American Mineralogist, Volume 71, pages 1441-1454, 1986

Transformation mechanisms between single-chain silicates

Ross JonN ANcnr-* Department of Earth Sciences,University of Cambridge, Downing Street, Cambridge CB2 3EQ, England

Arsrnlcr The single-chain silicate structures of clinopyroxene, bustamite, and pyroxenoids are compared. The possible types of transformations between these structuresare then char- acterizedin terms of the changein lattice type accompanyingthe structural transformation. Four types of transformation are shown to occur; these may be divided into two groups. Observationsmade by high-resolutiontransmission-electron microscopy on partially trans- formed samplesshow that the mechanism of transformation between clinopyroxene and the pyroxenoidsis one ofglide ofline defectson {001} ofpyroxenoid and on {lli} of clinopyroxene. The structural reorganization associatedwith the passageof such a line defectshows that it is not truly a partial dislocation,but that various different displacements of atoms occur in the core of the defect. Comparison of the structure of bustamite with that of the pyroxenoids shows that with the cell setting in which the latter possessa C-face-centeredlattice, bustamite has a lattice that is best describedas an interleaved complex of two -Flattices. This indicates that the relative positioning of the silicate chains in bustamite is different from that in the pyrox- enoids or clinopyroxene. Consequently,inversions in the solid state between bustamite and either clinopyroxene or pyroxenoid must proceed by mechanismsthat carry out the inversions in two steps.Observations made by high-resolution transmission-electronmi- croscopyon partially inverted samplesindicatethat the inversions from bustamite proceed by the propagation through the structure of line defectswith Burgersvectors of 7z[010]on the {102} planes (-41 cell) which create a wollastonite-like intermediate structure. This intermediate structure subsequentlyinverts to either clinopyroxene or pyroxenoid. The initial stageofthe inversions in the opposite direction is shown also to be the creation of this wollastonitelike structure, probably by the glide of line defects on the {001} planes of pyroxenoid or the { I I I } planes of clinopyroxene.

INrnonucrroN microscopy(unrnru) studies,there hasyet to be an attempt possible There are three main structure types for single-chain to relate the observed faults to transformation Indeed, since silicatesof formula MSiO3 (M: Ca,Mn,Fe,Mg):clino- mechanismsbetween the various structures. of pyroxenoids pyroxene, pyroxenoid, and bustamite. They are distin- most of the high-resolution observations material, the observedfaults guishedby the different relative positioning ofthe chains have beenmade on synthetic growth the result of incipient in each structure, that is, by their lattice types. Transfor- are more likely featuresthan mations between these phaseshave been the subject of phasetransformation. paper of single-chain several single-crystal X-ray studies during which one phase In this the three structure types reorganization is inverted to another by heating. Thesestudies have shown silicates are compared, and the structural possible is described. that the inversions are topotactic and pseudomorphous associatedwith each transformation After a review of the possible types of transformation (Dent-Glasserand Glasser, 196l: Morimoto eI al., 1966; of a number of Aikawa, 1979).The relative orientations of reactant and mechanism, the results of a nnrnlnl study partially presented.The com- product phaseshave beenused by theseworkers to derive inverted chain silicatesare with data atomic-scaletransformation mechanismsthat involve the bination of these unit-cell-scale observations is to be cooperativedifiirsion of cations.This derivation of atom- previously available from X-ray studies shown al. (1984) that ic-scalemechanisms from the relative orientation ofreac- consistentwith the suggestionofAngel et proceed propagation of tant and product phasesin a transformation is clearly not such transformations may by the starting material. satisfactory.And although the pyroxenoidshave beenthe line defectsthrough the structure of the subject of many high-resolution transmission-electron Stnucrunns The crystal structures of chain silicates may be de- * Present address: Geophysical Laboratory, Carnegie Institu- tion of Washington,2801 Upton Street,N.W., Washington,D.C. scribed as the stacking together of tetrahedral and octa- 20008.u.s.A. hedral layers (e.g., Pannhorst, 1979). In the case of py- 0003-004x/86/ | | Iz-t 44tso2.oo 1441 1442 ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES

Fig. l. Idealized chain configurationsofpyroxenoids projected onto (100). (a) Wollastonite. (b) Rhodonite. (c) Pyroxmangite. (d) Ferrosilite III. (e) Clinopyroxene. (f) Bustamire. roxenoids the oxygens coordinating these layers repeat unit with an infinite distancebetween consecutive approximate a close-packedarray; this plane is chosento wollastonite-like chain units. Unfortunately this has led, be (100).In this cell setting(due to Koto et al., 1976),the incorrectly, to the description of pyroxenoids as a struc- tetrahedralchains lie in the (100) planesand extend along tural seriesmade up of pyroxene and wollastonite slabs the c axis. The b axis defines the relative displacements (modules),clinopyroxene thus being composed of an in- ofadjacent chains within the (100) layers, and the lattice finite stack of pyroxene modules (e.g.,Koto et al., 1976; type definesthe relative positions ofchains in the adjacent Czankand Liebau, 1980). (100) layers. The idealized chain configurations within Although within a single (100) layer the relative posi- (100) layers of the observed pyroxenoid structures are tioning ofthe tetrahedral chains in clinopyroxeneand the shown in Figures la-ld. Each chain repeat unit consists pyroxeneJike portions of the pyroxenoid chains is the ofa number ofpyroxene-like pairs oftetrahedra, followed same(Fig. l), the relative position ofthe tetrahedralchains by three tetrahedra in a wollastonitelike configuration. in adjacent (100) layers differs in clinopyroxene and the Thus the pyroxenoid minerals have chain-length period- pyroxenoids (Fig. 2). In pyroxenoids the direction ofthe icities given by p : 2n + 3 (n integral): wollastonite (p : b axis is defined by the relative positions of the offset 3), rhodonite (p : 5), pyroxmangite and pyroxferroite tetrahedra of the wollastonite-chain units, and conse- (isostructuralwith p : 7), and ferrosilite III (p : 9). Min- quently a increaseswith increasingchain periodicity from eral stmctures with periodicities greater than nine are not 99' in wollastonite to 114' in ferrosilite III (Koto et al., known, but such repeats have been observed as short 1976). In order to properly compare the pyroxene struc- sequencesin partially disordered material. ture with that ofthe pyroxenoids, a ofthe clinopyroxene Clinopyroxenes have often been described as pyrox- cell should be increasedto I 20" by a new choice of b axis, enoids with formal periodicity of p : *, i.e., with a chain as indicated in Figure 2b. With this cell orientation the ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES t443

Fig.2. Tetrahedralchains in consecutive(100) layers in (a) pyroxenoidand (b) clinopyroxene.The shadedchains consist of Fig. 3. Singletetrahedral chains \ilith associatedoctahedral apex-downtetrahedra related to the left-handchain by the C-lat- bandsin (a)wollastonite and (b) a long-periodpyroxenoid. Note tice vectorsin eachstructure. Note that whenclinopyroxene is theassociation ofthe three-site-wideportions ofoctahedral band describedon axesparallel to thoseofthe pyroxenoids,it hasan with the wollastonitelikeunits in the tetrahedralchains. 1 lattice.

