THE AMERICAN MINERALOGIST, VOL. 48, MAY-JUNE, 1963

COMPARISON OF THE CRYSTAL STRUCTURES OF AND Dowarn R. Pneconl,q.Nl C. T. Pnnwrrr,2Massachusetts I nslituteof T echnology, Cambrid ge, M ass achus etts

Aesrnecr The structures of bustamite and wollastonite difier principally only in the relative arrangement of chains of tetrahedra. Both structures have a pseudomonoclinic cell, this unit having space group P21/rnin wollastonite and A2/m in bustamite.

INrnooucrtoN On the basis of a comparisonof optical properties, Sundius (1931) postulatedthat bustamite (CaMnSizOo)is Mn-rich wollastonite(CaSiOa). Schaller(1938, 1955) also concluded that bustamite had the wollastonite structure becauseof a closerelationship between the optical propertiesof the two . Berman and Gonyer (1937), using rotating-crystal

Tasln 1. Syuunrnv ,rNo Ulrrr-cnr-r Dere lon Busr.qurre aun Wolr-lsroNrrB

Woliastonite Bustamite Peacor Bustamite Peacor Buerger and Buerger and Prewitt

7.e4A r5.4r2L 7.rc6L b 7.32 7.157 7.r57 7.07 13.824 13.824 q 90"02' 89"29' 90031' R 95"22', 9405r1 9+"35', ^l 103026' I0zo56' t03"52',

Space group PI FI A1 photographs, found that their unit cells were similar, and concluded that they wererelated only by solid solution.Buerger, however, (1956) found that the unit cell of bustamite (Table 1) is closelyrelated to, but different from, the cell of wollastonite. He noted that there is a sort of super- structure relation between the two minerals. Liebau et al. (1958) con- firmed Buerger's unit cell and guessedthat the differencein structures is basedonly on a different ordering of chains and cations. The structure of bustamite has recently been determined and refined (D. R. Peacorand M. J. Buerger,in press).The structure of wollastonite was determinedby Mamedov and Belov (1956) and refined by Buerger

1 Present address: Department of Geology and Mineralogy, The University of Michi- gan, Ann Arbor, Michigan. 2 Present address: Centrai Research Department, E. I. Du Pont de Nemours and Co., Wilmington 98, Delaware.

588 BA STAM I TE AN D WOLLASTON IT E 589

^oi 7r i v o, .z:J o"u?'l-i l/(sir .61 V/siz 05

c"9r,uof,'zr {t::04 06 CoJ 75 so, CozoT

Frc. 1 Projection along 6 of the structure of wollastonite (space group PT). and Prewitt (1961).These structures are different but bear a very close relationshipto one another. DBscnrprroN oF STRUcTURES The face-centeredunit cell of bustamite may be transformedto an ,4- centered cell with the followine transformation matrix: t-+ -+ 0t I 0 1 0l t 0 0 -1J The,4-centeredcell bearsa closerelationship to the wollastonitecell, as does the face-centeredcell (Table 1). Since the relation between the structures of wollastonite and bustamite is clearer if bustamite is de- scribed in terms of the ,4-centered cell, all description of bustamite in this paper is referred to this cell. Atomic coordinates for both bustamite and wollastonite are listed in Table 2. The similarity showsthat the asymmetric units of eachstructure are essentially the same. The structures are shown projected along D in Figs. 1 and 2. Four wollastonite unit cells and two bustamite unit cells are shown. Ca or Mn atoms are shown as large open circles' inversion L centersat y:i, f; as small opencircles, and inversioncenters at y:0, as small solid circles.The structuresare the same in projection except for 590 D. R. PEACOR AND C. T, PREWITT minor coordinate shifts. The arrangement of the oxygen atoms of both structures approximates close packing in a crude way with an obvious layering parallel to (101). Layers consisting of Ca (wollastonite) or Ca

