American Mineralogist, Volume 6l, pages 878-888, 1976

The crystal chemistry and space groups of natural and synthetic titanites

JoHN B. HrccrNs,qNp Plur- H. Rrssn

Department of Geological SciencesVirginia Polytechnic Institute and State Uniuersity

B lacksburg, Virginia 2406 I

Abstract

The chemical substitutions of primary importance in natural t itanites are (Al,Fe)S+ *(F,OH)-eTia+ + 02-, where total Al * Fe is not greater than 30 mole percent, and Al isusually predominant. Electron microprobe analyses and bond strength considerations suggestthat Al and Fe occupy octahedral sites in natural titanites, whereas ubiquitous rare earthssubstitute for Ca.

The formation of out-of-step l inear domains parallel to [100] (the octahedral chain direc-tion) is favored by substitutions of Al * Fe for Ti. Pure CaTiOSiOn is primitive (P2Ja),withweak but sharp k + / odd reflections. Increasing Al + Fe substitution increases the frequencyof domains and the & + / odd reflections become diffuse, eventually disappearing as theaverage structure attains space Eroup A2/a.

The lattice parameters of titanites vary directly with the effective octahedral cation radius.As in zircon, partial metamictization of some titanites causes increases in unit-cell dimensionsand decreases in density, possibly accompanied by the inclusion of water in the recoil-damaged structure.

Introduction

The purpose of this investigation of t itanite (or"sphene," CaTiOSiOr) is to evaluate previous chem-ical analyses with regard to stoichiometry and chargebalance, to present new data obtained by electronand ion probe microanalysis, and to determine therelations between the cell parameters and the chem-ical substitutions of primary importance in naturaltitanites: (Fe,Al)3+ + (F,OH)- .r Ti4+ + O'?-. It isdemonstrated herein that the primitive space groupP2r/a is observed only in synthetic or chemically"clean" natural t itanites (< 3-4 mole percent sub-stituents for Ti), whereas those with ) 4 mole percent(Al+Fe) have diffuse reflections k + I odd whichbecome unobservable above -20 mole percent sub-stituents (space group A2/a). In addition, informa-tion on some of the rare earth elements and theeffects of metamictization on density, lattice parame-ters, and characteristic X-ray intensity yield underelectron bombardment are reported for selected spec-imens.

The first significant crystal chemical observationsof titanite were made by Zachariasen (1930), whofound that it was an orthosil icate with Ti in octahe-

dral coordination and Ca in an irregular seven- tonine-coordinated polyhedron. He explained chemicalsubstituents as isomorphous replacements in a struc-ture with general formula ABXS|O$ where I is alarge cation (Ca plus rare earths and Na) of radius0.67 to 1.35 A, B is an octahedrally-coordinated ca-tion (Ti plus Al, Fe, etc.) of radius 0.57 to 0.67 A, andX is 02-, F-, or OH-. Zachariasen concluded that,by partially replacing the underbonded O(l) oxygen(designated X) with F- or OH-, the strength of thebonds which reach this site from the neighboringcations would be more nearly equal to the charge ofthe average anion occupying the site, in addition tocharge-balancing other substitutions. Further struc-tural studies of t itanite were not undertaken unti l1968, but in the meantime numerous chemical analy-ses (e.g., Young, 1938; Prince, 1938; Jaffe, 1947) ap-peared along with some attempts to relate chemistryto physical properties.

Sahama (1946) analyzed seven titanites and con-cluded that natural specimens are "defect" (non-stoichiometric) structures, since according to hisanalyses, the amount of F and OH substitution wasindependent of the substitution of Ca and Ti bycations of other valences (Fig. la). Zabavnikova

878

0.49 0.50 0 5t o.52Prop'n Ti+ Fe +N + l t l ! + f f ig

CRYSTAL CHEMISTRY OF TITANITES

o.52

io.5o

o 49"o.49At.

o.50 0 5r o.52 0.53Prop 'n T i+ Fe +A l+ Nb + To + Mg

879

ac o.5o-o-o(L

: o.49<l-

@_ o .5 lg-o-otu

o.+effi

(/) o.ss

0.45o.45

At.

6 o.u,g-o-oL

(L

. . 0 . 5 0

c-o-o(L

. 0.50

0.50 0.55 0.60At . Prop 'n T i+ Fe+Al

o.52

0.49o.49 0.50 0.5t o.52 0.53

A t . P rop 'n T i+ Fe + A l

Ti in the octahedral site. Apparently they did notrecognize that the substitution of Fe for Ti shouldhave an effect on lattice parameterS opposite to thatof Al substituting for Ti, because the effective ionicradius of Fe8+ is greater than that of Ti, and theradius of Al is less. They also assigned significantamounts of Al to the tetrahedral (Si) site in certainsamples. Their data will be discussed later.

Although there is abundant information on titan-ite, most work does not successfully correlate phys-ical properties with chemical data. Attempts to corre-late optical properties with chemistry have beenfrustrated because of the complex substituent profileof natural titanites, especially at the minor and traceelement level. Similarly, density varies only slightlybecause only minor element substitution is involved,and in any case the most common substitution is Al(at. wt. 27) + Fe (at. wt. 56) for Ti (at. wt.48). Part of

o.65

FIc. l. Plot of atomic proportions Si versus atomic proportions of octahedral cations for titanites analyzed by (a) Sahama (1946), (b)Zabavnikova (1957), (c) Lee et a l . (1969), (d) th is study (Table I ) . The data points in a, b, and d al l fa l l wi th in the st ippled area of c.

