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COMPARISON of the CRYSTAL STRUCTURES of BUSTAMITE and WOLLASTONITE Dowarn R

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COMPARISON of the CRYSTAL STRUCTURES of BUSTAMITE and WOLLASTONITE Dowarn R THE AMERICAN MINERALOGIST, VOL. 48, MAY-JUNE, 1963 COMPARISON OF THE CRYSTAL STRUCTURES OF BUSTAMITE AND WOLLASTONITE 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 minerals. 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.
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