Talr,e 2. Coonorrlrrs exo Isornoprc'Irupnnerunr Flcrons or Arous rN Worresror.rrrn (ullrn varuns) ano Busrlurre (r,owrn vtr,uns). Fon Couranrsor, Busteurln z Coorurxarns ann Mulrrplrnn ev 2. coonarrerns?:ff g-f :T-",g'#'.ff,itxgJ""*Rrterrvn

Atom B

Car .1985 .4228 .7608 .41 Mnr .2018 .4284 .7466 .56 Ca, .2027 .9293 .7640 .45 Cat .1988 .9411 .7570 .69 Ca, .4966 .2495 .4720 .37 Mn, r/2 r/4 1/2 .56

Cat .5034 . /JUJ .5280 .J' Ca, 1/2 3/4 1/2 .,J

Sir .1852 .3870 .2687 1^ .1768 .3881 .2686 .34 Siz .1849 .9545 .2692 .24 .1715 .9434 .2650 .32

Sis .3970 .7235 .0560 .22 .3950 .7171 .04.36 .16

Or .4291 .2314 .8019 .48 .4316 .2400 .8054 .68 Ot .4008 .7259 .8302 .37 .4036 .7178 .8138 .57 o, .3037 .4635 .4641 .60 .3126 11a < .4586 .48 a .3017 .9374 .4655 .64 .3017 .9302 .4630 .50

o5 .0154 .6254 . /J+J .63 .0261 .6167 .7098 .64

a .UI/J .1319 .7353 -ll .0280 .1627 .66

Oz .2732 .5118 .0919 .Jl .2574 -5U4/ .0786 .57 Os .2713 .8717 .0940 .51 .2729 .8739 .0822 .29

Og .2188 . r784 .2228 .68 .1851 .1676 .2294 1.34 BLIS'I'AM I TE AN D WOLLASTONI TE 591

csrno

o

9z oo /ouat

o "2') " ot14

Mustoo co" t/o

Frc. 2. Projection along b of the structure of bustamite (spacegroup,4I). Pairs of chains shifted bv b/2 are shaded in.

,-) -/-

Frc. 3. Projection along c of the structure of wollastonite. The primitive triclinic cell is outlined with a light line. The pseudomonoclinic cell (space grottp Ph/m) is partially out- Iined with a dotted line, and mirror planes are indicated with a heavy line. D. R. PDACOR AND C. T. PREWITT and I,In (bustamite) atoms in octahedral coordination alternate with layers composedof Si atoms between the sheetsof oxygen atoms. The SiOr tetrahedra are arrangedin chains whoserepeat unit is three tetra- hedra and which are orientedparallel to the b axis (Figs.3 and 4).

Sussrnucrunn RBt,qttotss Both bustamite and wollastonite are characterizedby a prominent substructurewith period b/2 (Figs. l and 2). Ail atoms exceptSi3and Os are relatedto a secondatom by a shift of about bf 2.Note, for example,in Fig. 1, that Car and Ca2fall almost exactly over eachother, and differ in 1 by 0.51. The relation is imperfect for some pairs of atoms, as with Or and Or of wollastonite, where the shift (Ar, Ay, Az) is 0.03, -0.49, -0.03. Nevertheless,it is approximately true. The coordinatesof the substructureatoms are very similar in the two structures.Thus, with the exceptionof Sigand Os,the structureso{ wollastoniteand bustamite are approximately the same.Note too, that although Siaand Og within the same chain have different s and z coordinates,they differ along D by ap- proximately b/2. The difierencein orientation of the chains in the two structuresmay be viewed roughly as an interchangein the positions of thesetwo atoms in certain chains.