(1957) analyzed l8 specimens, and her analyses showexceptionally good stoichiometric balance between oc-tahedral and tetrahedral cations (Fig. lb). Lee et al.(1969) reported X-ray fluorescence analyses for ma-jor elements which differ significantly and dis-tressingly from other titanite analyses appearing inthe literature and in this work (Fig. lc). It is smallwonder, then, that chemistry and the cell parametersand optical properties measured on these titanites donot correlate well with each other.

In 1972 Cerny and Sanseverino determined latticeparameters for the six titanites analyzed by Sahama(1946), in addition to the specimen used in the struc-ture refinement by Mongiorgi and Sanseverino(1968), and an annealed metamict titanite previouslyexamined by Cerny and Povondra (1970). Theyplotted cell dimensions of four specimens versus thesum (Fe*Mg+Al) which presumedly substituted for

S A H A M A

b.o a a

o

a

ZABAVNIKOVA

L E E e t o l .

d.

THIS STUDY

o ao

880 J. B. HIGGINS AND P. H. RIBBE

the observed density variation is undoubtedly related tron probe, although the slope is 1.07, rather than

to metamictization, which may be accompanied by 1.00 as expected. This probably indicates a slight

incorporation of water into the recoil-damaged struc- systematic bias in one or the other methods, but it is

ture as in zircon (Griinenfelder et al., 1964), hardly significant at the levels detected (0.0 to 2.3wL

The present work has concentrated on obtaining Vo F). Qualitative mass spectrum scans were run forquantitative microprobe analyses of the major ele- selected samples to determine ratios of rare earths,

ments, Ca, Ti, Si, Fe, Al, and F, as well as Ce, the lead isotopes, and uranium and thorium. In particu-

most abundant of the rare earths. Precise lattice pa- lar, the La, Pr, Nd, Sm, and Eu to Ce ratios were

rameters have been determined for most specimens, measured and corrected for natural iostopic abun-and using X-ray precession methods, the class of dances in order to arrive at a quantitative estimate of

reflections k + / odd were found to vary from sharp total La-, Pr-, Nd-, Sm- and Eu-oxides relative to Ce,

to diffuse to absent with increasing substitution of Al which was quantitatively determined by electron mi-

* Fe for Ti. croprobe. Further rare earth, lead and U/Th isotope

data are forthcoming in a detailed ion-probe study in

Experimental procedures cooperation with J. R. Hinthorne'

E le c t ron mi c ro p robe analy ses

The titanites were analyzed by conventional meth-ods for Ca, Ce, Ti, Al, Fe, and F with an AnLmicroprobe, using the Eupe,oR VII computer pro-gram (Rucklidge and Gasparrini, 1969) for data re-duction. Na and Mg were not detected at the 0.05weight percent level in any of the specimens; Ta wasnot detected, but specimen l2 contained -0.7 weightpercent Nb.

Ion microprobe mass analysis

The same polished samples analyzed with the elec-tron probe were examined using an Applied ResearchLaboratories ion microprobe mass analyzer (Iuu,+)(Andersen and Hinthorne, 1972, 1973) at Hasler Re-search Center, Goleta, California.' The sample sur-faces were bombarded with a negatively-chargedprimary beam of monatomic oxygen ("O ) at 17 kV.In order to determine F concentrations quan-titatively, the secondary ion intensities of the "F+isotope and the soSi+ isotope were counted. Sil iconwas used for intersample normalization since second-ary ion intensity for a given element is dependent onmatrix factors and sample orientation. It was as-sumed for these analyses that sil icon is stoichiometricin natural t itanites (i.e., one out of every eight atomsper formula unit is sil icon). Fluorine contents weredetermined by both electron and ion microprobetechniques. The Inrua results were calculated fromthe work ing curve of Hinthorne and Andersen (1975)and correlate l inearly with those determined by elec-

1 Dr. J. R. Hinthorne and Mr. Ray Conrad are acknowledgedfor their generous contributions of time and effort in this part of

the investigation, which is continuing and will be reported else-where.

Lattice parameters

Unit cell parameters of nine specimens were ob-tained from X-ray powder patterns using BaFr,annealed at 800'C (a : 6.1971 + 0.002 A; D. A.Hewitt, personal communication) as an internalstandard. The patterns were recorded at 0.5' 20/min./inch, using monochromatized CuKa radiation.Two oscil lations were measured, and peaks wereindexed with reference to the structure refinement ofSpeer and Gibbs (1976). Unit weights were assignedto all reflections and the lattice parameters were re-fined using the least-squares program of Applemanand Evans (1973).

Density meqsurements

Densities, corrected for temperature, were deter-mined on several hand-picked samples, using a Ber-man balance with toluene as the buoyant medium.

Discussion of results

The results of electron microprobe analyses and

lattice parameter measurements are l isted in Table Ifor twelve natural titanites (selected from a larger

suite of 40 specimens) that exhibit a wide range ofsubstitution of Fe * Al for Ti.