generally able to accommodatelarger cations than those lattice ofclinopyroxene is body centered.Since the lattice in the P modules,because ofthe greaterdegree ofrotation type controls the relative positions ofthe tetrahedral chains, possible for the associatedwollastonite tetrahedral-chain it is clear that the chains in clinopyroxene are arranged units. The various stability fields ofthe stackingsequences differently from thosein the pyroxeneJike modules of the composing the pyroxenoid structures are thus strongly pyroxenoids.Ifa structure was composedentirely ofthese dependentupon the relative sizesand proportions ofoc- modules,alternate (100) layersof tetrahedralchains would tahedral cations. As the averagecation size is increased, have to be translatedby 7z[001]in order to reproducethe those phaseswith a larger proportion of wollastonite-type observedclinopyroxene structure. This argumentalso ap- slabsare stabilized with respectto those with less.Thus, plies to the P2,/c clinopyroxenessince these have the same in general, the chain-length periodicity ofthe stable py- general positioning of silicate chains as the C2/c struc- roxenoid phase increaseswith increasing pressure,de- tures; the displacive transformation between the two creasing temperature, and decreasingaverage radius of structures,when it occurs, consistsmainly of tetrahedral nontetrahedral cations (Simons and Woermann, 1982). fotatron. Consequently the chain periodicity of a structure may Interleaved with the (100) tetrahedral layers are layers changein responseto a changein one ofthese variables, of large cation sites that are genorally of octahedral co- giving rise to transformations betweenpyroxenoids, and ordination. The cation sites in pyroxenoids form bands between pyroxenoid and clinopyroxene. The various that run parallel to the tetrahedralchains, the bandsbeing transformations are best characteized by the change in two sites wide when associatedwith the pyroxene-like lattice type that accompaniesthe changein structure: portions of the tetrahedral chains and three sites wide when associatedwith the wollastonite-like chain units (Fig. Pyroxenoid - Pyroxenoid C - C 3). The structuresof the pyroxenoid minerals may there- Clinopyroxene - Pyroxenoid I - C fore be consideredas being formed from the stackingto- Pyroxenoid - Bustamite C- "F-complex" getherof two types of structure module. The wollastonite Clinopyroxene- Bustamite 1 - "F-complex." module, denoted W, is a one-unit-cell-thick layer of wol- lastonite cut parallel to (001), i.e., approximately perpen- Complications are introduced into possible transfor- dicular to the chain extension direction. The pyroxene- mation behavior by the fact that the bustamite structure, like module, P, may either be consideredto be the residual which has a chain periodicity of three, is more stableover structure left after removal of the W modules from rho- much of the (Ca,Fe,Mn)SiO,system than the wollastonite donite, or be derived from clinopyroxene by the glide of structure to which it is closely related. The structure of altemate octahedral + tetrahedrallayers by Yz[001]- one- bustamite, when describedon axes that parallel those of half of the chain repeat distance. Thus the P module is the pyroxenoid CI cell settings,does not possessa simple also a layer parallel to (00 I ) of pyroxenoid, but contains lattice type. Koto et al. (1976) described the resultant a pair oftetrahedra from eachsilicate chain. The observed lattice as a complex of two interleaved F-lattices. stacking sequencesare TuNsronuATIoN MECHANTSMS Wollastonite (W) p:3 In most ofthe oriented transformationsbetween silicate Rhodonite (WP) p: 5 minerals, the oxygen atoms tend to preserve their posi- Pyroxmangite (WPP) p :7 tions when possible,whereas the Si and other cations are FerrosiliteIII (WPPP) p :9. redistributed within this anion frarnework (Taylor, 1960). The transformations considered here are all between The cation sites associatedwith the W modules are structures that possessapproximately the same oxygen ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES packing, so that cation displacement is the only major PvnoxeNorn ro PYRoxENoro (C - C) structural changeassociated with eachtransformation. The Aikawa (1979) studied the inversion between pyrox- two types of mechanisms proposed for these transfor- mangite and rhodonite by single-crystalX-ray techniques. mations achieve these changesin diferent ways. He proposed that the inversion proceededby the coop- Dent-Glasser and Glasser (1961) described the exper- erative diffusion of two-fifths of the Si and other cations imental inversion of rhodonite to what they termed "wol- into sites that are vacant in the pyroxmangite structure, lastonite." The mechanismproposed by Dent-Glasserand and that the relative orientation ofthe reactant and pro- Glasser(1961) for this transformation requiresthe move- ducephases is controlled by the preservationofthe oxygen ment of Si atoms, with minor displacementsof the oxygen layerscoordinating "dense-zones"of cations common to atoms. A similar mechanism of cooperative cation dif- both structures. fusion was proposed by Morimoto et al. (1966) for the Angel et al. (1984) derived a possibleshear mechanism johannsenite inversion of (a Ca-Mn clinopyroxene iso- primarily for the inversion ofjohannsenite to bustamite typic with diopside) to bustamite in which the relative and then applied the same principles to inversions in- orientation ofjohannsenite and product bustamite is con- volving the pyroxenoid structures.It should be pointed trolled by the preservation of the oxygen layers coordi- out that, as a consequenceofa misunderstandingofthe nating "dense-zones"of cations common to both struc- precisestructural relationship betweenclinopyroxene and tures. This type ofmechanism has also been proposedfor the pyroxenoids, the Burgers vectors derived in the ap- the transformation of rhodonite to bustamite (Morimoto pendix to Angel et al. (1984) for the pyroxenoid-to-py- et al., 1966) and for that between rhodonite and pyrox- roxenoid inversions are not vectors ofthe oxygen sublat- mangite(Aikawa, 1979). tices of the structures and are therefore incorrect. They The secondtype ofmechanism proposedfor thesetrans- are now derived correctly for the idealized structures. formations employs the passageof partial dislocations The periodicity of pyroxenoids is defined by the fre- through a structure to create an atomic configuration that quency with which the wollastonite modules appear in product phase. locally resemblesthat of the Dislocations the stackingsequence. Each ofthese modules contributes were originally proposedas a microscopic explanation of one offset tetrahedron to each tetrahedral chain (Fig. l), process the of slip in metals. The Burgers vector of a and the principle ofthe derivation ofthe shearmechanism dislocation is defined as the displacementcreated by the is to createone such offset tetrahedron in a pyroxeneJike dislocation between slipped and unslipped material. It is portion of one chain. As describedin Angel et al. (1984), thus a measureof the displacementof all the atoms of the this tetrahedron is generatedby the movement of a tet- structure causedby the passageof one dislocation. But in rahedral chain into a position offset from that related by the caseof more complex materials such as the silicates the Clattice vector. The Burgersvector ofthe shear re- studied here, the processof shearby a dislocation creates quired for such a movement is the sum ofthe same Clattice energeticallyunfavorable environments for some atoms, vector as for the clinopyroxene-to-bustamite inversion and these undergo a further redistribution into more fa- (Angel et al., 1984), Vdl7}l, and the equivalent of the vorable sites(synchroshear; Kronberg, 1957).The passage offset vector, Yr[011]""..On axes parallel to those of the of line defectshas already been shown to be the mecha- pyroxenoids (Fig. 2b) this offset vector in clinopyroxene nism by which polytypically disordered bustamite elim- becomesV+l}l2l. The formal descriptionof this offsetvec- inates stacking mistakes (Angel, 1985). And the obser- tor will vary not only from pyroxenoid to pyroxenoid vation of stacking faults in spinel from the PeaceRiver becauseofdiferences in chain periodicity, but also within meteorite (Price et al., 1982) is consistentwith the trans- a pyroxenoid becauseofthe deviations ofthe tetrahedral formations betweenspinel and spinelloids [specificallythe chainsaway from the idealizedstructures considered here. beta phaseof (Fe,Mg)rSiOoin this caselproceeding by the Alternatively, these deviations may be consideredto be passageof line defects. Electron microscopy of similar accommodated by the structural relaxation that follows shockedmaterial from the Tenham chondritic meteorite the shear. In the idealized structures the ofset may be (Putnis and Price, 1979) and of synthetic material (Lacam rewritten in terms of the chain periodicity p asthl0l4/pl, et al., 1980)supports the proposalby Poirier (1981)that giving the Burgersvectors of the shearsin each structure the olivine-to-spinel inversion alsoproceeds under certain type as conditions by a shear mechanism. In the following sectionsthe four types of transforma- Wollastonite Vep 3 a/31 (203) (as tion characterizedby the change in lattice type) be- Rhodonite V+123 4/5) (205) tween the single-chainsilicates are examined in more de- Pyroxmangite rhl2 3 4/71 partially Q07) tail. Evidence from HRrEM observations of Ferrosilite III Vql23 4/91 (209). inverted samplessuggests that thesetransformations pro- ceedby the passageofline defects.The orientational re- lationship betweenthe phasescreated by this mechanism The slip planesare derived by a similar transformation is approximately the sameas that predicted by the dense- of the slip plane in clinopyroxene, which becomes{ I 0l } zone theory. on the /-centered cell, and by the constraint that the Bur- ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES 1445 gers vector of a glissile dislocation must lie in the slip odicity faults, especiallyof thosewith the rhodonite struc- plane. Note that the slip planesare slightly inclined to the ture, but no direct evidence of the mechanism of for- (001) interfaces of the W and P modules of which the mation of these faults was found. However, a possible pyroxenoids are composed,and therefore shear on these mechanism is suggestedby micrographs of a rhodonite planes would cut across the modules and convert one produced in a longer annealing run. Figure 4a shows a module type to the other. Calculation shows that if rho- number of line defectswithin a rhodonite grain that run donite and pyroxmangite are intergrown with these slip parallel to the incident electron beam (along [10]) and planes and with their b axes parallel, the relative orien- that terminate the (001) chain-periodicity faults. An en- tation of the two phasesis the same as that predicted by largement of an area of this micrograph is provided in the dense-zonepresewation theory. Figure 4b, and the structural interpretation based upon image matching using the snnt-r(Simulation of High-Res- Observations olution Lattice Images) program (O'Keefe and Buseck, The easiesttransformation to study is that betweenrho- 1979) is drawn in Figure 4c. The rows of bright spots in donite and pyroxmangite becauseof the low temperature the micrograph are associatedwith the wollastonite-like at which the inversion takes place.Aikawa (1979) carried units within eachtetrahedral chain, so that the periodicity out annealing experiments to invert natural pyroxman- faults consist of two W modules without an intervening gites to rhodonite. Despite undergoing partial oxidation P module, i.e., a short portion of two unit cells of wol- of the Mn2+ and Fe2+in the pyroxmangite, the product lastonite structure. For the MnSiO. composition, such a rhodonite showed the orientation relationship to the py- structure is not stable, and this sample appearsto be in roxmangite predicted by the dense-zonetheory of Mori- the processof eliminating such faults by the passageof moto et al. (1966). Although Aikawa (1979, 1984) was line defectsthrough the structure.At eachdefect it would able to demonstrate that many natural intergrowths of appear that local atom displacementsoccur, possibly us- pyroxmangite and rhodonite also exhibit this orientation ing the empty core of the line defect as a fast diftrsion relationship, Ried and Korekawa (1980) reported fine- path. The concept of a Burgers vector of a shear is not scaleintergrowths and chain-periodicity faults in plrox- strictly applicable to such a defect, but bearing in mind enoidsthat are oriented slightly differently. Unfortunately that the line ofthe defect is parallel to a Clattice vector, the other HRTEMstudies of pyroxenoids reported in the the actual atomic reorganization may well be that which literature (Czank and Liebau, I 980; Jeffersonet al., I 980; was described by the shear mechanism derived above. Jeffersonand Pugh, 1981) do not give sufficient details The only differencelies in the planeson which the defects for the preciseorientation relationshipsto be ascertained. travel through the structure; the slip plane of the shear Thesechain-periodicity faults and intergrowths generally mechanismwas (205) in rhodonite,but is obviously(001) show very little strain, but Ried and Korekawa (1980) in the micrographs,while consideration of the structural noted that they areoften terminatedby dislocationswithin rearrangementrequired at the defectsof Figure 4 indicates the crystal,which suggestedthat transformationsmay pro- that atom displacementswith componentsnormal to the ceed by the passageofpartial dislocations. (001) planes are necessary.Such atom rearrangements In order to ascertainthe type of mechanism operative within the core of the line defect will also include the during pyroxenoid-to-pyroxenoid transformations, a se- movement of the octahedral cations, which was previ- ries of inversion experiments was carried out with syn- ously assignedto the final mechanisticstep ofsyncroshear thetic material of MnSiO, composition. A glasswas pre- after the passageofa partial dislocation. pared from a mixture of silica and manganesesulfate by Similar line defectswere also observedin the products the samemethod as describedin Angel (1984).A series of the very short hydrothermal annealing runs of the of hydrothermal devitrification runs was then carried out, MnSiO, glass,in which longerchain periodicitiesare being which produced samples consisting of various mixtures eliminated (Fig. 5). It therefore seemslikely that the py- of rhodonite and pyroxmangite, the starting material for roxmangite-to-rhodonite inversion carried out in these the inversion experiments coming from a two-week run experiments also proceededby the passageof these line at l.5 kbar and 650'C. Although theseconditions are with- defects.This would account for the increasingdensity of in the rhodonite stability field, Hnrenr of material from chain-periodicity faults in the pyroxmangite, and if such this run showed that it contained an appreciablepropor- defectsnucleated at the wrong relative spacings,they would tion ofpyroxmangite and had a very low density ofstack- generatethe WW-type (00 I ) chain-periodicity faults being ing faults. This material was then annealedunder pure Ar eliminated in Figure 4. (to prevent oxidation) at higher temperaturesto invert the ' pyroxmangiteto rhodonite. Cr-rNopvnoxENE To PYRoxENoID (I C) Observationsmade on the products ofseveral annealing The replacementof clinopyroxene by pyroxenoid was runs show that the proportion of rhodonite increasesrel- discussedat length by Angel et al. (1984), who proposed ative to pyroxmangitewith increasedannealing time with- that the transformation mechanismwas one of dislocation in the rhodonite stability field. The residualpyroxmangite glide followed by cation synchroshear.The slip plane may also appearsto have an increaseddensity of chain peri- be derived by considering which lattice plane of clino- 1446 ANGEL: TRANSFORMATIONS BETWEEN SINGLE.CHAIN SILICATES

