O O I O @ O O ( 7 ) Oo ( r ) N

A l @ @ @ o c O N O F F . o O I O I S ( 9 Q E Eq - q a ? \ q C q I n q q q q C g u ? q q IF O O o O C T O T O l ( D O + O O o O N O O Io o N

F O @ F @ N ( 7 J O @ N F O I \ t @ F C Oq r q a ? o ? o l o q I t c l t u ? g q C q \ u ? I Io o o o o @ N | | ( 9 o t o o o o A l o | |( Y , ( 9 N

N N O o F O N N O C T F F C I ( o ( D bc ? q i r o q o ? o ? | | \ \ u ? q q C q o l < q I Io o o o o 6 b I l N o * o o 0 0 ( \ l 0 | |O C D N

F F N O O N O A I O - - F ( ' r N O ) @F F F A I C D b F | | * ( ' ) F - o o O O * ( g | |a , i c t c t d c i c i d I l d o + d c i o o 6 i c i | |(t (r, N

qEN$q€8€s$E( o 0 0 0 a l o t N o o N o

a D ( r ) t @ F t t @ r O < f F Oq q - q q q \ q - q \s o o o c ) c ) C o o o c ) o

O N N C ) r r ) O l O a O N Fo l q r a ? q q u ? | q q \o O O O F N O I O N ON F ( t N

o N t s < t O O C \ l @ ( O @ F -6 O O F N t O t s O O 6 l @F j d d d d @ - d d o i c ;o l t A l

o O @ t i F - \ t O )t F N O ( o 6 r F . ,r o o o o c t d dN S

F @ N t s t i F o ( o @ c o Fq q C C r r r r r q \N O O O ( D \ t C r O O ( " r Oo ( \ l N

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+ o o s O @ O F F O N ( ? ) O OT 6 O O O O t s r r ) O O O o ) O O O+ d c i c t c i o i o d d d c i o + t o

O F N O O T O ( t r F F t s ( ? ) O O@ o o o o b @ o o o o r o o o+ d o o o A i d c t d d c i o + < t

o @ N o oo o o o oc d d d + < o

F @ N O Oo o r o o od d i o c i @

r N ( o O Oo o o o od d o + d

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* N * h F ( 7 ) t \ F @ N N@ O O O t 6 ( t F F F ( 9

F i d d d c i g F - < j d f l i c i

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TABLe 2. Chemical compositions of the warwickite crystals usedfor structure refinement

BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE l 383

were made +250lo beyond the peak scan range. Threestandard intensities and orientations were monitored ev-ery 2 h and 200 reflections, respectively; no sigrrificantdeviation was noted during the data measurement.

The equivalent pairs (hkl) and (hkl) were measured inthe 2" < 0 < 33'range, k and / starting from zero. Theresulting intensities were corrected for background, Lo-rertz-polaization, and absorption effects following thesemiempirical method of North et al. (1968), and equiv-alent pairs were averaged (discrepancy factor R""- :0.013-0.041). A reflection was considered as observed ifI > 3oI.

Structure refinement was carried out using SHELX-76(Sheldrick, 1976) and starting from the atomic coordi-nates and isotropic displacement factors of Moore andAraki (1974), modified for the difference in orientation(Pnma). Scattering and anomalous dispersion factors weretaken from the International Tables for X-ray Crystallog-raphy (1974). Neutral B, half-ionized O, and fully ionizedM cation scattering factors were used; as the scatteringfactors for Ti4*, Fe3+, and Cr3+ are similar, only Fe3+ andMg2+ scattering curves were used at the Ml and M2 sites.The final difference-electron density map showed onlyrandom variations, with maxima between t0.5 e/A3 and+0.3 e/A3 for all samples. The atomic coordinates anddisplacement factors for the seven refined samples arelisted in Table 4; Table 5 reports selected interatomicdistances and Table 6,' the observed and calculated struc-ture factors. Mean atomic numbers at the octahedral sites(estimated by structure refinement and by electron probemicroanalysis) are listed in Table 7.

