THE AMERICAN
MINERALOGIST,
VOL. 48, MAY-JUNE,
1963
COMPARISON OF THE CRYSTAL STRUCTURES OF
BUSTAMITE AND WOLLASTONITE
Dowarn R. Pneconl,q.NlC. T. Pnnwrrr,2Massachusetts
I nslituteof T echnolo
gy, Cambridge,M assachus
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 comparison of optical properties, Sundius (1931)
postulatedthat bustamite (CaMnSizOo)is Mn-rich wollastonite(CaSiOa).
Schaller(1938,1955)also concludedthat bustamite had the wollastonite
structure becauseof a closerelationshipbetweenthe 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
Buerger
b
q
R
^l
Space group
Bustamite Peacor
and Buerger
7 . e 4A
7. 3 2
r5.4r2L
7. 1 5 7
13.824
89"29'
9405r1
I0zo56'
7. 0 7
90"02'
95"22',
103026'
FI
PI
Bustamite Peacor
and Prewitt
7 . r c 6L
7.r57
13.824
90031'
9+"35',
t03"52',
A1
photographs, found that their unit cells were similar, and concluded that
they were related 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 superstructure relation between the two minerals. Liebau et al. (1958) confirmed 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 determined by Mamedov and Belov (1956) and refined by Buerger
1 Present address: Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, Michigan.
2 Present address: Centrai Research Department, E. I. Du Pont de Nemours and Co.,
Wilmington 98, Delaware.
588
BA STA M I TE AN D WOLLASTONIT E
^v o i 7 r i
589
o"u?'l-i
o, .z:J
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 ,4centered cell with the followine transformation matrix:
t-+ -+
I 0
t 0
0t
1
0l
0 -1J
The,4-centeredcell bearsa closerelationshipto the wollastonitecell, as
does the face-centeredcell (Table 1). Since the relation between the
structures of wollastonite and bustamite is clearer if bustamite is described 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 each structure
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
centersat y:i, f; as small open circles,and inversioncentersat y:0, L as
small solid circles.The structures are 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
Mnr
.1985
.2018
.4228
.4284
.7608
.7466
.41
.56
Ca,
Cat
.2027
.1988
.9293
.9411
.7640
.7570
.45
.69
Ca,
Mn,
.4966
.2495
.4720
1/2
.37
.56
Cat
Ca,
.5034
1/2
. /JUJ
.5280
1/2
Sir
.1852
.1768
.3870
.3881
.2687
.2686
.34
Siz
.1849
.1715
.9545
.9434
.2692
.2650
.24
.32
Sis
.3970
.3950
.7235
.7171
.0560
.04.36
.22
.16
Or
.4291
.4316
.2314
.2400
.8019
.8054
.48
.68
Ot
.4008
.4036
.7259
. 71 7 8
.8302
.8138
.37
.57
o,
.3037
.3126
.4635
11a <
.4641
.4586
.60
.48
a
.3017
.3017
.9374
.9302
.4655
.4630
.64
.50
o5
.0154
.0261
.6254
.6167
. /J+J
.7098
.63
.64
.UI/J
.1319
.1627
.7353
-ll
.0280
.2732
.2574
. 5 11 8
-5U4/
.0919
.0786
Os
. 2 71 3
.2729
.8717
.8739
.0940
.0822
.51
.29
Og
.2188
.1851
. r784
.1676
.2228
.2294
.68
1.34
a
Oz
r/2
r/4
3/4
.J'
.,J
1^
.66
.Jl
.57
BLIS'I'AMI TE AN D WOLLASTONI T E
591
csrno
o
9z oo
/ouat
o
"
"2')
ot14
o
Musto
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 outIined 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 tetrahedra 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 except Si3and Os
are related to a secondatom by a shift of about bf 2.Note, for example,in
Fig. 1, that Car and Ca2fall almost exactly over each other, 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 approximately b/2. The difierencein orientation of the chains in the two
structures may be viewed roughly as an interchangein the positions of
these two 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 outlined 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 wolIastonite lies in the relative arrangement of the silicate chains with respect to the planes of octahedrally coordinated Ca and Mn ions. Note
first that the arrangement of Ca and Mn ions in bustamite is approximately 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 inversion-relatedchains nearest the 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 chains are not shifted along 6. Tetrahedra
in silicatechainswhich are shifted by b/2 are shadedin Fig.2.
CauoN Onopnnqc
Schaller(1938)statesthat 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(Buergerand Prewitt,
1961) atom scalefactors were refined. The refinement showed that the
composition of the material used as a source for intensity data very
nearly approached the ideal composition, CaSiOa. The bustamite
(ideally CaMnSLOo)whosestructure was refinedby Peacorand Buerger
(1962) contained a slight excessof Ca relative to Mn. In wollastonite the
Ca atoms are distributed over three generalpositions.The Ca and Mn
are ordered in bustamite, with one Ca and one Mn on inversion centers
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 each structure 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 atoms in
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 reflected 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. Compoundswhosecompositionsare intermediatebetweenbustamite and wollastonite still must be investigated to determine the limits and mechaTasr,B 3. CenoN-oxvcrn
INrrn.q,rolrrc Drsraxcns
Bustamite
Mnr-or
oz
Or
05
06
Oz
Av.