Site assignments of substituents

Understanding the crystal chemistry of t itanite re-quires that the elemental substituents be correctlyassigned to their respective coordination polyhedra in

the structure. The average unit cell of an l-centeredtitanite contains six nonequivalent crystallographicsites, three of which are occupied by anions and three

by cations. The O(2) and O(3) sites [and O(4) and

O(5) in primitive titanitesl are occupied by oxygenatoms bonded to the tetrahedral cation. The O( 1) site

CRYSTAL CHEMISTRY OF TITANITES 88r

is located at the shared octahedral vertex and is occu-pied primarily by oxygen but may contain smallamounts of F andlor OH (see below). Fluorine iscommon and although OH was not determined quan-titatively, ion microprobe mass scans indicated thepresence of H in all specimens (cl Beran, 1970).

The tetrahedral site in natural titanites is occupiedby Si, although several workers have assigned smallamounts of Al to the tetrahedral site to make upapparent deficiencies of Si in their analyses. In thiswork, however, all Al has been assigned to the oc-tahedral site, and it has been assumed that the tet-rahedral site is occupied only by Si. These assump-tions may be justified in two ways: (l) by examiningthe effect of substituting a trivalent cation in thetitanite structure in terms of Pauling's (1960) elec-trostatic valence rule, and (2) by observing that thereis essential stoichiometric balance between tetrahe-dral Si and the sum of the presumed octahedralcations-Ti, Al, Fe, Ta, and Nb.

Pauling's second rule states that in a stable ionicstructure the valence of each anion, with changedsign, is exactly or nearly equal to the sum of thestrengths of the electrostatic bonds to it from theadjacent cations. In the titanite structure with com-position CaTiOSiOo the average bond strengths atthe O(2) and O(3) sites are slightly greater than 2.00while the O(l) site is strongly underbonded. Thislocal electrostatic imbalance can be improved by sub-stituting trivalent cations in octahedral sites alongwith the substitution of a monovalent anion (F orOH) in the O(l ) site to retain overall charge balance.Figure 2 illustrates the changes in Pauling bondstrengths and formal charges resulting from thecoupled substitution M3+ + (F,OH)'- = Ti4+ + 02-.The average bond strength to the O(2) and O(3) sitesreaches a value of 2.00 at almost the same composi-tion at which the charge at the O(1) site is equal to thebond strength received at the O(l) site. Taylor andBrown (1976) have also rationalized the coupled sub-stitution in terms of Brown and Shannon (1973) bondstrengths at the O(l) site. Both models lead to theconclusion that monovalent anions are most likely tosubstitute for oxygen in O(l) and that concomitantlytrivalent cations substitute for Ti in octahedral coor-dination.

Natural titanites have been reported to incorporatea number of other metals in their structures includingNb, Ta, Cr, V, Mn, Mg, and Sn (Sahama, 1946;Clark, 1974), and it is assumed that these cationssubstitute for Ti in the octahedral site. Althoughmany previous wet-chemical analyses report divalent

iron in natural titanites, there is no independent ana-lytical evidence for it. In this work all Fe has beenassumed to be trivalent and located in the octahedralsite; a Mdssbauer study of an Fe-rich titanite isclearly desirable.

In order to compare the analyses of previous work-ers (Sahama, 1946; Zabavnikova, 1957; Lee et al.,1969) with those of this work and to determine theeffect of assigning all Fe and Al to the octahedral site,a plot of atomic proportions of Si versus atomicproportions of the sum of the octahedral cations ispresented in Figure l. Assuming that all tetrahedralsites are fil led with Si and that all Fe * Al is octahe-drally coordinated, the data points should fall on the45o line on each graph. The proximity of the datapoints to the 45o line for the analyses of Zabavnikova(1957) and this study suggests that assigning Ti * Fe-| Al to the octahedral site is reasonable with respectto octahedral-tetrahedral stoichiometry, and thatwithin the experimental errors of chemical analysis(even at the level of L2Vo of the amount present ofany element) there is no need to assign Al to thetetrahedral site. In light of the plots for Zabavnikova(1957) and this work, the spread of data points ofSahama's (1946) analyses seems to indicate less accu-rate determination of some of the metals, but theanalyses of Lee et al. show an extremely large devia-tion and must be regarded as unreliable.

Stoichiometric charge balances have been calcu-lated successfully for other orthosilicates (cf. Jones etal., 1969), but for titanite the amounts of (OH)-substituting for 02-, entered in Table I as OH",;", arein all cases less than 0.7 weight percent. The signifi-cance of these numbers is highly questionable for thefollowing reasons: (l ) not all the possible cation sub-stituents were analyzed for (e.9., the heavy rareearths) and (2) the cumulative estimated standarderrors in microprobe analyses for Ca, Ti, and Si maybe as high as I to 2 percent of the amount present,with larger relative errors for the more minor ele-ments.