Fig. 4. Transformation ofstacking sequencesin rhodonite by line defects that run parallel to the incident direction ofthe electron beam, [110]. (a) Note the "en echelon" arrangementof the line defectsthat lie on every (001) plane of the rhodonite. (b) Enlargement ofa portion ofthe micrograph in (a), together with a structural interpretation (c). Note that interpretation ofthe structure in the core ofthe defect is not possibleat this resolution. pyroxenebest matchesthat of(001) ofpyroxenoids and noids have been reported in Hnrnr'r studies of synthetic has the same centering of lattice points. The latter con- material. In a study of ferrosilite III, Czank and Simons dition ensuresthat the relative positioning ofthe chains (1983) found long periods of pyroxene-like structure in- in a pyroxenoid structure formed by the inversion of cli- tergrown with the pyroxenoid with {lll}"o- parallel to nopyroxeneis correct.Figure 6 showsthat the t I I I1 planes {001}eyd.A more detailed study of similar intergrowths of clinopyroxene fulfill these conditions, this being the by Ried ( I 984) confirmed the orientation relationship pre- slip plane of the mechanism proposed by Angel et al. dicted by the proposedshear mechanism and showedthat (1984), the Burgers vectors of the dislocations being the displacement vector associatedwith the chain-peri- v4<23T). odicity faults in the clinopyroxene was V+(01l). Sincethe Intergrowths of clinopyroxene with various pyroxe- addition ofa Clattice vector, 7z[II0], and Yr[0I1] is equal ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES 1447

