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Effects of Compositional Variation on the Crystal Structures Of American Mineralogist, Volume 73, pages 798-808, 1988 Effects of compositionalvariation on the crystal structuresof pyroxmangite and rhodonite LrNoa R. hncxxny Corning GlassWorks, SullivanPark FR-51, Coming, New York 14831,U.S.A. Cn.q.RLrs W. BunNrr.qlr Department of Earth and Planetary Sciences,Harvard University, Cambridge,Massachusetts 02138, U.S.A. Ansrnlcr As adjacent members of the pyroxene-pyroxenoid polysomatic series, constructed of fragments of the wollastonite (W) and pyroxene (P) structures,pyroxmangite (WPP) and rhodonite (WP) possessvery similar structureswith comparablesite distortions and cation ordering patterns. The two structuresrespond quite similarly to cation substitutions. Oc- tahedral and tetrahedral distortions generally lessenand the silicate chains straighten as larger cations substitute for smaller. Both structures exhibit limited stepwiseordering of cations over the octahedral sites,with large cations preferentially entering the sites on the edgesof the octahedralbands. Detailed structural responsesto cation substitution generally parallel those observedin pyroxenes. The characteristics of the inner octahedral sites strongly influence several structural parameters,including the sizesand configurations ofthe outer polyhedra. There is, how- ever, no well-defined mean cation size limit that differentiatesrhodonite from pyroxman- gite from pyroxene. Structural parametersfor both rhodonite and pyroxmangite structures changesmoothly with composition and produce only minor structural adjustments.These adjustments,however, produce localized higher-energystructural configurationsthat clus- ter at the boundariesbetween the W and P modules of the structures.Such configurations include a strongly kinked tetrahedral chain, short Si-Si distances,and a highly distorted octahedron. In contrast, the P-P boundary in pyroxmangite is virtually distortion-free. Concentrationsof strain energyat W-P boundaries likely play a major role in controlling phasetransformations in this system. INrnooucrroN structed by appropriately stackingthese wollastonite (W) Pyroxenesand anhydrous pyroxenoids have the gen- and pyroxene (P) modules, thereby deriving wollastonite eral chemical formula MSiO3, where M most commonly (W), rhodonite (WP), pyroxmangite (WPP), ferrosilite III is Ca, Mg, Fe, and Mn. Their structuresconsist of single (WPPP), and pyroxene (P). The structures of rhodonite chains of silicate tetrahedra arrangedin layers parallel to (n: 5) and pyroxmangite (n : 7) areillustrated in Figure (100) that alternate with layers containing bands of di- l. Bustamite, another three-repeatpyroxenoid, is based valent cation octahedra;oxygen atoms are approximately on a different linkage ofoctahedral and tetrahedral layers closest packed (Prewitt and Peacor, 1964). The various and thereforeis not a member of this series.This concept structuresdiffer in the periodicity of the tetrahedralchain is discussed more fully by Koto et al. (1976) and by and in the correspondingarrangement of the octahedrally Thompson (1978). coordinated cations. In this manner they constitute a A number of workers have addressedthe observed structural seriesthat has been classifiedaccording to the temperature,pressure, and compositional stability limits number n of tetrahedrabetween ofsets that intemrpt py- that exist for each pyroxenoid structure (e.g., Akimoto roxene-likechain configurations(Liebau, 1962).Pyrox- andSyono, 19721'Ito,l972;MareschandMottana, l9T6; ene representsone end member of this structuralseries: Ohashi and Finger, 1978;Brown et al., 1980).Akimoto a pyroxenoid with no offsets(n: oo). and Syono (1972) determined that a pyroxenoid of When the entire structural configuration of octahedral MnSiO. composition undergoessuccessive polymorphic bands and tetrahedral chains is considered, these struc- transformations from rhodonite to pyroxmangite to cli- tures are seento constitute a polysomatic series,which is nopyroxene and ultimately to garnet-type structures as defined as a group of distinct structures constructed of pressureincreases at constanttemperature. A similar trend different numbers of slablike portions of end-member is observedwith respectto composition: As mean cation structures-in this case,of wollastonite and clinopyrox- size decreases,structures with increasing n predominate. ene. All members of the pyroxenoid seriesmay be con- These observations raise an intriguing question: Can 0003-004x/88/0708-0798$02.00 798 PINCKNEY AND BURNHAM: PYROXMANGITE AND RHODONITE 799 TreLe 1. Crystal-structurerefinements of pyroxmangiteand rhodonite Symbol Composition Samole source Reference Pyroxmangites CWB-P (Feo&Cao,3Mno@Mgoor)Sioo Apollo11 site. Burnham(1971) Roth-P (MnolsMgo6s)SiOg Synthetic-- D. Rothbard(unpub.) FH-P (Mno51Mgo€)Siog Synthetic" Fingerand Hazen(1978) ntro (Mno@FeooTMgo @Cao@)SiO3 Japant Ohashiand Finger(1975) AJ-P (Mnor?Mgoozoaool)SiOg Japantf Pinckneyand Burnham(1988) Nar-P MnSiO. Synthetic+ Naritaet al. (1977) Rhodonites FH-R (Mno6rMgo ss)SiO3 Synthetic.