The large seven-coordinated cation site in the titan-ite structure is occupied primarily by Ca; however,previous chemical analyses and the ion microprobeanalysis carried out during this study indicated that anumber of trace elements, particularly rare earths,probably occupy this site. As mentioned in our dis-cussions of the Iuun method, we determined theratios of certain rare earth isotopes to the 1a0Ce iso-tope by measuring peak heights on mass spectralscans, At this stage we have reported only La, Pr, Nd,Sm, and Eu as REE in Table l; others are present,

882 J, B, HIGGINS AND P. H. RIBBE

Tesln 1. Chemical analyses and lattice parameters of titanites

Specinen No.

Or lg lna l No. *

L 2

usNM R26731 BMNH 1966,501

Goschener A lp . San Qu ln t in , Ba jaS w i t z . C a l i f . , M e x i c o

3 4

BMNH 1968,124 USNM 820360

Okehanpton, Piednont,Devon, Eng. I ta1y

) o l

vPr&su-27 uscs 04-065 S-2

Capahal ina, Henet Quad., Yl i j j l i rv l ,BrazlL Callfornia Finland

s t o 2Ca0Ce2O 3RE203***T io2Fe2O 3A120 3F (Oltsa1.)- (F=0)

t o E a a

(excluding 0H."1")

s iCaREET iFe

T1+Fe+AlFEffective OcEa-hedral Radius (l)

a (A)b(l)c( l )B ( " )v ( i 3 )

3 0 . 6 128.57n . d . * *n . d ,3 9 . 4 3L . L 40 . 090 . 0 0 ( 0 . 3 )0 . 0 0

99 .84

1 , 0 0 01 . 000n . d ,o .9690 . 0 2 80 . 0 0 41 . 0 0 10 . 0 0 0

0 ,606

7 . 062 (3 )8 . 708 (4 )6 . s57 (2 )1 1 3 . 7 8 ( 2 )3 6 9 . 0 ( 2 )

30 . 7028 .75n . d .n . d .3 8 . 8 00 . 3 7I . 010 .07 (0 .3 )0 . 03

9 9 . 6 7

1 , 0 0 0I . 0 0 3n . d .0 . 9 5 00 , 0 0 90 . 0 3 90 . 9 9 8o. oo7

n . d . n . d .0 . 9 5 6 0 . 9 5 50 . 0 1 0 0 . 0 0 90 . 0 4 5 0 . 0 4 71 . 0 1 1 1 . 0 1 1

99 .92

1 .0000 . 9 9 4o . o 2 70 . 9 4 40 . 0 2 60 . 0 5 81 . 0 2 80 . 033

0 . 6 0 2

7 . 0s 3 ( 1 )8 . 702 (1 )

113 .83 (1 )3 6 7 . 8 ( 1 )

97 .80

1 . 0000 . 9 8 90 . 0 0 6o . 9 2 40 . 0250 . 0 5 21 . 0 0 1o.o24

o . 602

7 . 057 (2 )8 . 70s (3 )

1 1 3 . 8 9 ( 2 )368 . 2 ( 1 )

WXIGHT PERCENT OXIDES

30 . 34 29 . 99 30 . 44 29 . 82 30 . 0928 .60 28 .40 28 .41 27 .65 27 .77n . d . n . d . n . d . 0 . 8 0 0 . I 8n . d . n . d . n . d . 1 . 5 4 0 . 3 138 .59 38 .10 37 .84 37 .45 36 .970 , 4 2 0 . 3 4 0 . 8 0 1 . 0 2 1 . 0 11 . 1 5 1 . 1 9 1 . 1 7 L . 4 6 1 . 3 40 . 2 8 ( O . 2 ) 0 . 1 3 ( 0 . 4 ) 0 . 1 " 0 ( 0 . s ) 0 . 3 r ( 0 . 2 ) 0 . 2 3 ( 0 . 4 )0 . r 2 0 . 0 5 0 . 0 4 0 . 1 3 0 . 1 0

9 9 . 2 7 98 . 10 98 .72

ATOMS PER UNIT CELL NORMALIZED TO ONE SILICON

0 .602 0 .602

1 . 000r . 010

1 . 0 0 01 . 004

1 . 0 0 01 . 0 0 0

0 . 9 3 50 . 0 2 00 , 0 4 5I . 0 0 00 .010

0 . 6 0 2

7 .O6L(2)8 . 7 0 6 ( 3 )6 . s s s ( 2 )113 .93 (3 )3 6 8 . 3 ( 1 )

0 .029 0 . 0 1 4

0 . 6 0 2

7 . 0 6 1 ( 1 )8 . 708 (1 )6 . s 5 3 ( 1 )1 1 3 . 8 6 ( 1 )3 6 8 . s ( 1 )

I,ATTICE PARAMETERS

not deternlned

* USNM = U.S. Natlonal Museun; BMNH = Brit ish Museum of Natural Hlstory; VPI&SU = VPI&SU Collecttons; USGS =Unl ted Sta tes Geo log ica l Survey ; S-2 = Sahama (1946) Spec lnen No. 2 .

* * Detec ted on ion n ic roprobe bu t on ly p resent a t background leve l (<0 .01 / ) on e lec t ron mic roprobe.