TEM study

Finely ground warwickite crystals were dispersed onholey carbon films supported on a Cu grid and coatedlightly with C; they were examined with a Philips 4007transmission electron microscope operated at 100 keVwith C" : 2.8 mm and a resolving power of + A. High-resolution images were obtained with underfocus condi-

I To obtain a copy of Table 6, order Document AM-91-465from the Business Ofrce, Mineralogical Society of America, 1130Seventeenth Street l.IW, Suite 330, Washington, DC 20036,U.S.A. Please remit $5.00 in advance for the microfiche.

J /S4S3S1

MgO 28.26 30.82CaO 0.06MnO 0.05 0.14At,o3 0.94 0.36Cr,O" 0.84 11.88Fe"O" 44.48 26.21Tio, 2.o2 6.62ZtO2 0.03 0.05Nb2o5 0.06 0.25B,O" 22.45 23j9sio, 0.86 0.42Mg 4.27 4.55Ca 0.01Mn 0.01Ar 0.11 0.04Cr 0.07 0.93Fe+ 3.39 1.95Ti 0.15 0.49ZrNb 0.01r6rt 9.00 9.00B 3.92 3.96si 0.09 0.04r3,4r> 4.01 4.00o 16.00 16.00

31.43 32.58

0.05 0.0s1 .19 0.559.06 8.20

27.54 25.576.64 8.960.09 0.07

23.15 23.370.85 0.644.61 4.75

0.14 0.060.71 0.632.04 1.880.49 0.66

8.00 8.003.93 3.950.08 0.064.01 4.01

16.00 1 6.00

31.31 32.35 30.020.03 0.04 0.040.11 0.03 0.040.21 0.79 0.522.O0 0.80 11.71

34.58 33.00 28.918.05 9.16 5.090.05 0.010.11 0 .o2 0 .10

23.24 23.29 22.960.30 0 50 0.624.62 4.73 4.46

0.09 0.060.06 0.922.44 2.17ojr

Tt

0.01o.o20.162.580.60

8.00 8.01 8.003.97 3.94 3.940.03 0.05 0.064.00 3.99 4.00

1 6.00 16.00 1 6.00

Note.' Sample 56 from jumillite; the remaining samples from carbonatit+like veins.

atite-like rocks and by the general indications of thestructure refinement; Mn was assumed to be Mnr+. Silicawas assigned to tetrahedral coordination (for explanation,see the crystal chemistry section).

X-ray data collection and refinement

Seven chemically homogeneous warwickite crystalsfrom the carbonatite-like vein were selected for the in-tensity data collection. Systematic absences are consistentwith space group Pnma. The crystals were mounted on aCAD4 diffractometer operating at 52kV and 40 mA withgraphite-monocromated MoKa X-radiation. Unit-cellparameters were obtained by least-squares refinement us-ing 25 reflections 18' < 0 < 25 (Table 3). Starting fromthe results of an o/0 plot based on ten selected reflections(Einstein, 1974), all intensities were measured using theo-scan technique (scan width 0.8-1.8). Background counts

TlaLE 3. Crystallographic data and parameters of the studied warwickite samples

DimensionsSample (mm)

R*^ P6/V, . N . * x100 x100

vtA.1tAl

D

tA1IA1s1s2s3S4J C

S6S7

0 . 0 5 x 0 . 0 6 x 0 . 0 90 . 0 2 x 0 . 0 3 x 0 . 1 50 . 0 3 x 0 . 0 4 x 0 . 1 30 . 0 2 x 0 . 0 2 x 0 . 1 30 . 0 2 x 0 . 0 3 x 0 . 1 . |0 . 0 1 x 0 . 0 1 x 0 . 0 60 . 0 3 x 0 . 0 4 x 0 . 0 8

470 455349 348403 400328 328343 343288 268432 430

2.123.061.302.232.544.072.40

2.461.882.321.871.793.842.63

9.246(1)9.22612)e.230(s)9.228(1)9.246(3)9.242(4)9.25s(1 )

3.0927(2)3.0860(7)3.0906(8)3.0840(5)3.0e93(5)3.098(1)3.0941 (8)

9.384{1) 268.39.369(1) 266.89.378(3) 267.59.371(1) 266.79.378(2) 268.79.383(1) 268.79.402(1) 269.3

Note.' samples as in Table 2; R"r = )"-?I' tt- - t^"t. Shown are crystal size, number of unique and observed reflections, agreement factors,

and lattice parameters. 2*2i' t*'