t1001

a t0101 b t0-111 Fig. 6. (a) The (001)lattice section ofa pyroxenoid:lattice parameterstaken from pyroxmangite.O) A { 1l I} latticesection of johannsenite(clinopyroxene). Lattice spacings in Angstriims areindicated. Fig. 5. Line defects(some circled) running parallel to [1I0] in a disorderedpyroxenoid. stamite. Dent-Glasser and Glasser (1961) described the inversion ofrhodonite upon heating,but becauseoftheir to t/+l23ll, this also is consistent(see, for example,Ame- belief that the inversion product was wollastonite and not linckx and Van Landuyt,1976, p. 95) with the proposed bustamite, their mechanismof cation difirsion partly gen- mechanism. Even so, these observations were made on eratesthe wollastonite structure (Prewitt and Peacor, 1964). synthetic material in which the microstructuresdescribed Morimoto et al. (1966) also studied the inversion of var- are growth features,and do not necessarilyconstitute evi- ious chain silicatesto the bustamite structure. However, dence for the transformation mechanism. inconsistenciesappear in the discussiongiven by Mori- Veblen (1985) reported observations made by rrnreu moto et al. (1966); the diagrams from which the dense- on a natural johannsenite that clearly showspetrographic zone preservationtheory is derived are all ofwollastonite, evidence of having undergone partial inversion to rho- rather than bustamite, while Figure l2b of their paper, donite. His observations of pyroxenoid lamellae parallel which is labeled bustamite, is identical to Figure l5b, to the { I I I} planesof the clinopyroxenematrix are strong describedas wollastonite. It would therefore appear that evidence in favor ofthe shear mechanism proposed by insteadofproducing bustamite, the transformation mech- Angel et al. (198a). Similar chain-periodicity errors were anisms proposed by Morimoto et al. for both the clino- observedoccasionally in natural johannsenite (Fig. 7a) in pyroxene-to-bustamite(1 * f'-complex) and the rhodo- which longer periodicities correspondingto {001} pyrox- nite-to-bustamite (C - .F-complex) inversions produce enoid spacingsoccur parallel to the {l 111 planes of the (incorrectly) the wollastonite structure. clinopyroxene matrix. Some of the material was experi- Becauseof the differencesin lattice types of bustamite mentally annealedin a Mn solution below the temperature and clinopyroxene,the dislocation mechanismfor the in- of inversion to bustamite in order to demonstrate that such microstructures are the product of a partially com- pleted transformation. The increasein the Mn content of the structurehas promoted the creation ofextensive stack- ing disorder on {llI} of the clinopyroxenematrix (Fig. 7c). The associatedselected-area electron-diffraction pat- tern (Fig. 7b) showsmaxima correspondingto wollaston- ite and rhodonite, togetherwith diffuse streaksparallel to the (l I l) plane normal of the clinopyroxene.By contrast, difraction patterns from clinopyroxene annealedin CaCO, solution show strong maxima from wollastonite alone (Fig. 7d), whereasthe maxima correspondingto longer chain periodicities are very difuse or absent. The orientation relationshipobserved in these samplesis (l0l) of cli- nopyroxene parallel to (100) ofpyroxenoid as expected from the matching of the two lattice types. The disorder is therefore characterizedby short sequencesof pyrox- enoid mineral structuresintergrown with clinopyroxene, with {00 I } ofthe pyroxenoidsparallel to one ofthe { I I 1} Fig. 7. Stacking disorder in clinopyroxene parallel to {1lI} planes of the clinopyroxene. owing to partial inversion to pyroxenoid. (a) Naturaljohannsen- ite. (b) Selected-areaelectron-diftaction pattem of this material Busrnvrrrn TRANSFoRMATIoNS after annealingin Mn solution, and (c) the correspondingimage. (d) Selected-areaelectron-diffraction pattern from a similar spec- As was the case for - - the C C and C l transfor- imen annealedin a Ca solution. Note the predominanceof wol- mations discussedabove, two types of mechanism have lastonite diffraction maxima (arrowed) over those from other been proposedfor the inversions ofchain silicatesto bu- pyroxenoids. t448 ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES

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'!-..:: :,L: :

.-.+a. .1.--a.