* * * Ca lcu la ted f ron . rMMA spec t ra l scans , us lng-139La [na tura l abundance, 99 .9%l , la lp r [1oOZ] , 1q3Nd tLz .2Z l ,r + / S m I 1 5 . 0 % ] , r 5 l E u [ 4 7 , 8 2 ] r a t l o e d t o r q u c e [ 8 8 . 5 2 ] . T h e r a l i o s w e r e c o r r e c t e d f o r i s o t o p l c a b u n d a n c e s , a n dweight percent RX2O3 was deternined 1n proportion to welght percent Ce2O3 as neasured by electron mlcroprobe.

but their primary peaks are overlapped by LaO, CeO,PrO, NdO, SmO, EuO, and other monoxide peaks.

Stoichiomet ry of synthetic CaTiO SiO n

Analysis of a CaTiOSiOn crystal synthesized byD. A. Hewitt in this laboratory and used by Speerand Gibbs (1976) in their crystal structure analysis,shows a slight deflciency (-3Vo of the amount pres-ent) with respect to Ca, when the formula is nor-malized to five oxygens. This probably resulted fromthe glass being slightly offstoichiometric compositiondue to the presence of discrete grains of CaSiO, foundby microprobe analysis in this specimen.

Latlice pqrameters

The lattice parameter ranges observed for naturaltitanites are'. a :7.039-7.088 A; b : 8.643-8.740 A; c= 6.527-6.58a A; p : l l3. '14- l l4. l5 ' (Table I andCerny and Sanseverino,1972). These ranges are con-

sistent with the small amounts of chemical sub-stitution observed. Since the major chemical sub-stitutions in natural t itanites occur in the octahedralsite, changes in lattice parameters are directly relatedto changes in the average size of the cation occupyingthis site. The M-O bonds associated with this site arealigned more or less parallel to the three crystallogra-phic axes, so that changes in the effective size of theoctahedral cation should be discernible more or lessequally along all three crystallographic axes. In orderto observe the expected relationship between chem-istry and lattice parameters, some function of thechemistry of the octahedral site is required to plotversus lattice parameters. Cerny and Sanseverino(1912) plotted the sum of the atomic proportions ofFe * Al + Mg versus lattice parameters, and eventhough substituting Fe and Al for Ti has oppositeeffects on the size of the octahedron, they obtained agraph which changed in the expected direction by

CRYSTAL CHEMISTRY OF TITANITES

Tenlr 1, continued

883

SpeciEen No.

Ortglnal No.

Locality

8 9

VPI&SU-44 VPI&SU-43

Cape North, Cape Grisons,B r e t o n I s . . N . S . S w i t z .

10vPr&su-38

Ceeron Brook PlutonC a p e B r e t o n I s . , N . S .

11 11 (heated)

wl&su-42 wr&su-42Renfrew Co. Renfrs Co.

canada canada

' L2 t2 (heatetl)t

vPr&su-41 wr&su-41Plercevll le Plercevil le

New York New York

IIEIGHT PERCENT OXIDES

s io2CaoCe20 3REzO 3Tro2Fe203A120 3F (oHcalc)- (r=0)

Tota l(exc lud ine OH - )-

c a l c '

s lCaRXETiFeA1Ti+Fe+A1FNb

Effective Octa-hedral Radtus (l)

30.44a7 04n . d .n . d .36.790 . 5 92 . O 70 . 4 6 ( 0 . 4 )0 . 1 9

98.t2

1 .0000 , 9 8 4n . d .0 . 9090 ,0150 .0801 .0040 .048

0 .600

30 , 7028.54n . d .n . d .36 .770 . 48z . 4 L

0 . 1 8 ( 0 . 7 )0 . 0 8

99 .00

1 .0000 , 996n , d .0 . 9010 . 012o .0921 .0050 . 019

0 . 598

29.992 7 , 4 80 . 7 70 . 9 1

3 6 . 1 9l- . 18L . 2 20 . 3 7 ( 0 . 3 )0 . 16

9 8 . 5 9

29.83 30.5627 .OL 27 .680 . 2 8 0 . 2 80 . 3 6 0 . 3 5

30.79 3t .922 . 7 6 2 . 6 93 . 6 7 3 . 8 51 .50 (0 .4 ) 1 .50 (0 .4 )0 . 6 3 0 . 5 3

95 .56 98 .2L

r.000 1.0000 .9700 .0080 .7850 .0660. 1491 .0000 .146

7.088(2) 7.Osz<L)8 .740 (2 ) 8 .690 (2 )6.584(2) 6.542(L)113 .95 (3 ) 113 .98 ( r )372 .7 ( L ) 366 .3 ( r )

2 . 2 7 ( 0 . L ) 2 . 2 7 < o . 3 )

30 .7527.960 .48o . 7 4

30 .392 . 9 24 . 9 3

10.4728.040 . 4 80 . 7 4

30.7 62 . 6 J

5 . 4 6

0 . 9 5

100.49

0 . 9 5

101. l0( inc lud lnc 1 .0 v t .Z Nb"O. )

ATOMS PER IJNIT CELL NORMALIZED TO ONE SILICON

I .0000 . 9820 . 0200 . 9070 .0450. 0481 .0000 . 039

0 .603

7.060(2)8 . 7 2 4 ( 2 )6 .565 (2 )r13. 80 (2 )369 .94 (1)