I 384 BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE

Tlsle 4. Atom coordinates and equivalent isotropic and anisotropic displacement factors [4'?] for the studied warwickites

B4

Sample 5101 0.0212(2)02 0.2469(2103 0.2387(2104 0.0091(2)M1 0.11564(5)M2 0.10278(7)B 0.1678(3)Sample 5201 0.0202(2102 0.2463(2103 0.2386(2)04 0.0098(2)M1 0.11569(6)M2 0.10218(9)B 0.1658(4)Sample 53ol 0.0208(2)02 0.2465(2103 0.2376(2)04 0.0101(2)M1 0.1 1s67(6)M2 0.10266(9)B 0.1674(4)Sample 5401 0.0203(2)02 0.2468(2)03 0.2388(2)04 0.0090(2)M1 0.115s6(6)M2 0.10245(9)B 0.1672(4)Sample Ss01 0.0209(2)02 0.2467(2)03 0.2371(2)04 0.0101(2)Ml 0.11495(6)M2 0.10218(9)B 0.1673(3)Sample 5601 0.0192(5)02 0.2478(5)03 0.2372(6)04 0.0099(s)Ml 0.11s0(2)M2 O 1024(2)B 0.1672(9)Sample 57

0.8648(2)0.7465(2)0.0064(2)0.3870(2)0.5688s(5)0.1 8992(6)0.8741(3)

0.8680(3)0.7469(2)0.00s7(2)0.3841(3)0.56890(6)0.1 9020(7)o.87s2(4)

0.8674(3)0.7471(2)0.0054(2)0.3835(3)0.56875(6)0.1 9032(8)0.8750(4)

0.8669(3)0.7475(2)0.00s3(2)0.3842(3)0.56885(6)0.1 9035(8)0.8748(4)

0.8676(3)o.7481(2)0.0050(2)0.3834(3)0.56951(6)0.1 90s8(8)0.8755(4)

0.8662(8)0.7484(6)0.00s9(6)0.3824(8)0.5696(2)0.1901(2)0.873(1)

0.8671(3)0.7470(3)0.00s3(3)0.3843(3)0.s6856(7)0.19040)0.8740(5)

o102o3a4M 1M2B

0.0210(2)0.2470(3)0.237s(3)0.0095(2)0.1 1 ss2(7)0.1 025(1 )0.1 663(4)

0.250.25o.250.250.250.25o.25

o.250.250.250.250.25o.25o.25

0.250.250.25o.25o.250.250.25

o.25o.25o.250.250.250.25o.25

0.25o.25o.25o.25o.250.250.25

0.25o.25o.250.250.250.250.25

0.250.25o.25o.25o.250.250.25

1.03(5)1 .1 0(6)1.05(6)1.00(5)1.08(1)1.1 9(2)0.90(7)

1.39(6)1.29(7)1.35(7)1.21(6)1.27(211 .17(3)1 .5(1 )

1.42(6)1.38(7)1,29(711.35(6)1.21(211.13(3)1 .3 (1 )

1.37(7)1 .41(8)1.44(8)1.26\7)1.16(2)1.06(3)1 .5 (1 )

0.68(6)0.73(7)0.75(7)0.66(6)0.s2(210.41(3)0.60(e)

0.7(1)0.4(1)0.6(1)0.ss(s)0.38(5)0.50(8)0.8(1)

0.66(7)0.62(8)0.63(8)0.66(7)o.62(2)0.57(3)0.6(1)

0.93(6)1.10(7)0.s6(6)0.e0(6)1.',t7(2)1 .12(3)0.75(8)

1.46(7)1.36(8)1 .41(8)1.44(711.58(2)1 41(3)2.3(1)

1.s8(8)1.64(8)1.64(8)1.80(8)1.66(2)1.42(3)1 .9(1)

1 .1 1(7)1.3e(s)1.42(s)1.24(711 .1 5(2)1.02(3)I Q f l t

0.46(9)0.54(7)0.s5(8)0.61(7)0.56(2)0.33(3)0.46(9)

0.41(6)0.26(8)

0.37(8)0.43(e)0.33(9)0.75(e)0.57(2)0.41(4)0.5(1)