Fig. 8. Lattice sectionsperpendicular to the chaindirections of (a) wollastoniteand O) bustamitein the cell settingof Koto et al. (1976).The horizontalarrow indicates the relativeoffset of the two -FJattices.(c and d) The samelattices with the cell settingofPeacor and Prewitt(1963). The opencircles represent lattice points displacedby one-halfof the lattice repeatperpen- dicularto the sectionrelative to thoserepresented by filled circles. version ofjohannsenite to bustamite proposed by Angel et al. (1984) necessarilyconsists oftwo stages.The first stage is the creation of a wollastonite intermediate phase : by the passageof line defects,with b Vq(2311,through Fig. 9. Faultsin a bustamitesample annealed in the stability the structure on the { I I 11 planesof the clinopyroxeneas field of wollastonite.The faultslie on the (102)planes of busta- for the clinopyroxene-to-pyroxenoid (1 - C) transfor- mite andhave a fault vectorof y2[010]. mations described above, followed by the inversion of this intermediate to bustamite. The inversion of pyrox- enoid to bustamite would follow a similar mechanism; was introduced by Peacor and Prewitt (1963) and is il- first, the creation of the wollastonite intermediate by the lustrated in Figures 8c and 8d. The wollastonite cell is mechanism for pyroxenoid-to-pyroxenoid (C - C) in- now primitive, while bustamite has an l-face-centered versions, followed by inversion to bustamite. In the re- cell with a c parameter double that of wollastonite. It is verse direction the transformations require the inversion now clear that the bustamite structure may be derived of bustamite to wollastonite as a first stage,followed by from that of wollastonite by the translation of alternate transformation to the product structure. Thus both types (001) layersby Yz[010],one halfofthe chain-repeatdis- of inversion mechanism require the existenceof an in- tance along the chains. This relationship led Angel (1985) termediate phase with the wollastonite structure during to propose that wollastonite and bustamite were two inversionsbetween bustamite and either clinopyroxeneor members of a seriesof polytypes basedon the stackingof pyroxenoids. prismatic structural units along the a and c axes of wol- lastonite. Polytypic stackingvariation along a is well known Wollastonite and bustamite in wollastonite, and Angel (1985) found intergrowths of It is clear from Figure I that although the individual wollastonite and bustamite with (001) common to both tetrahedral chains in bustamite have the same configu- phases.Transformations between these polytypes proceed ration as those in wollastonite, their relative positions by the passageof 7d0l0l line defectson (ft0l) planes; in even in a single (100) layer are different. Figures 8a and Angel (1985) it was noted that, becauseof the pseudo- 8b compare the lattices of wollastonite and bustamite in monoclinic symmetry of the structures,glide of Yz[010] projection down the chain-extension direction. This shows defects on (l0l) of wollastonite and (102) of bustamite that ifthe cell setting used for pyroxenoids is applied to would be an alternative to glide on (001) and would also bustamite, the lattice type is nonconventional.Koto et al. generatethe other structure. Such defectswere observed (1976) described it as an interleaved complex of two in an initially fault-free Fe-rich bustamite that was an- F-lattices with a relative displacement of the origins of nealed in the stability field of wollastonite (Fig. 9). These t/a(a + b). Although such a cell has advantages in com- isolated (102) faults are the beginning ofthe inversion to paring bustamite with the pyroxenoids, it is inconvenient wollastonite, since in the region of a fault the relative to use, so although the classification of the transformations positions of the tetrahedral chains is that of the wollas- involving bustamite will retain the term "F-complex," an tonite structure. alternative cell will be used to discuss structural details The structural reason for the transformation between of the transformation. wollastonite and bustamite proceeding by shear on the This simpler cell setting for wollastonite and bustamite (102)planes ofbustamite, or (l0l) of wollastonite,is quite ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES 1449