0 .9700 .008o . 1 7 60 .0690. r450 . 990o.t49

1.000o . 9 7 40 .015o . 7 4 30 .0720. t891.004o.2360.015

0 .593

1 .0000 .9860 .0150 .7590 .0700 .2111.0400 .2360 .015

0 .5920 .597 0 .596

LATTICE PA.RAMETERS

a(A)b (l)c (l)B ( ' )v ( 83 )

not detemlned 7 .085 (4 ) 7 .0s5 (8 )8 .714 (s ) 8 .680 (10 )6 . s72 (3 ) 6 . s56 (7 )r14 .15 (5 ) 113 .93 (9 )37O.2 (2 ) 357 .0 ( s )

r Speclmens heated under vacuuu at l lOO'C for three hours.

using data only from Fe-poor titanites. In this studyan effective octahedral cation radius (abbreviatedEOCR) has been calculated using the following equa-t ions:

EocR : 0.605 - (0.07s Allsi) + (0.04 FelSi)

where 0.605 is the radius of quadrivalent Ti in six-fold coordination, AllSi and FelSi are the atomicproportions of Al and Fe normalized to one silicon,and 0.075 and 0.04 are the respective differences be-tween the radii of Al and Ti and Fe and Ti (effectiveionic radii from Shannon and Prewitt, 1969,1970). If_more Fe than Al is present, EOCR is greater than0.605 since Fes+ is larger than Tia+, whereas if moreAl is present, EOCR is smaller than 0.605. Effectiveradii of octahedral cations have been calculated forthe specimens listed in Table I and those of Cernyand Sanseverino (1972) for which lattice parametershave been reported. They range from 0.587 to 0.606A. Figure 3 illustrates the relation between the chem-istry of the octahedral site and lattice parameters ofour specimens, Hewitt's synthetic titanite, and the

eight specimens of Cerny and Sanseverino. Severalfactors probably affect the scatter in data points,including the assumption that only Fe, Al, and Ti arepresent in the octahedral site and that minor changesin chemistry of the calcium site have no effect onlattice parameters.

As a matter of passing interest, the b cell dimensionreported by Takenouchi (1971) is in disagreementwith all others values of b for synthetic titanites. Thisaberrant value can be traced to the misindexing of the140 peak as 2li, 133 as 402, and25l as 404 (J. A.Speer, personal communication).

Metamict titanites

During the initial plot of lattice parameters versusEOCR it was noted that two specimens (nos. I I andl2) had lattice parameters inconsistent with the otherspecimens. The glassy appearance of specimen l1 andthe broad diffraction peaks in its powder patternsuggested that it might be partially metamict. Con-firmation of its metamict character was obtained byion microprobe analysis which showed significant

884

M o l e % ( F , O H ) i n O ( l ) S i t e

Mole % Mts in Oc lohedro l S i le

100 80 60

Mo le % T i t a i n Oc tohed ro l S i t e

FIc. 2. Change in Pauling bond strength and formal charge atthe O(l), O(2), and O(3) anion sites with substitution of luf+ *(F,OH)- for Tia+ * O'z-.

Th232 and I-J"', in addition to radiogenic lead. Be-cause Holland and Gottfried (1955) had found thatdensity decreased while the cell dimensions increasedwith increasing metamictization of zircon, and be-cause Cerny and Sanseverino (1972) had reported acompletely metamict titanite in which the structurewas restored after heating for three hours at 800"C,we decided to anneal those of our specimens whichhad discrepant lattice parameters. Specimens I I and12 were heated at l l00oc for three hours in evac-uated sil ica-glass tubes, and upon cooling were rean-alyzed with the electron microprobe. Lattice parame-ters of the heated specimens were redetermined(Table I ). Cell edges of both specimens decreased andare represented on Figure 3 by the data points con-nected by nearly vertical dashed lines. Several otherspecimens were also heated under identical condi-tions: specimen l0 registered only slight changes inlattice parameters, but specimens I and 5 showednone.

Another interesting change which was noted after

J, B. HIGGINS AND P H. RIBBE

6oo 7

8.750

8.730

I 670

I 650

^ zo80

oE|r

E 7.060

ocD! lo '

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Ef f ective Ocfohedrol Rodius

Frc. 3. Variat ionofeffect iveoctahedralcation radiuswitha, D,and c cell edge. Triangles represent data from Cerny and Sanseve-rino (1972), squares are data from this work. Open symbols in-dicate unheated specimens and filled symbols represent heatedspecimens. The regression line was calculated using only data fromheated specimens.

ot 0(2) ond O(3)

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CRYSTAL CHEMISTRY OF TITANITES

heating specimen I I is related to the microprobeanalyses. Specimen I I had consistently yielded lowoxide weight percent totals (-957o) before heating;however, after heating the weight percent of eachelement analyzed increased by 2 to 5 percent of theamount present, yielding a total of -98 percent. Inorder to be sure that this effect was not due to chem-ical zonation or other properties peculiar to onegrain, several grains of unheated and heated samplewere analyzed side by side and all heated grainsshowed the same increase in elemental percentageswith respect to the unheated samples. This phenome-non at first suggested to us that density differences inthe metamict and recrystallized titanite might have aneffect on the characteristic X-ray intensity yield of theconstituent elements; the recrystallized grains yieldedhigher X-ray intensities for all elements checked. Butof course it is possible that the relatively higher X-rayyields simply resulted from a loss of water uponheating. The densities of the unheated and heatedsample l2 were determined with the Berman balanceand were found to be 3.510 g/cms and 3.585 g/cm8,respectively. This 2.1 percent increase in density uponheating agrees well with the observed 1.7 percentdecrease in cell volume. Although specimen 12 didnot show any change in composition upon heating,possibly because it did not contain water, the de-crease in lattice parameters is consistent with theobserved increase in density.