Frc. 4. Projection along c of the structure of bustamite. The,4-centered triclinic cell is outlined with a light line. The pseudomonoclinic cell (space grorry A2/m) is partially out- lined with a dotted line, and mirror planes are indicated with a heavy line. BU S'IAMITE AN D WOLLASTONITE

Anna.NcBlrpur or CuatNs ol TprnanBona The primary difference between the structures of bustamite and wol- Iastonite lies in the relative arrangement of the silicate chains with re- spect to the planes of octahedrally coordinated Ca and Mn ions. Note first that the arrangement of Ca and Mn ions in bustamite is approxi- mately the same as the arrangement of Ca ions in wollastonite' This is easily seenin Figs. I and 2, and is discussedin more detail under cation ordering.Chains of tetrahedra are arrangedin sheetsparallel to (101) in both structures. Figures I and 2 show that alternating chains within a single sheet are related by inversion centers. However, the pair of inver- sion-relatedchains nearestthe origin in Figs. t and 2 is arranged similarly in each structure and can be viewed as a common structural unit. The end-centeringtranslation in bustamite is partially a reflection of a shift of magnitude bf 2 ol successivepairs of chains of tetrahedra within the sheetsparallel to (101),relative to the pair shown nearestthe origin. In wollastonite,however, the chainsare not shifted along 6. Tetrahedra in silicatechains which are shifted by b/2 are shadedin Fig.2.

CauoN Onopnnqc Schaller(1938) states that mineralsin this group have beenfound with "NInO ranging from a few per cent . . . to a maximum of 33 per cent'" During refinementof the structure of wollastonite(Buerger and Prewitt, 1961) atom scalefactors were refined. The refinementshowed that the composition of the material used as a source for intensity data very nearly approached the ideal composition, CaSiOa. The bustamite (ideally CaMnSLOo)whose structure was refinedby Peacorand Buerger (1962) containeda slight excessof Ca relative to Mn. In wollastonite the Ca atoms are distributed over three generalpositions. The Ca and Mn are orderedin bustamite, with one Ca and one Mn on inversioncenters and two Ca and two Mn in general positions. In addition, evidence strongly suggestedthat the "excess" Ca of bustamite replaced l\lln at the position NInr rather than the special position 1\lln2. Although all Ca or Mn atoms of eachstructure have approximately the same coordinates,there is not complete equivalenceof positions. The Caz and l\[nz atoms of bustamite on inversion centers are equivalent to the Ca3atom in a generalposition in wollastonite.The Mnr and Car atomsin general positions of bustamite, however, are not equivalent to Car and Cagof wollastonite. Half of the Mnr atoms of bustamite occupy positions in the structure similar to those of Car of wollastonite, and half occupy positionssimilar to Caz.The sameis true of Car of bustamite. This is re- flected in the difference in distribution of inversion centers in (101) 594 D. R. PEACORAND C. T. PREWITT layerscontaining Ca and 1\{natoms, as can be seenin Figs. 1 and 2. Com- poundswhose compositions are intermediatebetween bustamite and wol- lastonitestill must be investigatedto determine the limits and mecha-

Tasr,B 3. CenoN-oxvcrn INrrn.q,rolrrc Drsraxcns

Bustamite Wollastonite

Mnr-or 2.499A Car-Or 2 548A oz 2.286 Oz 2.437 Or 2.163 or 2.324 05 2.tM 05 2.302 06 2.041 Oe 2.272 Oz 2.335 Oz 2.412 Av. 2.245 Av. 2.383 Car-Or 2.437 Ca:-Or 2.316 Oz 2.531 Oz 2.369 Or 2.298 Or 2.421 Or 2.382 os 2.5Ol Oo 2.302 oo 2.316 Os 2.358 Os 2.406 Og 2.899 Av. 2.388 Av. 2.384(excluding O) Mn:-2Or 2.215 Car-Ot 2.439 2Ot 2.15+ Oz 2 '349 2on 2.241 Os 2.429 Av. 2.203 or' 2 335 Or 2.441 Oo' 2.349 Os 2.642 Av. 2.390 (excludingOr) Caz-2Oz 2.344 Ca:-Or 2.439 20, 2.412 Oz 2.349 2oa 2.421 Og 2.429 20s 2.891 Or' 2 335 Av. 2.392 (excludingO) On 2.441 On' 2.349 Os 2.642 Av. 2.390 Sir-O3 1.628 Sir-o: 1.618 os 1 587 Os 1'572 Oz 1.645 Oz 1.659 Os 1.616 Os 1.647 Av. 1.619 Av. 1.624 Si2-O4 1.626 Si2-O4 1'617 Oo 1.585 Oa 1 581 Os 1.647 Oa 1.650 Os 1.613 Os | 637 Av. 1.618 Av. 1.621 sL-or 1.600 siS-or 1.599 Oz 1.595 Oz 1.599 Oz 1.660 Oz 1.665 Os 1.671 Os 1.673 Av. 1.632 Av. 1.634 BUSTAMITE AND WOLLASTONITE 595 nism of solidsolution in each.It is possible,ifnotprobable,thatmetastable intermediate compounds exist which have partial or complete disorder both in Ca and Mn and in silicatechain distributions.