1.02(7)0.92(8)1.25(8)1 .1 8(7)1.02(211 .1 7(3)0.s(1)

1.50(8)1 .3(1)1 .8(1)1.26(8)1.04(2)1.o2(4)1 .6(1)

1.24(811 .3(1)1 .11(9)0.93(7)0.66(2)0.60(3)0.9(1)

1.88(s)2.0(1)1.s(1)1.40(9)1.1 5(3)0.e1(4)1.9(1)

0.6(1)0.77(8)1 . 1 ( 1 )0.83(8)0.48(2)0.43(3)0.6(1)

o.29(7)0.6(1)

1 .0(1)0.9(1)1 .2(1)0.8{1)0.64(3)0.66(5)0.7(1)

1.14(7)1.28(8)0.95(7)0.eq6)1.04(2)1.29(3)1.02(9)

1 .21(8)1 .21(9)0.81(9)0.s3(7)1.1 9(2)1.07(4)0.6(1)

1.44(e)1.15(9)1.13(9)1.31(8)1 .31(2)1.36(4)1 . 1 ( 1 )

1 .12(9)0.8(1)1 0(1)1 .1 5(8)1.16(2)1.25(410.7(1)

0.70)0.74(s)0.63(s)0.53(7)o.52(21o.47(4)0.7(1)

0.43(7)0.6(1)

0.6(1)0.5(1)0.40)0.42(9)0.65(3)0.66(4)0.5(1)

-0.12(6)0.06(5)0.02(s)0.04(6)

-0.08(2)0.03(2)0.06(9)

-0.36(8)0.20(6)

-0.34(6)0.0s(7)

-0.06(2)0.07(3)0.2(1)

-0.24(8)o.24(7)

-0.38(7)0.13(8)

-0.05(2)0.00(3)0 .1(1)

-0.14(8)o.22(7)

-o.28(710.18(8)

-0.06(2)-0.06(3)

0 .1(1)

-0 .1 (1)-0.30(7)-0.10(6)-o.'t2(7)-0.06(2)-0.02(3)-0 .1 (1)

-0.01(6)-0.20(7)

-0.01(8)0.06(7)

-0.14(7)0.04(8)

-0.o7(2)0.00(3)0.2(1)

Note.'SamDles as in Table 2.'The form ol the anisotropic displacement factor is expl-1/4(Prlta'2 +

fl"": 0.. + zhla'dBnll. Symmetry restraints for temperature factors: -Bl, :

tions using a 30 pm objective aperture. The microanal-yses were done with the energy-dispersive X-ray detectorsystem EDAX 9100; this instrumental configuration madepossible the detailed correlation of chemistry and micro-structure.

CHnurcar, RELATToNS

The warwickite samples show wide compositionalvariation (Tables I and 2, Fig. 3). The greatest chemicalvariations are in CrrO. (0.00-13.45 wto/o; 0.00-1.07 apfu),

Fe,O,(22.6V50.63 wto/o; 1.66-3.94 aptu), and TiO2(0.21-10.15 wto/o; 0.02-0.74 apfu). MgO is in the range 25.68-33.20wto/o(3.95-4.82 apfu), and Mn, Ca, Zr, Nb, Al, andSi are minor elements. These variations occru within in-dividual samples and even within individual zoned grains.In most zoned crystals, Cr, Ti, and Mg increase from coreto rim, with Fe decreasing dramatically. This composi-tional variation could be related to the chemical changesinduced in the differentiating carbonatite-like magmas bythe opaque oxides. These crystallized earlier than or con-

BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE

Taele 5. Selected data from structure refinement of warwickite samples

l 385

s7S6J Cs4DJS1

B-o1 [A]B-O2 tAlB-O3 tAl(B-o) tAl

1.358(3)1.404(3)1.404(3)1.389

1.34s(4)1.413(411.394(4)1.384

1 3s6(4)1.404(4)1.383(4)1.381

1.3s8(4)1 .401(4)1.390(4)1.383

1.357(4)1.402(4)1.375(4)1.378

1 .369(10)1 .401(1 2)1.390(1 2)1.387

1.347(4)1.408(5)1.401(s)1.385

M 1 - o 4 ( x 1 ) t A lM1-o4 (x2) tAlM1-o2 (x1) tA lM1-O3 (x2) [A](M1-O) [A]OQE

1.970(2)1.973(1 )2.062(212.1 33(1 )2.O411 .0196

1.988(2)1 .978(1 )2.058(2)2.1 30(1 )2.0441 .0166

1.992(2)1 .984(1 )2.063(2)2.1 39(1 )2.0501 .0160

1.991(3)1.973(1 )2.066(2)2.1 31 (1 )2.0441 .0167

1.997(3)1 .983(1)2.o71(212.1 s3(1 )2.0571 .0158

2.007(7)1.983(3)2.079(5)2.150(4)2.0591 .0158

1.se1(3)1.982(2)2.072(312.142(212.O521 .0166

M2-o1 (x2) tAlM2-o4 (x1) [A]M2-o3 (x 1) tAlM2-o2 (x2) tAl(M2-O) [A]ooE

1.992(1)2.042(2)2.132(2)2.1464't)2.0751.0124

1.988(1)2.007(2)2.139(2)2.149(1)2.0701 .01 10

1 .99s(1 )2.003(2)2.136(2)2.147(2\2.07',|1 .0108

1 .987(1 )2.010(3)2.142(2)2.145(2)2.0691.0112

1.998(1)1.s99(3)2.142(2)2.155(2)2.O751.0104

1.985(4)1.997(7)2.130(6)2.14s(412.0661 . 0 1 1 4

1.998(2)2.016(3)2.14s(3)2.149(212.0761 . 0 1 1 0

Nofe.'Samples as in Table 2. Bond distances and octahedral quadratic elongation (OOE) as in Robinson et al. (1971).

temporaneously with warwickite, removing Fe from themelt and causing an increase in concentration of Cr andprobably Ti (at a slower rate) in the melt. The warwickitesamples from the jumillites have very low Cr (Table l,samples l-7; Table 2, sample 56).

Two main substitutions occur in the studied warwick-ites: (l) Mg,* + Tio* = 2Fe3* and (2) Fe3* + Cr3*. Theflrst substitution characterizes the crystal chemical rela-tions of all the analyzed crystals (Fig. 3a); it seems toprevail in warwickite with low Cr3+ content (Cr3+ < 0.2apfu) (Figs. 3b and 3c). Substitutions I and 2 both occurin samples with Cr3+ content in the range 0.20 < Cr3+ <0.65 apfu. The substitution mechanism Mgz+ * Tia+ +2Crr+ seems to prevail in warwickite with Cr3+ > 0.6apfu and Fe3+ ( 2.3 apfu (Fig. 3c).

Cnysru, cHEMTcAL RELATToNSThe seven crystals for which structures were refined

cover a wide range of octahedral (M) composition (Table2). The M cations occupy the Ml and M2 octahedra (Fig.l), which share edges to form chains of four units M2-Ml-Ml-M2 rotated at 60. to each other in the (010) plane.The chain interstices are occupied by (BOr)3- triangleswhich share the O atoms (Ol, 02, 03) with the Ml andM2 octahedra. 04 is shared by octahedra only. The iso-tropic displacement factors of samples l-4 are quite high,and this may be ascribed to some positional disorder. In

most samples, Ml and M2 have lower values with respectto O and B.

Triangular and tetrahedrally coordinated sites

The B-Ol bond distance is always smaller than B-O2and B-O3 in Jumilla samples. Previous studies on war-wickite related the smaller B-Ol distance to electrostaticbond strength (Venkatakrishnan and Buerger, 1972;Moore and Araki, 1974; Norrestam, 1989). Ol is linkedto B and M2, whereas 02 and 03 are linked to B, Ml,and M2. Furthermore, the octahedral composition seemsto affect mainly B-Ol and B-O3 individual bond dis-tances (1.345 A < S-Ol < 1.369 A; 1.375 A . B-O3 =1.404 A), whereas B-O2 remains almost unchanged.

Si in warwickite has been assigned to the tetrahedralsites by analogy with cuspidine. By substituting a (SirO,)6-group for a (BrOu)6- group, the sequence . . . B ! B tr Btr. . . along the [010] direction is changed to the sequence. . . Si O Si tr Si O Si . . . as for cuspidine (Merlino andPerchiazzi, 1988). It was not possible to determine if the(SirO,)6- groups are randomly distributed or if they formcuspidine-type domains within warwickite, as the Si con-tent is too low.