Effects of chemical substitution on space groups ofnatural and synthetic titanites

Modern refinements"of the crystal structure of tita-nite are reported by Mongiorgi and Sanseverino

(1968) for a natural A2/a titanite and by Speer andGibbs (1976) and Taylor and Brown (1976) for syn-thetic CaTiOSiOr, which has space group P2'/a atroom temperature but inverts by a displacive trans-formation to A2/a ar 220 + 20"C. The structure hasbeen described as infinite parallel chains of corner-sharing TiOu octahedra cross-linked by SiOr tetra-hedra. Two oxygens of each SiO. group are shared byadjacent octahedra in a chain, and the other twotetrahedral oxygens are shared with octahedra in ad-jacent chains. Thus each tetrahedron cross-links withthree different octahedral chains. Large cavities inthis framework of corner-sharing octahedra and tet-rahedra are filled with calcium which is coordinatedby seven to nine oxygen atoms (c/ Taylor andBrown, 1976, Fig. 3).

Robbins (1968) synthesized CaTiOSiOr and its ger-manium analog, CaTiOGeOr, by crystallizing meltsof high-purity oxides on an iridium wire loop in aplatinum crucible. Examination of these substancesby the X-ray precession method showed that bothcrystallized in space group P2r/a instead of A2/a,thepresumed space group for natural titanite. Robbinspostulated that his synthetic CaTiOSiOr might con-tain "two symmetrically inequivalent formula units"related by a face-centering translation, and that if adisorder amounting to a random succession of thetwo types of units were introduced, the resulting aver-age structure would be face-centered. He also pre-dicted that natural titanites with low water and halidecontent would be primitive.

Recently Speer and Gibbs (1976) refined the struc-ture of CaTiOSiOn crystallized from high-purityoxides at I150"C for 5 weeks in a platinum crucible.

FIc.4. A portion ofthe octahedral chain in P2r/atitaniteshowing alternating long and shortTi-O(l) bonds. Courtesy ofJ. A. Speer,

886 J. B. HIGGINS AND P. H, RIBBE

Mole percent

Speclmen No* Fe A1

Teslr 2. /< + / odd reflection data for titanrtescontainine Al + Fe in octahedral coordination

concomitantly substituting for oxygen at the O(l)site, may facilitate domain formation (Taylor andBrown, 1976). If a large number of these oppositelyoriented domains were present in an octahedralchain, the positional parameters of the octahedralcations would be "averaged" by X-rays, and the oc-tahedral chain would appear to contain (Ti + Al +Fe) atoms equidistant from O(l) atoms. If most ofthe octahedral chains in the structure were similarlyaffected, the lattice would appear centered (A2/a).Substitution of smaller amounts of (Al * Fe) for Timight result in the formation of fewer domains, andthis partial linear disorder would be evidenced byplanes of diffusely scattered X-ray intensity per-pendicular to the octahedral chains. These planesappear as diffuse streaks when viewed edge on in an

k + 1 o d d

0 . 0 0 . 0 s h a r p2 . 8 0 . 3 s h a r pz . u 4 . ) o a r r u s ez . ) ) . b d a r r u s e

3 . 6 2 , 8 d i f f u s e3 . 6 2 , 5 d i f f u s e7 . I 1 8 . 8 a b s e n t7 . 0 L4 .6 absen t

*Specimen nunbers are taken from Table 1,except for 13 (Speer and Glbbs, 1976) and34 and 35 which are f rom Carson Pass,Cal i fornla, and whose analyses are notreported ln th ie study.

Crystals of this material exhibit weak "difference"reflections (k + I odd) consistent with space groupP2r/a, a subgroup of A2/a. Their refinement in-dicated that the primary difference between theprimitive synthetic material and ,4-centered titanitesis the position of the titanium atom in the TiO.octahedron. In the natural t itanite refined by Mon-giorgi and Sanseverino (1968) the titanium is locatedmidway between two O(l) atoms, whereas in theprimitive synthetic material, the titanium atom is dis-placed along the octahedral chain direction (Fig.4)giving alternate short (1.766 A) and long (1.974 A)Ti-O(l) bond lengths. In one chain, the long-shortalternation is in the [100] direction, while in adjacentparallel chains related by centers of symmetry thedisplacement is in the [100] direction (cf. Fig. 4 inTaylor and Brown, 1976).

