and Wo (CaSiO.) Crystals of Fs.rWoru and Fs"uWo2u ..... 2,825(7'. 2. r27. 2.89r.
2.509. 24.8 [8-coord. ] 18.1 l7-coord.l. 1 2 , 8. 16.9. 2.302
American Mineralogist, Volume60, pages423434, 1975
The Effect of Ca-Fe Substitutionon the ClinopyroxeneCrystal Structure Yosslrezu
OHnsut,
Geophysical Laboratory, Carnegie Institution oJ Washington, Washington, D. C. 20008
Cnnnus W. Bunuseu, Department of Geological Sciences, Haruard Uniuersity, Cambridge, Massachusetts 02 I 38 eNn Lennv W. Ftrqcnn Geophysical Laboratory, Carnegie Inslitution ol Washington, Washington, D. C. 20008 Abstract The major structuralchangeresultingfrom Ca-Fe substitutionin four clinopyroxenesof intermediate composition between hedenbergiteand ferrosilite, measured at room temperature, is in the size and shape of the M2 polyhedron, whereas the Ml polyhedron remains essentially unchanged. The decreasein the average size of the M2 polyhedron associated with a compositional changefrom hedenbergiteto ferrosilitecauseskinking of the tetrahedralchain and concomitant increasesin the out-of-plane tilting of the basal face of the tetrahedron' As the M2 polyhedron decreasesfurther in size,the spacegroup changes from C2/ c to P2r/ c at about FsroWorocomposition. When the composition changesfrom FserWoruto FsrooWoo,the A chain extends and finally reversesits kink direction. The anisotropic temperature factors for theseintermediatecompositionsmay be explainedas a result of the effectsof positional disorder.
the P2rlc and C2/c structuresare that (l) there is distinct silicatechainin one crystallographically only Most mineralscan form solid solutions,either whereastwo typesof the chains structure, the C2/c limited or complete,and thus somesitesin the unit exist in the P2rlc structure,and (2) the M2 site' is cellsof mineralsare occupiedstatisticallyby more eight-coordinatedin the C2/c end members(diopthan one atomic species.If thereis a large difference side: Clark, Appleman,and Papike, 1969;hedenin the ionic radii of the atomsoccupyingthe site,the bergite: Veblen, 1969, and Cameron et al, 1973) crystalstructuremay changewith a changein ocand six-coordinatedin the P2'l c end members cupancyof the site and thus in the apparentsize (clinoferrosilite:Burnham, 1967; clinoenstatite: of the coordinationpolyhedron.One of the best Morimoto, Appleman,and Evans,1960). mineralogicalexamplesof multiple occupancyis the composition, of intermediate In clinopyroxenes wheresubstitutionof Ca for Fe or augite-pigeonite, M2 site is very irregular as a result of multiple ocMg is involved. cupancyof the largeCa and smallFe or Mg atomsin In the pyroxenequadrilateral(diopside-enstatitethe site. In a structural refinement of a pigeonsystem)the clinopyroxene ferrosilite-hedenbergite from Mull, Scotland, com- ite, Mgo.3eFeo.u2Cao.orSiOs spacegroupsare P2rfc for Ca-poor(pigeonite) (1970) reported a very Giiven and Morimoto positionsand C2/c for Ca-rich(augite)compositions M2 coordinationpolyhedron,which they at room temperature.The major differencesbetween irregular concludedwas an averageof eight and six coordinations. Giiven (1969) studied the relationship t Nomenclature for the pyroxenes is that proposed by Burnham betweenthe averagesize of M2 and kinking of the et al (1967'). Introduction
424
OHASHI, BURNHAM, AND FINCER