INrBnarourc DrsrANcES All averageCa-O distancesare remarkably similar (2.388+.005 A) in both structures as shown in Table 3. The comparison of Si-O distancesis particularly interesting.The averageof all Si-O distancesis 1.623A in bustamite and, L626 A in wollastonite, the difference being well within the standard deviation. All average Si-O distancesfor Sir and Si: are al- most exactly equal, and Sia-Odistances are uniformly larger than Sir-O and Si2-Odistances in both structures. There is an excellent correlation of individual Si-O distances with co- ordination of oxyg^en.For instance, all Si-Oz and Si-Os distances are greater than 1.64 A, I.673 A being the largest. Both Oz and Os are co- ordinated to two Si atoms, from which they receive a total bond strength of.2, and to one Ca or Mn atom, from which they receivea bond strength of ]. The excessof bond strength (|) is thus compensatedby unusually large Si-O distances. All Si-O6and Si-Oodistances are less than 1.59 A, L572 h beingthe smallest.These oxygen atoms are coordinatedto one Si atom from which they receivea bond of strength 1 and to two Ca or Mn atoms from which they receivea bond of strength $. Thus there is a bond deficiency,], which results in unusually short Si-O distances. The only major difierencesin coordination between the two structures involve Og.This is coordinated to Sir and Sizin both structures, but Si-O distancesare largerin wollastonite.In addition, Osis coordinatedto both Ca atoms in bustamite,but with very large Ca-O distances,and to only one Ca in wollastonite,at a shorterdistance.

PsBupol,roNoclrNrc CBus Ito (1950) noted that the angle a of the wollastonite unit cell is very nearly 90oand that (140),which is normal to (100),approximates a mirror plane. From these data he suggestedthat the triclinic wollastonite cell is made up of "twinned or otherwise juxtaposed" monoclinic cells. In- dividual monoclinic units are related by the glide b/4 or -b/4. He noted that the monoclinicunit, which he called protowollastonite,must have eitherof the spacegroups P2/mor Ph/m. Figures 3 and 4 are projections along c of the structures of wollastonite and bustamite respectively. Prewitt and Buerger (in press) noted that wollastonite has a pseudomonoclinic cell with space group Ph/m. The cell is partially outlined on the right side of Fig. 3 with a dotted line- whereasthe mirror planes are indicated by doubly heavy lines. The tri, 596 D. R. PEACOR AND C. T. PREWITT

clinic cell is outlined on the left with a lighter line. Figure 4 is an equivalent diagramof bustamite.The basicrepeat unit is the samein bustamiteas in wollastoniteli.e., two chainsrelated by a 21axis. The bustamite pseudo- monocliniccell is ,4-centeredhowever, with spacegroup A2/m. In wol- lastonite 21axes parallel to b are aligned along c. In bustamite there is a similar relation except that 2t and 2-fold axesalternate along c.

Acr

Manuscripl receired.,December 13, 1962; acceptedJor publication, Jonuary 7, 1963.