Linear domains in titanite

J. A. Speer (personal communication, 1973) exam-ined a natural titanite from Capahalina, Brazil andnoted diffuse streaks in place of the k * / odd reflec-tions which distinguish the primitive from the cen-tered lattice type, and as a result Speer and Gibbs(1976) modified Robbins (1968) domain theory fornatural t itanites, proposing that natural t itaniteswhich exhibit diffuse or no reflections of the type k */ odd consist of domains of the P2r/a titanite relatedby a half-turn parallel to b. They also suggested thatsubstitution of Al and Fe for Ti in natural t itanitesmight favor domain formation. An octahedral sitecontaining Al or Fe might serve as a boundary so thatl inear domains of octahedra on either side wil l con-tain Ti atoms displaced in opposite directions alongthe chain, and charge-balancing monovalent anions,

Ftc. 5. Zero-level a*b* precession photographs of: a. Specimen I

with sharp ft * / odd reflections (610, 410) indicating a primitive

lattice (space group P2r/a). b. Specimen 5 showing diffuse /< * /

odd reflections. c. Specimen I I showing no k * I odd reflections or

diffuse streaks indicating a centered lattice (space group A2/a).

I J

1

6

343511t 2

o

b

# . r {

CRYSTAL CHEMISTRY OF TITANITES 887

a*b* precession photograph (Fig. 5b) but are moredramatically imaged by electron diffraction (Fig. 6).

Concentration of chemical substituents related to spacegroup

In order to confirm the relationship between spacegroup and chemistry, eight t itanites were chosen froma group of thirty analyzed specimens for study by theX-ray precession method. They represent the ob-served range of chemical compositions with respectto Al and Fe substitution for Ti. (Table 2). Figure 5ais an a*b* zero-level precession photograph of a natu-ral t itanite from Gcischener Alp, Switzerland, whichcontains 2.8 mole percent Fe and 0.3 mole percent Al.Reflections of the type k * / odd are present and arerepresented on the photograph by the 410 and 610reflections. Very faint diffuse streaks are associatedwith each k + / odd reflection indicating the presenceof at least some domain texture. Figure 5b is a similarphotograph of a titanite from Capahalina, Brazil,which contains 2.0 mole percent Fe and 4.5 molepercent Al. The photograph exhibits only diffuse

streaks in place of k * / odd reflections and indicatesa higher frequency of domains than the previousspecimen. Electron diffraction patterns of the Ca-pahalina titanite (Fig. 6) emphasize the continuousnature of the streaks which, as has been previouslymentioned, are actually planes of diffuse X-ray in-tensity viewed edge on. There are unexplained nodesof intensity observable in these planes. Due to theweak intensity of the streaks it was not possible toimage the l inear domains. The precession photographin Figure 5c of a titanite from Piercevil le, New York,containing 7.1 mole percent Fe and 18.8 mole percentAl, exhibits no k + / odd reflections or diffuse streaks.The absence of k + / odd reflection data indicatesextensive domain formation and the "average" struc-ture ofthis titanite appears to have a centered lattice.

Conclusions

The results of these X-ray investigations indicatethat natural t itanites exhibit reflections consistentwith primitive and centered as well as intermediatestructures. The space group of both synthetic and

Fig 6 Electron diffraction patterns of specimen 5 illustrating diffuse planes of intensity normal to

[100]. Nodes are present in diffuse streaks between ft * / reflections. Courtesy of Miss Florence Lee.

888 J. B, HIGGINS AND P. H. RIBBE

natural titanites at the unit cell scale is P2r/a, but theformation of domains due to chemical substitution inthe octahedral chains causes the average structure ofmost natural titanites to be "disordered" relative tosynthetic and nearly pure natural specimens, produc-ing an increasing diffuseness in k * / odd reflectionsand resulting in an apparent absence of these reflec-tions in the most (Al + Fe)-rich titanites.

Summarizing the earlier portion of this study, it isobvious that the chemical substitutions of primaryimportance in natural titanites are (Al,Fe)3+ *(F,OH)- e Tio+ + O'-, where Al * Fe is not greaterthan -30 mole percent, and Al is usually pre-dominant. Electron microprobe analysis and bondstrength considerations suggest that Al occupies theoctahedral site in natural titanites, whereas ubiqui-tous rare earth elements substitute for Ca. Prelimi-nary ion microprobe studies indicate that Pb, U, andTh are present in all but two specimens examined,both of which are from pegmatites.

The lattice parameters of titanites vary directlywith the effective octahedral cation radius, except forthose specimens in which partial metamictization hascaused increases in unit-cell dimensions and de-creases in density. Annealing at I100'C for threehours restores these structures so that their latticeparameters are consistent with nonmetamict titanites.

AcknowledgmentsThe authors are particularly grateful to Dr. J. A. Speer of

Virginia Polytechnic Institute and State University and Dr. J. R.Hinthorne of Hasler Research Center for their contributions to X-ray and ion probe studies, respectively. J. A. Speer, G. E. Brown,and W. A. Dollase cri t icized the manuscript. Dr. C. A. Andersenof Applied Research Laboratories provided access to the IMMAfacility, and the Research Division of Virginia Polytechnic In-stitute and State University provided funds for that work and all ofthe computing. Specimens were contributed by the U.S. NationalMuseum, the British Museum (Natural History), Drs. P. Cerny,D. H. Eggler, R. A. Wiebe, D. A. Hewitt , J. A. Speer, and Mr.D. Gottfried. This project was supported by Grant DES 7l-0M86A03 from the Earth Science Section of the National ScienceFoundation.

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Manuscript receiued, October 14, 1975; acceptedfor publication, May 28, 1976.