tetrahedralchain.Takeda(1972)discussed variation (all synthesizedat 20 kbar and 950"C), and Fs.uWoru of M2-O bondsin clinopyroxenes asa functionof Ca (6 kbar and I l40oC) have been examined using the contentand showedthat the M2 polyhedronbecomes Buerger precessioncamera. Class b reflections (h + k very irregular,thushighly unstable,in the middleof = odd) are exhibited by Fs.rWoruand some crystals the solid solution.He proposedthat irregularityof of FsroWoro,whereaslong-exposurephotographs of the M2 polyhedralshapeprovided a structural ex- F s . u W o r u , F s r o W o s o ,a n d t h e o t h e r c r y s t a l s o f planationfor the presenceof a miscibilitygap. In FsroWorodo not show these b reflections.Thus the their crystal structural study of six end-member (metastable) boundary at room temperature between clinopyroxenesat high temperatures,Cameronet al the space groups P2rlc and C2/c is close to the (1973)explainedthe increased solidsolutionbetween FsroWo2ocomposition on the join hedenbergitethe Ca-poor and Ca-rich clinopyroxenesat high ferrosilite. At low pressures the clinopyroxene temperatureby the change in M2 coordination of this composition is not stable relative to the associated with the P2r/c-C2/c phasetransition. assemblagefayalite * tridymite (Bowen, Schairer, The presentstudyis concerned primarilywith the and Posnjak, 1933; Lindsley and Munoz, 1969). effectsof chemicalsubstitutionon the structure No exsolved phases are detected on long-exposure studiedat room temperature. To eliminatethe struc- precessionphotographs, and thus the crystals are tural effectsof possiblechangesof Mg-Fe distribu- consideredto be homogeneous. tion in the cationsites,the binaryjoin hedenbergite U n i t - c e l l p a r a m e t e r s o f c r y s t a l s o t h e r t h a n (CaFeSirO.)-clinoferrosilite (FerSirO.)was selected. FsroWo2o were measured using a back-reflection Therefore,thesestructuresreflectsolely a difference precision Weissenbergcamera. Data for both CuKa, betweenCa and Fe contents.Furthermore,if theMl and Ka, wavelengths were refined by the leastsiteon thejoin studiedis occupiedonly by Fe atoms, squares method and included corrections for film as expectedfrom its polyhedralsize,it is possibleto shrinkage,camera eccentricity,and specimenabsorpisolatethe effectsof changesof the M2 siteon the tion (Burnham, 1962). Unit-cell parameters for otherpartsof the structure,especially the tetrahedral FsroWoro were determined by a lattice-constant chain configuration. refinement subroutine for a four-circle diffractometer. This subroutine, which is a part of the Geophysical Laboratory diffractometer system, has Unit Cell and SpaceGroup some new features: auto-centering of reflections and Syntheticsinglecrystalsof clinopyroxenes on the least-squaresrefinement of the orientation matrix join were kindly sup- from which the unit-cell parameters (Table l) are hedenbergite-clinoferrosilite plied by Dr. D. H. Lindsley. Synthesistechniques calculated (Finger, unpublished; Gabe, Alexander, were describedby Lindsley and Munoz (1969, and Goodman, 1970).Analysis of the clinopyroxene Appendix).Crystalsof FsruWoru,2 FsroWoro, FsruWoru lattice deformation due to chemical substitution on this join is given elsewhere(Ohashi and Burnham,
Experimental
r973).
TesI-r l. Unit-Cell Parameters and Unit-Cell Volume of Ca-Fe Clinoovroxenes
e (4> (A) b
ed)
B (des.) y (43)
*Parenrhesized
Measurementand Reductionof X-Ray Intensity Data
Fs65Wo35
Fs75SoZ5
9.812(1)* 9 . 0 4 9( 1 ) s . 2 3 3( 1 ) ros.34(1) 448.r(1)
9 . 78 r ( 2 ) 9 . 0 72 ( 2 ) 5 . 2 4 6( 2 ) 106.s5(2) 446,2(3)
9.760(6) 9 . 0 5 7( 8 ) 5,234(3) 1 0 6 . 2 8( 5 ) 444,r