10004 ,)* *

l-f ",t

500A

-t-F-a**

Fig. 10. (a) High-resolution micrograph of a wollastonite lamella within a clinopyroxene matrix, viewed down [110] of the clinopyroxene.The offsetofthe (l l0) fringes acrossthe fault is indicated. (b) Severalwollastonite larnellaeparallel to a single {1 I l} plane ofa clinopyroxene grain. (c) Inclined { I I I } lamellae ofwollastonite in a clinopyroxene gtain and (d) subgrains ofpolytypically disordered bustamite in the same grain. The bustamite is oriented with [031] approximately parallel to the electron beam, the subgrainshaving mutual misorientations of up to 10o. clear. In both structures theseare the closest-packedplanes consistedof short sequencesof longer chain periodicities and will thus be far more favorable for the passageof ofdisordered pyroxenoids (seeFig. 7a). The observations dislocationsthan the (00l) planes,which arethe slip planes made on the products of a seriesof isothermal annealing suggestedby simple comparison of the two stacking se- of this material at 900"C (under Ar to prevent oxidation) quences.The observation of such faults in bustamite an- afe now presented. nealed in the stability fields of clinopyroxene or pyrox- The first stage in the inversion is the appearanceof enoid is therefore interpreted as the beginning of the isolatedfaults parallel to the { I I 1} planesofjohannsenite, formation of a wollastonite-like structure as an interme- as predicted by the inversion mechanism. These faults diate step in the transformation from bustamite to cli- commonly run acrossthe width of the flakesproduced by nopyroxene or pyroxenoid. It is to prove the existenceof crushing the sample for nnrsM for distancesof several such an intermediate structure that the following obser- thousandingstrtims. However, they are not usually single vations are presented. lamella of wollastonite; for example the fault in Figure lOa appearsto be 28 A wiOe,corresponding to the length Clinopl'roxene-bustamite(1 - I-complex) of four chain-repeat units of a dreierketten single-chain For the study ofthe inversion ofclinopyroxene to busta- silicate. In view of the difficulties in interpreting image mite, a number of annealingexperiments were carried out contrast around faults, it would be wrong to concludethat using natural johannsenite almost completely free of this was indeed a lamella of four chain-repeat units of stackingfaults. The few found were parallel to { I 11} and wollastonite without further evidence. In this case the 1450 ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES evidenceis provided by the displacementacross the fault that a wollastonite structure with the johannsenite com- of the lattice fringes of the clinopyroxene matrix. These position (ideally CaMnSirOu) will be highly thermody- displacementscan be shown to be those that would be namically unstable with respect to bustamite, owing to produced by the introduction of four wollastonite-like the presenceof the relatively small Mn cation. The slow chain units into eachchain, or overall by the introduction rate of nucleation and growth of the lamellae may also be of four VqQ3T)shears into the structure. related to this instability, or alternatively to the need to In specimensthat have undergoneannealing for longer diffuse Mn away from, and Ca to, the lamellae.A further periods of time, these { I I 11 lamellae become more nu- limitation on the coherent growth of the lamellae within merous. As seenin Figure l0b, all of the faults within a the clinopyroxenematrix is the volume differencebetween singlegrain lie parallel to one single {lll} plane ofthe the two phases,and the reliefofthe strains introduced by clinopyroxene, rather than on both of the symmetry-re- the volume increase on inversion may account for the lated planes,a phenomenonfirst noted by Veblen(1985) relative misorientations of the subgrains seen in Figure in johannsenite being replaced by rhodonite and pyrox- l0d. mangite. Such orientational selectivity must be due to the Samplesof Ca-rich bustamites from Broken Hill, New loss of the symmetry elementsin the point group of the South Wales,Australia, provided an opportunity to study clinopyroxenewhich relatethe { I I I 1planes to one another. the initial stagesof the reversetransformation, from bu- This could occur throughout an entire grain of the cli- stamite to clinopyroxene.These bustamites have exsolved nopyroxene,which thereby makes growth processesnon- clinopyroxeneby a mechanismthat involves solution and equivalent on the two previously symmetrically equiva- reprecipitation via fluid inclusions (Wilkins and Sverjen- lent planes, or it may occur at the point of nucleation of sky, 1977); HRrEMshows that the bustamite matrix has the line defectsthat give rise to the faults. In this casethe numerous faults parallel to the (102) planes. Figure 1la most likely causeof this orientational selectivity is a phe- clearly shows that these (102) faults have an associated nomenonVeblen (1985) termed "templating." In a poly- displacementvector of Vz[0l0], and low-resolution images crystalline specimenone of the adjacent grains may, be- (Fig. I lb) indicate that they are produced by the passage causeofthe relative orientation ofthe two grains, create of dislocations through the bustamite matrix. The pres- a grain-boundary environment that favors fault nuclea- enceof (102) faults, which are far more common in these tion on just one of a setof symmetry-relatedplanes. Tem- than in any other natural samplesstudied, appearsto be plating from adjacentgrains is the favored explanation in related to their thermal history. Experimental annealing this casebecause orientational selectivity is observed in of thesebustamites increased the fault density to the point the inversion to bustamite ofpolycrystallinejohannsenite, where streakingwas apparent parallel to the (102) plane whereasin their single-crystalexperiments where the only normal on the k-odd rows of electron-diffractionpatterns "neighboring grains" were small quantities of surfaceox- (Fig. I lc). The faults therefore representthe first stageof ides, Morimoto et al. (1966) found both possibleorien- the solid state rnechanism of the exsolution of clinopy- tations of bustamite inverted from johannsenite. roxene from bustamite, being the creation of the wollas- The need for cation synchroshearand relaxation ofthe tonite-like intermediate structure. structure created by the passageof a single line defect appearsto favor the formation of lamellae with widths - correspondingto a few chain repeatsofwollastonite, rath- Pyroxenoid-bustamite(C I-conplex) er than single faults. These multiple width faults will de- Apart from the special caseof the transformation be- velop becausethe existenceof the initial single fault will tween wollastonite and bustamite, the only other trans- createa favorable environment for the nucleation of sub- formation of this type that occurs is that between rho- sequentline defects.However, becausewide wollastonite donite and bustamite. The inversion of rhodonite to lamellae are not found, there must be a critical width bustamite is extremely rare in natural material, the only beyond which it is unfavorable for the lamellae to grow evidence for this being a few occurrencesof a two-rho- by further passageofline defects. donite assemblagedescribed by Abrecht et al. (1978). In Figure lOc shows a set of {ll1} faults in a thinned natural samplesthe transformation occursfar more com- specimenofjohannsenite; a few thousandtrngstrdms away monly in the opposite direction as the result of metaso- the same original grain of clinopyroxene has inverted to matic alteration ofbustamite during which Ca in the struc- bustamite (Fig. l0d). The bustamite has formed as ture is replaced by Mn, and more rarely bustamite may subgrainsthat show relative misorientations of up to 10". unmix at low temperaturesinto an assemblageof clino- Eachsubgrain shows extensive polytypic stackingdisorder pyroxeneplus rhodonite (Abrecht et al., 1978).However, on (100), which is eliminated during further annealingby the easiestway to study the transformation experimentally the passageof further line defects through the structure is to invert rhodonites with compositions closeto the Ca- as describedby Angel (1985). Theseobservations indicate rich limit of its stability field by annealingat temperatures that there then follows a period of rapid gowth of the abovethe miscibility gapbetween the two phases(Abrecht wollastonite nucleated on the lamellae, together with an and Peters, 1980). Such experiments have been carried almost simultaneous inversion to bustamite. That this out twice before in order to study the transformation by inversion is rapid is hardly surprising when one recalls X-ray diffraction techniques and have been repeated ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES 1451

Fig. ll. (a) High-resolution rnicrograph of two (102) faults in a bustamite sample that has exsolved clinopyroxene and (b) a similar set offaults at lower resolution, with terminating dislocations.(c) Selected-areaelectron-difraction pattem ofa [2] Il section of a bustamite heated in the stability field of clinopyroxene.Note the streakingof the k-odd layersparallel to t* (l 02).

here in order to study the transformation mechanism by that generatedthem can only be assumedto be identical HRTEM. to those involved in the pyroxenoid-to-pyroxenoid in- The study by Dent-Glasserand Glasser(1961) and the versions. However, it should be noted that a single fault interpretation of their results by Morimoto et al. (1966) such as this cannot be produced by a purely diffirsive have been shown to be mistaken, becauseof the conclu- mechanism becausethe number of tetrahedra in each chain sion of Dent-Glasserand Glasser( 196I ) that the inversion has been increasedwithin the stackingfault, whereasthis product was wollastonite rather than bustamite. Yaman- is possibleby a shear-typemechanism. In longerannealing aka and Tak6uchi (1981) also studied this inversion by runs and those at higher temperatures,the quantity of heating natural rhodonite, but their primary aim was to bustamite is greater,but no increasein the density of faults investigate the structure of the Mn-rich bustamite pro- in the residual rhodonite grains is observed.From this it ducedby the inversion. Unfortunately they did not report may be concluded that, as in the transformation from the orientation of the product bustamite relative to the clinopyroxene to bustamite, the rate-determining step is rhodonite, but did note that the morphologiesof the prod- the formation of narrow lamellae of wollastonite inter- uct compared to that of the starting material suggested mediate within the host phase.Once a small critical vol- that an oriented transformation had taken place.Yaman- ume of the intermediate structure has beenformed, it acts aka and Tak6uchi (1981) proposed an inversion mecha- as a nucleusfor the rapid inversion of the remaining rho- nism of cation movement within the (100) planessimilar donite matrix 10 bustamite. to that proposedby Morimoto et al. (1966), but as in that Bustamites from the bustamite-rhodonite suite de- paperthe figuresofYamanaka and Tak6uchi(1981) are scribedby Abrecht et al. (1978) were examinedby nnreiur of wollastonite rather than bustamite. for evidence of the solid-state inversion mechanism of Similar annealingexperiments were carried out in order bustamite to rhodonite, or of the unmixing of bustamite to study the inversion by nnrrr"r. The material used for to rhodoniteplus clinopyroxene.In [211] sectionsofthe these experimentswas a well-crystallized natural rhodo- bustamite coherent lamellae were found that lie parallel nite that HRrEM showed to be free of chain-periodicity to the (102) planesof the matrix (Fig. l3a). The associated faults, but that did contain a few (100) twins. A seriesof selected-areaelectron-diffraction pattern (Fig. I 3b) shows annealingruns was carried out on this material, under Ar maxima correspondingto bustamite,together with streak- to prevent oxidation, at temperatures believed to be ing parallel to the (102) plane normals on the ft-odd rows, within the bustamite stability field. The first stageof the indicating random Yz[010]displacements in the structure. inversion is the creation ofchain-periodicity faults in rho- In addition there are also weak maxima on these k-odd donite that consist of three tetrahedra. i.e.. wollastonite- rows that cannot be attributed to bustamite, even in like W modules. Figure 12 shows a single chain-perio- twinned orientations, but which are indexablein terms of dicity fault in a specimenslightly inclined from the I l0] a wollastonite unit cell (Fig. l3c). The lamellae are there- axis, so that the fault lying parallel to (001) is slightly fore wollastonite oriented with (101) parallel to (102) of inclined. Unfortunately no partially formed chain-period- the bustamite matrix, i.e. with the closest-packedplanes icity faults were observedin thesesamples so the defects of the two structuresparallel. Note that the relative ori- r452 ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES

1:; 1::, .,T :;

-.,jLo-JLoo_loJ!_1_

Fig. 12. A singlewollastonite-like chain-periodicity fault (W) .:::, .;il oo* parallelto (001)in a rhodonitespecimen slightly inclined from - L ll 101. Fig. 13. (a) Wollastonitelamellae lying on the (102)planes of bustamite.(b) Selected-areaelectron-diffraction pattern of the entation is such that the wollastonite lamellae are in a specimenin (a)and (c) the indexing of thispattern. Upper indices twin orientation with respect to a wollastonite structure correspondto wollastonite,lower onesto bustamite.(d) The with the samestacking sequence along [00] as the busta- relative orientationsof the wollastoniteand bustamitein real mite matrix. In the T,G stacking vector notation often spacein projectionon (001). usedto describethe wollastonite polytypes (Henmi et al., 1978), and which was extended to bustamite by Angel (1985),a (T) polytype of bustamite has (102) lamellae also the first stageof the reversetransformation (i.e., to consisting of the (G) polytype of wollastonite. As a con- bustamite) in which coherentlamellae of wollastonite are sequenceof this twin relation, the b axis of the product formed within either clinopyroxene (Fig. 8) or rhodonite wollastonite is parallel to -b of the matrix bustamite (Fig. (Fig. l2). The formation of the wollastonite intermediate l 3d). is therefore an example of the operation of the Ostwald step rule: In any phase transformation or reaction the Discussion kinetically most favorable sequenceof structures will form, In the analysis of possible mechanismsfor the trans- rather than thoseinvolving the greatestpossible reduction formations involving the bustamite structure, the exis- in free energy.In the caseof solid-state transformations, tence of an intermediate phasewas postulated for struc- this is equivalent to the statement that transformations tural reasons. However, the need for an intermediate proceed via the structures that are most easily formed structure can also be interpreted in kinetic terms. If the within the matrix of the starting material. Quite clearly transformation from bustamite to either clinopyroxeneor this principle could be employed to suggestpossible in- rhodonite were to be carried out by a single-stepmech- termediate phasesin other mineral transformations. anism in the solid state, then this step would have to be the direct nucleation and growth of the new phasewithin CoNcr,usroNs the bustamite matrix. The different arrangement of the In this paper the possibletransformations betweenthe tetrahedralchains within the structureswould result in an single-chain silicates in the (Ca,Mn,Fe,Mg)SiO3system incoherent,or at best semicoherent,interface between the have been distinguishedby the changein lattice type ac- matrix and the nuclei. This in turn would lead to a high companying each transformation. This change corre- activation energy for the formation of such nuclei and spondsto the re-arrangementof the main structural com- thus a low overall rate for the transformation. By contrast ponent of these structures, the silicate chains. The the two-step mechanismdescribed in this paper has much observations made by HRTEMshow that the transforma- lower activation barriers and henceresults in a faster rate tions classifiedasC - Cand 1 - C(pyroxenoid-pyrox- of transformation. In forming the intermediate structure enoid and clinopyroxene-pyroxenoid)proceed by the pas- in a bustamite matrix, the first-neighbor coordinations of sage of line defects through the structure of the starting all the atoms remain essentiallythe same,with only sec- material to create the product phase. When a similar ond- and higher-neighbor coordinations being changed. mechanism was consideredfor the two transformations The interface between wollastonite lamellae and busta- involving the bustamite structure, it was shown that a mite matrix is therefore fully coherent, and the creation single-stepmechanism of this kind was not capable of of this surfacedoes not make any significantcontribution reproducing the required lattice type, i.e., the correct dis- to the activation energyof the transformation. Similarly, tribution of the silicate chains. Careful re-examination of the secondstage ofinversion from intermediateto product the alternative diffusive topotactic mechanisms previ- phase may proceed in a fully coherent manner. This is ously proposed for these transformations (Morimoto et ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES I 453 al., 1966) showed that these too create the wollastonite Abrecht, J., Peters,Tj., and Sommerauer,J. (1978) Manganifer- structurerather than that ofbustamite. Both ofthese types ous mineral assemblagesof Ravinella di Sotto, Valle Strona, Memorie di ScienzeGeologische, 33,215-221. inter- Italy. of mechanism were shown to therefore require an Aikawa, N. (1979) Oriented intergrowth of rhodonite and py- mediate phasein order to carry out the transformations. roxmangite and their transformation mechanism. Mineralog- This phase was confirmed to be wollastonite by unrervr ical JournalofJapan, 9,255-269. observations.Its formation can be interpreted as a con- - (1984) Lamellar structureof rhodonite and pyroxmangite Mineralogist, 69, 270-27 6. sequenceof the Ostwald step rule, which indicates that intergrowths. American Amelinckx, S., and Van Landuyt, J. (1976) Contrast effectsat kinetic factorsmay be important in determining the mech- planar interfaces.In H.-R. Wenk, Ed. Electron microscopy in anism of thesetransformations. mineralogy. 68-1 12. Springer-Verlag,Berlin. This study has also demonstrated the difficulties in- Angel, R.J. (1984) The determination of the johannsenite-busta- volved in characterizingsolid-state transformation mech- mite equilibrium inversion boundary. Contributions to Min- eralogy and Petrology, 85, 272-27 8. anisms in minerals, even with the use of electron mi- - (1985) Structural variation in wollastoniteand bustamite. croscopy.These problems mainly arisefrom the transient Mineralogical Magazine, 49, 37-48. nature of the partially transformed statesand the need to Angel, R.J., Price, G.D., and Putnis, A. (1984) A mechanismfor distinguish suchmicrostructures from other artifacts,such pyroxene-pyroxenoid and pyroxenoid-pyroxenoid transfor- of Minerals, 10,236-243. asgrowh faults in the starting material and faults induced mations. Physicsand Chemistry Czank, M., and Liebau, F. (1980) Periodicity faults in chain by specimen preparation. The latter two problems were silicates: A new type ofplanar lattice fault observed with nnrrvr. avoided in this study by the careful characteization of Physicsand Chemistry of Minerals, 6, 85-93. starting materials by Hnrer'r and by preparing specimens Czank, M., and Simons, B. (1983) High resolution microscope for electron microscopy by both crushing and ion-beam studies on ferrosilite III. Physics and Chemistry of Minerals, prep- 9,229-234. thinning. It is unlikely that thesetwo very different Dent-Glasser, L.S., and Glasser, F.P. (1961) Silicate transfor- aration methods would introduce the sametypes of faults mations: Rhodonite-wollastonite.Acta Crystallographica,14, into the specimens,and comparisonof observationsmade 8 l 8-822. on samples prepared by both methods did allow some Henmi, C., Kusachi, I., Kawahara,A., and Henmi, K. (1978) 7T Fuka, Okayama Prefecture.Mineralogical specificartifacts of the crushingof samplesto be identified wollastonite from JournalofJapan, 9, 169-181. as such. Jeferson, D.A., and Pugh, N.J. (1981) The ultrastructure of py- However, the assumption remains that the quenched- roxenoid chain silicates, IlL lntersecting defects in a synthetic in faults observed in samplesrecovered from a seriesof iron-manganesepyroxenoid. Acta Crystallographica, A37, 28 l- isothermal annealing experimentsdo representthe com- 286. D.A., Pugh, N.J., Alario-Franco, M., Mallinson, L.G., plete Jefferson, sequenceoftransformation states.Indeed this study Millward, G.R., and Thomas, J.M. (1980) Ultrastructure of provides examplesin that although the wollastoniteJike pyroxenoid chain silicates,I. Variation ofchain configuration intermediate structurehas beenobserved, the rnechanism in rhodonite. Acta Crystallographica,A36, 1058-1065. by which it is transformedto and from clinopyroxeneand Koto, K., Morimoto, N., and Narita, H. (1976) Crystallographic pyroxenes pyroxenoids. As- rhodonite was not directly observed. Rather it was de- relationshipsofthe and Japanese sociation of Mineralogists, Petrologistsand Economic Geol- duced that thesetransitions were likely to proceedby the oglstsJournal, 7 l, 248J54. propagation of line defects.Similarly, becauseof the ap- Kronberg, M.L. (1957) Plastic deformation of single crystals of parently rapid transformation rates,no transition was ob- sapphire: Basal slip and twinning. Acta Metallographica, 5, servedbetween the microstructuresshown in Figures l0c 507-524. M., and Poirier, (1980) Olivine glass possible Lacam, A., Madon, J.P. and l0d. Until it is to study suchtransformations and spinel formed in a laserheated, diamond anvil high pres- in situ in the electron microscopeat high resolution, the sure cell: An investigation by transmission electron micros- results of a study such as this may be subject to reinter- copy.Nature, 288, 155-157. pretation. Morimoto, N., Koto, K., and Shinohara, T. (1966) Oriented transformation of johannsenite to bustamite. Mineralogical Acrhlowr,nDcMENTS Journal of Japan, 5, 44-64. O'Keefe, M.A., and Buseck, P.R. (1979) Computation of high Thanksare due to Dr. AndrewPutnis for his supervisionof resolution rru imagesof minerals. American Crystallographic this project,and to him, Dr. Mike Bown,and Dr. David Price 15, 27-46. paper. Association Transactions, for their helpfulcomments on draft versionsof this The Pannhorst, W. (1979) Structural relationships between pyrox- reviewby Dr. Ian McKinnon contributedmuch to the organi- Neues fiir Mineralogie Abhandlungen, 135, l- presented were enes. Jahrbuch zationof the material here.Natural specimens 17. kindly donatedby ProfessorT. G. Vallanceof theUniversity of Prewitt, C.T. (1963) Comparisonof the crystal Dr. Abrecht Mineralogical Peacor,D.R., and SydneyGeology Department, J. ofthe of bustamite and wollastonite. American Mineral- Basel andDr. G. Chinner,curator of the structures Instituteof University, ogist,48, 588-596. Harker collectionof the Earth SciencesDepartment, University Poirier, J.P. (1981) On the kinetics ofthe olivine-spinel transi- of Cambridge.This work was carriedout with the supportof a of the Earth and Planetary Interiors, 26, 179- Natural EnvironmentResearch tion. Physics researchstudentship from the l Great Britain. 87. Council of Prewitt, C.T., and Peacor,D.R. (1964) Crystal chemistry of the pyroxenesand pyroxenoids.Arnerican Mineralogist, 49, 1527- RnrnnnNcns r542. Abrecht,J., and Peters,Tj. (1980)The miscibilitygap between Price, G.D., Purnis, A., and Smith, D.G.w. (1982) A spinel to rhodoniteand bustamite along the join MnSiOr-CaouMnooSiO,.g-phasetransformation mechanism in (Mg,Fe)rSiOn.Nature, Contributionsto Mineralogyand Petrology,74,261-269. 296,729-73r. t454 ANGEL: TRANSFORMATIONS BETWEEN SINGLE-CHAIN SILICATES