Octahedrally coordinated sites

The mean atomic number at each cation site, calculat-ed from the electron probe analysis, agrees quite well with

TABLE 7. Mean atomic number for octahedral sites, determined by structure refinement and microprobe analysis for the studiedwarwickite samples

s1 s2 s3 s4 s5 s6 s7

M1 XrefM2 XrefMl + M2 Xref'Ml + M2 EPMA"-

10.28(3) 1 0.15(3)7.92(2) 7.27(3)

36.40 34.8436.45 34.95

1 0.06(3)7.17(31

34.4634.50

s.82(4)7.14(3)

33.9234.08

10.00(4)7.4o(4)

34.8035.02

9.50(10)7.36(s)

33.7234.44

1 0.1 3(5)7.43(4)

35.123s.29

Note.' Sampfes as in Table 2', )kel: X-ray refinement; EPMA: electron microprobe analysis (Z: 4).' 2 x ( M 2 + M 1 ) .'* Sum of octahedral cation electrons.

a

a

Oo o

S . o

l 386

1.2

Cr

BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE

2.08

Ti

0.6

o| 2.06

Mg 'o' .018 OGE

Fig. 4. Mean (M-O) [A] vs. OQE for Ml (open squares) and

M2 (filled squares) octahedra.

M2 octahedra (Ml-O :2.041-2.059 A, M2-O :2.066-

2.076 A, OQE,, : 1 .0160-1.0196, OQEM' : 1 .0104-1.0124:' Table 5).

With increasing (Ml-O) and (M2-O) distances, thedistortion parameter decreases (Fig. a). Furthermore, in

Fe-rich warwickite (sample Sl), the average distance is

smaller and the distortion is higher than in Mg-, Ti-richwarwickite.

The individual bond distances M-Ol and M-O4 are

smaller than the mean (M-O) distance at both sites. Theincrease in Ti in the Jumilla warwickite samples causesthe decrease in M2-O4 (r : -0.898, Fig. 5) and a cor-responding increase in the M1-O4 distance. Thus, it seemsthat the occurrence of Ti at M2 induces the substitutionof Fe for Mg at Ml. Moore and Araki (1974) suggest thatTi in Tirich warwickite preferentially substitutes at Ml.

MrcnosrnucruREs

Although the low R values obtained for structure re-

finements support a well-ordered atomic arrangement,many crystals examined during the preliminary investi-gations showed structural disorder and chemical zoning.

Bovin et al. (1981), in a study of structural defects in

Fe

Ti0.6

oa' o t

o

o

ooo coo o.o! o

-.:....

o ' g l l .^e

a e a

1 O

t do .oa

Cr 1'2

Fig. 3. t, *. t, "r,

Cr vs. Fe (b); Ti vs. Cr (c) (atoms pfu).Open circles refer to the profile of a zoned crystal (sample M6).Filled circles refer to unzoned warwickite crystals and to themean value of zoned warwickite crystals.

that from the X-ray refinement (Table 7). The agreementis not as good for sample 56, perhaps because the chem-ical analysis is the average ofdeterminations on severalcrystals from the same rock sample. The site refinementsindicate that Ml is enriched in (Fe,Cr) relative to M2, inagreement with the results of Norrestam (1989).

The octahedral distortion OQE (Robinson et al., 1971)was used to describe the deformation of octahedra. TheMl octahedra are smaller and more distorted than the

s 6 l

s5 ts 4 t

l l5 J 5 Z

0.E

Ti

0.0 2.03 M2_ 04

t*

Fig. 5. Ti (atoms pfu) content vs. M2-O4 individual bonddistance [A].