- Fingerand Hazen(1978) MT-R (Mno665Mgo 315)Si03 Synthetic.- Murakamiand Tak6uchi(1979) Peac-R (Mno75Mgo rscao 1o)SrO3 Balmat,NY$ Peacoret al. (1978) OF-R (Mnosr Feo oTMgo o6Cao ou)Siog Japanf Ohashiand Finger(1975) Nar-R MnSiO3 Syntheticf Naritaet al. (1977) ' Lunarmicrogabbro. " Synthesizedby J. lto. t Taguchimine: regionally metamorphosed manganese ore deposit. tt Aiiro mine:mineralized manganese ore lens in chert. + Synthesizedby Akimotoand Syono(1972). $ Metamorphosedsedimentary evaporite sequence. the phaserelationships be readily understood in terms of though this correlation was briefly examined by Mura- structural attributes identifiable in each phase?The sta- kami and Tak6uchi (1979), the exact nature ofthe rela- bility of micas, for example, is related to the geometric tionship has not been defined. Nor has there been any fit of the octahedral and tetrahedral sheets(Hazen and systematic study, similar to the plT oxene studies of Ohashi Wones, 1972, 1978); if the octahedral layer becomesso et al. (1975) and Ribbe and Prunier (1971), regardingthe large that normal tetrahedral rotations and polyhedral specific efects of compositional variation, temperature, distortions can no longer compensatefor it, then the sheet or pressureon these crystal structures. silicate will be renderedless stable relative to alternative This paper examinesthe effectsof compositional vari- structures.We might anticipate that there could be sim- ation on the crystal structuresof pyroxmangite and rho- ilar constraints inherent in the linkages ofthe octahedral donite, concentratingprimarily on Mn-Mg substitutions. bands and tetrahedral chains ofpyroxenoids. Becausethere is substantial compositional overlap be- tween the Mn-Mg pyroxmangite and rhodonite stability Pntvrous woRK fields along the MgSiOr-MnSiO, join, thesecompositions Although the chemical and structural relations among are particularly suitable for examining any developing the pyroxeneshave been studied intensively (e.g., Cam- structural instabilities. Accordingly, data from previous eron and Papike, l98l), analogousstudies for the pyrox- crystal-structure refinements of pyroxmangite and rho- enoids are relatively scarce. Ohashi and Finger (1975) compared the structures of pyroxmangite and rhodonite and noted the close topologic and configurational corre- spondencebetween certain cation polyhedra in the two structures.Ohashi and Finger (1978) studied the role of the octahedral cations in the crystal chemistry of the three- tetrahedral repeat hydrous and anhydrous pyroxenoids and demonstrated that the different stacking configura- tions oftetrahedral and octahedral layers determine the resulting cation-ordering patterns and limit the extent of solid solution. Very little has been written, however, concerning the relationship between bulk composition and structural stability of the intermediate pyroxenoids. Liebau (1962) noted that the chain configuration in pyroxenoids is ap- parently a function ofthe averagesize ofthe octahedrally coordinated cations. More specifically,based on the ob- servation (e.g., Freed and Peacor, 1967) that pyroxene stability dependson the relative size of the Ml octahe- Rhodonite Pyroxmongite dron, Tak6uchi (1977) suggestedthat the sizesof the cat- Fig. l. Projections ofthe rhodonite and pyroxmangite struc- ions in the inner Ml-like octahedra of pyroxenoids are tures onto (100). Octahedral(M) sitesin both structuresare iden- primarily responsible for their relative stabilities. Al- tified by number. After Ohashi and Finger (1975). 800 PINCKNEY AND BURNHAM: PYROXMANGITE AND RHODONITE donite (listed in Table l) have been assessedalong with Site those from a new refinement of a nearly pure MnSiO, Occupancy' pyroxmangite. Data for pyroxferroite (Burnham, l97l) are also included in the analysis. We employ the CI setting for the pyrxoxenoids; data Ml(3,4) taken from previous refinementsbased on the PI setting have beenconverted into their CI equivalents.The series ofAppendix TablesAl through A,5compiles selecteddata for pyroxmangite and rhodonite taken or calculatedfrom the crystal-structurerefinements listed in Table l. Srnucrunq.L vARIATToNS M6(2) Cell expansion For the most part, the pyroxmangite and rhodonite Mg structures respond very similarly to compositional vari- Mn ation. Cell parameters of both structures, for example, increasefairly smoothly with the substitution of Mn for Mg, although small amounts of Ca presentin natural rho- M5.M7 donites cause marked increasesin the a and D cell di- mensrons.
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