Wollastonite
2 . 4 9 9A
2.286
2.163
2.tM
2.041
2.335
2.245
Car-Or
Oz
or
05
Oe
Oz
Av.
2 548A
2.437
2.324
2.302
2.272
2.412
2.383
2.437
2.531
2.298
2.382
2.302
2.358
2.899
2.384 (excluding
O)
Ca:-Or
Oz
Or
os
oo
Os
Av.
2.316
2.369
2.421
2.5Ol
2.316
2.406
2.388
Mn:-2Or
2Ot
2on
Av.
2.215
2.15+
2.241
2.203
Car-Ot
Oz
Os
or'
Or
Oo'
Os
Av.
2.439
2 '349
2.429
2 335
2.441
2.349
2.642
2.390 (excludingOr)
Caz-2Oz
20,
2oa
20s
Av.
2.344
2.412
2.421
2.891
2.392 (excludingO)
Ca:-Or
Oz
Og
Or'
On
On'
Os
Av.
2.439
2.349
2.429
2 335
2.441
2.349
2.642
2.390
Car-Or
Oz
Or
Or
Oo
Os
Og
Av.
Sir-O3
os
Oz
Os
Av.
1.628
1 587
1.645
1.616
1.619
Sir-o:
Os
Oz
Os
Av.
1.618
1'572
1. 6 5 9
1.647
1.624
Si2-O4
Oo
Os
Os
Av.
1.626
1.585
1.647
1.613
1 .618
Si2-O4
Oa
Oa
Os
Av.
1'617
1 581
1 .650
| 637
1.621
sL-or
Oz
Oz
Os
Av.
1.600
1.595
1.660
1.671
1.632
siS-or
Oz
Oz
Os
Av.
1.599
1.599
1. 6 6 5
1.673
1.634
BUSTAMITE AND WOLLASTONITE
595
nism of solid solutionin 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 almost exactly equal, and Sia-Odistances are uniformly larger than Sir-O
and Si2-Odistancesin both structures.
There is an excellent correlation of individual Si-O distances with coordination 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 coordinated 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 being the smallest.Theseoxygenatoms are coordinatedto one Si
atom from which they receivea bond of strength 1 and to two Ca or Mn
atoms from which they receive a 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 larger in 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 shorter distance.
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),approximatesa mirror
plane. From these data he suggestedthat the triclinic wollastonite cell is
made up of "twinned or otherwise juxtaposed" monoclinic cells. Individual monoclinic units are related by the glide b/4 or -b/4. He noted
that the monoclinic unit, which he called protowollastonite,must have
either of the spacegroupsP2/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 linewhereasthe 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
diagram of bustamite.The basicrepeatunit is the samein bustamiteas in
wollastoniteli.e., two chainsrelated by a 21axis. The bustamite pseudomonoclinic cell is ,4-centeredhowever, with spacegroup A2/m. In wollastonite 21 axes parallel to b are aligned along c. In bustamite there is a
similar relation except that 2t and 2-fold axesalternate along c.
Acr<NowTBuGMENTS
The authors wish to express their appreciation to Professor M. J.
Buerger for suggestingthis problem. This work was sponsoredby a grant
from the National ScienceFoundation. Computations were performed
in part on the IBM 7090computer at the M.LT. Computation Center.
Rnrnnrncns
BrnunN, I{ap.-nv aNo Fonrsr A. GoNvnn (1937) The structural lattice and classification of
bustamite. Am. Minerol. 22, 215-216.
Buntcnn, M. J. (1956) The arrangement of atoms in crystals of the wollastonite group of
metasilicates. Proc. Nat. Acad. Sci.42, 113-116.
C. T. Pnnwrrr (1961) The crystal structures of wollastonite and pectolite.
Proc. Nat. Acad.. Sci,.47, 1884 1888.
Ito, T. (1950) X-ray Studies on Polymorphism. Maruzen, Tokyo.
Ltnr,tu, F., M. SrnuNc aro E. Tnrr.o (1958) Uber das System MnSiOrCaMn(SiOr)2.
Zeit. anorg. al,l,g.Chem. 297, 213-225.
Maurnov, Kn. S. aNn N. V. Brr,ov (1956) Crystal structure of wollastonite. Dokl. Akad.
I[oa] SSSR, lO7, 463-466.
Pencon, Donar.l R. .txo M. J. Burncnn (in press) Determination and refinement of the
crystal structure of bustamite, CaMnSizOo. Zeist. Krist.
Pxnwrlr, C. T. axo M. J. Bunncnn (in press) A comparison of the crystal structures of
wollastonite and pectolite. Am. Mi.neral.
Scnar,r,nn, Walnnuln
T. (1938) Johannsenite, a new manganese pyroxene. Am. Mi,neral,.
23, 575-582.
--(1955) The pectolite-schizolite-serandite series. Am. Mineral, 40, 1022-1037.
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488-518.
Manuscripl receired.,December 13, 1962; acceptedJor publication, Jonuary 7, 1963.