> ! tI

i1hs6 l

s60s5tr

s70s3tr

s2 ttrs4

b

oo,̂%

r a ooo loo^ o a o i lo

Fig. 6. Selected area electron diffraction pattern (a) and elec-tron micrograph (b) of a warwickite crystal oriented with b par-allel to the electron beam. Distances between the white spotscorrespond to the cell parameters a and c. One isolated defectand one pair of defects are marked by arrows.

oxyborates with the general formula tel\{rtr1lQr, de-scribed the structures as a sequence of n octahedral layers(n : 2, 4, 6 for ludwigite, orthopinakiolite, and tak6u-chiite, respectively) between slip (s) or twin (t) planes.The structure of warwickite may thus be indicated as . . .2t2t . .. , in which the twin axis (or rotation axis) is per-pendicular to (100) and passes through the center ofa Btriangle. The most common structural defects of thewarwcikite samples examined occur in (100), changingthe sequence from the type .. .2t2I ... to the type . . .2t22t . .. . Figure 6a shows a typical selected area electrondiffraction (SAED) pattern ofwarwickite crystals orientedwith b parallel to the electron beam. It is characterizedby well-defined spots, indicating a low proportion ofde-fects. The corresponding high-resolution electron micro-graph is shown in Figure 6b, in which the periods of whitedots define the a and c cell translations. The defects ob-served in a number of crystals are of the same kind asillustrated in Figure 6b and occur randomly in the se-quence. They are produced by the shift ofa row ofwhitedots (Yza + Y+c) with respect to an adjacent row of theundisturbed structure. In places, this defect is repeatedtwice (Fig. 6b), generating a sequence ofthe type . . .2t222t. . . . As shown in Figure 7, this sequence gives rise tochains eight octahedra wide, linked by chains oftwo oc-tahedra that retain their original orientation. It is not pos-sible to discern diferences in the topologies of the Mland M2 polyhedra in the new arrangement. Apparentlythe appearance of the structural defects is not controlledby the chemical composition (as revealed by electron mi-croprobe analyses made on diferent zones of the samecrystal); it is probable that it is induced in the growingcrystals by some impurity (Bovin et al., l98l).

CoNcr,usroNsThe warwickite samples from Jumilla are chemically

variable and are characterized by high Fe and Cr con-

BIGI ET AL.: Fe- AND Cr-RICH WARWICKITE r 387

t +

- +

- - +

t +

Fig. 7 . Schematic representation of the pair of defects shownin Figure 6. The structure of warwickite is viewed down the baxis. Black and open circles are as in Figure l. The omission oftwo consecutive twin operations, t, generates chains eight octa-hedra wide, linked by chains of two octahedra.

tents. The occurrence of Cr-rich varieties is unique forthe warwickite family and is also rare in the natural oxy-borates (Hawthorne, 1986). It is reasonable to hypothe-size the existence of natural Cr and Fe end-members ofthe warwickite family. The warwickite samples studiedshow the following main substitutions: Mg2+ * Tia+ +2Fe3+ and Fe3+ + Cr3+. Coupled substitutions occur atboth Ml and M2 octahedral sites; Fe and Cr are concen-trated in Ml, whereas Mg and Ti dominate at M2. Pre-sumably the occurrence of Ti at M2 is locally coupled toMg at Ml.

Many crystals are chemically zoned and show planar(100) defects, causing variations in the octahedral chainIengths. In oxyborates having the general formulat51M3l3lBO5, different structures are generated through theregular repetition of similar structural defects (Bovin etal., 1981). The possibility of generating comparable struc-tures through the repetition of such defects also seemsreasonable for warwickite. These defects are not relatedto variation in pressure and temperature conditions ofcrystallization, as the analyzed crystals come from thesame veinlet and they seem to be independent of thechemical composition for the crystals. Probably these de-fects are induced in the growing crystals by some impu-rity.

AcxNowr,nocMENTs

We wish to thank S. Merlino of Pisa University and G. Venturelli ofParma University for useful discussion and critical reading of the manu-script. Thanks are also due to P.B. Moore and F.C. Hawthorne for theirhelpful srrggestions and stylistic improvements. The Centro Interdiparti-mentale di Calcolo (CICAIA) and Centro Grandi Strumenti (CGS) ofModena University are acknowledged for their technical support and theCNR for the use of the ARL-SEMQ electron microprobe at the Istitutodi Mineralogia e Petrografia, Universite di Modena. Financial support

t +

I 388

was given by Ministero della Universita della Ricerca Scientifica e Tec-nologica and by CNR (grant 89.00360.05).

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Mar.n;scnrvr RECEIVED Aucusr 10, 1990MeNuscrrrr AccEPTED Apnrl 6, 1991