Putnis, A., and Price, G.D. (1979) High pressure(Mg,Fe).SiO. silicates and aluminates. Journal of Applied Chemistry, 10, phasesin the Tenham chondritic meteorite.Nature, 280,217- 3r7-323. 2r8. Veblen, D.R. (1985) rEM study ofa pyroxene-to-pyroxenoidre- Ried, H. (1984) Intergrowth ofpyroxene and pyroxenoid: Chain action. American Mineralogist, 70, 885-901. periodicity faults in pyroxene.Physics and Chemistry of Min- Wilkins, R.W.T., and Sverjensky,D.A. (1977) The role of fluid erals.10.230-235. inclusionsin the exsolutionofclinopyroxene in bustamitefrom Ried, H., and Korekawa, M. (1980) Transmission electron mi- Broken Hill, New South Wales, Australia. American Miner- croscopy of synthetic and natural funferketten and siebener- alogyst,62, 465-474. ketten pyroxenoids. Physics and Chemistry of Minerals, 5, Yamanaka, T., and Takduchi, Y. (1981) X-ray study of the rho- 35l-365. donite-bustamite transformation. Zeitshrift fiir Kristallogra- Simons, B., and Woermann, E. (1982) Pyroxene-pyroxenoid phie,157, 131-145. transformations under high pressure. In W. Schreyer, Ed. High pressure researchesin geoscience,529-536. E. Schweizer- bart'sche Verlagbuchhandlung, Stuttgart. Mexuscnrsr RECETvEDSEprnMsen 20, 1985 Taylor, H.F.W. (1960)Aspects of the crystalstructures ofcalcium MnNuscnrsr AccEPTEDJurv 8, 1986