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Miissbauer Spectra of Synthetic Hedenbergite-Ferrosilite Pyroxenes'

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Miissbauer Spectra of Synthetic Hedenbergite-Ferrosilite Pyroxenes' Amefican Min.eralogist, Volume 58, pages 850-868, 1973 MiissbauerSpectra of Synthetic Hedenbergite-ferrosilitePyroxenes' ERrc Dowry, U. S. Geological Suruey, Washington, D. C. D. H. LrNnsrEys Carnegie Institution ol Washington, llashington, D. C. Abstract The hedenbergite-ferrosiliteseries may be taken as representativein some respects of other sections across the common pyroxens quadrilateral. Data are reported for eight clinopyroxenes synthesized at a temperature above the augite-pigeonitesolvus, plus two orthopyroxenesnear ferrosilite in composition.Clinopyroxenes less calcic than about Fse,Wor were found to have the pigeonite structure (p21/c) on quenching. The assignmentof doublets and the interpretation of area ratios in the M<issbauerroom- temperature spectra is clarified with the hypothesis that a distinct doublet arises from each of the four different types of next-nearest-neighborconfigurations for iron cations in site Ml. These four types arise from random distribution of calcium and iron in M2. Overlap of some of these doublets with the M2 doublet causesthe area anomaly (overestimation of M2 area) previously noted when only two doublets are fitted. This overlap apparently also influences area ratios of liquid-nitrogen temperature spectra of high-calcium pyroxenes, but in these spectra, it does not seem to be possible to fit all the different Ml doublets, owing to their close superposition.This makes it questionablewhether very accurate site distributions for high-calcium clinopyroxenescan be obtained from Mtissbauerspectra, even at liquid-nitrogen temperature. The quadrupole splitting of both M sites can be rationalized with Ingalls' crystal-field model. The Ml site, becauseof its low distortion, is very sensitive to small changes in its environment, hence the several doublets for different next-nearest-neighborconfigurations. Variation in quadrupole splitting of the distorted M2 site, though less pronounced than that of Ml, probably representslarger structural variations, becauseat high distortion, quadrupole splitting is less sensitive to small changes. The curve of M2 quadrupole splitting versus composition resemblesin its shape the augite-pigeonitemiscibility gap, providing a possible illustration of the crystal-chemicalbasis for thrs gap. Inhoduction relations obtained with as many different experi- The notorious complexity of the phase relations mental techniques as possible. We report here a and crystal chemistry of the pyroxenes suggeststhat reconnaissancestudy of a series of synthetic pyrox- there may be a great deal of information about enes by Mrissbauer resonancespectroscopy of iron. geologic processes in pyroxene assemblages.This The principal emphasis of most previous Mcjss- complexity also makes it desirable to have informa- bauer work on pyroxenes has been the measurement tion about pyroxene crystal chemistry and phase of order-disorder relations of iron and magnesium over the two nonequivalent metal sites, Ml and M2. 'P"blt."tt.n authorizedby the Director,U.S. Geological For some compositions, order-disorder may be con- Survey. sidered independently of the other problems of the ePresent address:Department of Geologyand Institute crystal chemistry of pyroxenes. Study of this sort of Meteoritics,The Universityof New Mexico,Albuquerque, began with orthopyroxenes (space grovp Pbcq), NewMexico 87131. sPresent address: Department of Earth and Space since they may be regarded as essentially a solid Sciences,State University of New York, Stony Brook, solution of enstatite (MgSiOr) and ferrosilite New York 11790. (FeSiOa). The first work was by X-ray diffraction 850 MOSSBAUERSPECTRA OF SYNTHETIC PYROXENES (Ghose, 1965), but emphasishas shifted where 20 kbar should be representativeof more magresian possible to Mdssbauer spectroscopybecause of sectionsof the pyroxenequadrilateral at lower pres- greaterprecision and rapidity (e.9.,Virgo and Haf- sures. ner, 1968, 1969,1970;Dundon and Hafner, 1971; A room-temperatureMijssbauer study of a series Saxenaand Ghose, 1970, 197 l) . of clinopyroxeneson the join hedenbergite-ferrosilite Somework in the samevein has also been done was reportedby Dundon and Lindsley (1967), and on clinopyroxenes,augite (spacegroup C2/c) and preliminary results for the present study at liquid pigeonite(P2r/c), especiallysince the return of the nitrogentemperature (90"K) were given by Dowty lunar samples(Bancroft and Burns, 1967a;Hafner and Lindsley (1970). More extensivework, in- andVirgo, 1970;Hafner et al, 197l) . Interpretation cluding particularly room temperature spectra, has of Mcissbauerspectra of clinopyroxenes,however, is shown that some suggestionsmade in the latter re- somewhatmore complicatedthan that of orthopy- port were probablyunwarranted; this report should roxenes, since clinopyroxenesoften consist of an be consideredas supersedingthe earlier work. exsolutionintergrowth of two phases.Clinopyroxenes also containconsiderably more calcium than ortho- Experimental pyroxenesas well as somewhatlarger quantities of Synthesis minor elements such as aluminum, titanium and manganese.AIso, certain technicalpoints have not Pyroxenesused in this study were synthesized yet been establishedconclusively for clinopyroxene from mixturesof Fe-sponge,Fe2O3, quartz (from Lis- spectra,particularly the assignmentof peaks and bon, Maryland) or SiO2-glass,and (exceptfor pure the relative rates of absorption, or recoil free frac- FeSiOs) CaSiOepreviously made from CaCOgand tions, of the two sites.One objectiveof the present quafiz. (Details of the starting chemicalsand weigh- study, concernedwith a seriesof magnesium-free ing procedure have been given by Lindsley and pyroxeneson the join hedenbergite(CaFeSizOo)- Munoz, 1969,p.320).In mostcases these starting ferrosilite (FeSiO.3),was to provide some back- materialswere initially reactedin evacuatedsilica- ground for general clinopyroxenestudy in the form glasstubes at 900-950"C to mixtures of fayalitic of assignmentof peaksin the spectra,and interpreta- olivine, silica, and (for all but the most iron-rich tion of the area ratios of the peaksfor the two sites. compo,sitions) calcium-rich metasilicate. Approxi- A second objective was to try to provide some mately 0.7 g of the pre-reactedmixes were then crystal chemical information which might not be sealedinto silver capsules(12.7 mm long by 6.35 obtainable with other methods, through study of mm outer diameter) togetherwith 20 mg distilled the hyperfineparameters, quadrupole splitting, and H2O, and 1,to 2 mg excesssilica glassto saturate isomershift. the vapor phase.Initial attemptsto synthesizeiron- The choice of the join hedenbergite-ferrosiiite rich pyroxenesyielded products with severalpercent over similar but more magnesiansections in the magnetite,presumably caused by the difiusion of pyroxene quadrilateral (diopside-hedenbergite-fer-hydrogenthrough the silver capsules.Addition of rosilite-enstatite)is advantageousfor severalreasons. trace amountsof acetoneto the starting materials First, subsolidusphase relations on this join are com- inhibited this oxidation.Each capsulewas then re- parativelywell known, whereaslittle work has been acted for approximately2 hours at 20 kbar and done in the interior of the pyroxenequadrilateral. 950'C in piston-and-cylinderapparatus. Two of'the Second,restriction to two kinds of cations,Fe and pyroxenes,FSaz.s and FS1s,s,wero synthesizedby Ca, reducesambiguity in the interpretationof Mciss- repeatedruns in smaller (-0.01 g capacity) cap- bauer area ratios. Third, the relatively high iron sules. content facilitatesMcissbauer study. Our pyroxenes Other than pyroxene,the run productswere pri- were all made at 20 kbar, not becausewe wanted marily fayalite and silica, and occasionally very to study high-pressurephases, but becausethis sup- minor amountsof magnetite.Products were ground pressesfields of pyroxenoid and of fayalite plus and rerun until the content of unreacted fayalite silica, which would have interfered with synthesisof plus silica was judged by microscope examination a completeseries of pyroxenesat lower pressure.In to be well under I per cent. Microprobe examination respect to many of the crystal-chemicalproperties of pyroxenes synthesizedunder similar conditions here of interest,the hedenbergite-ferrosilitejoin at hasshown that compositionalvariations are less than E52 E. DOWTY, AND D. H. LINDSLEY 3 per cent CaSiOs. Incomplete reaction would tend the full width at half height. Flowever, experimental to make the pyroxenes slightly more calcic than their lines are usually more or less nonideal. In fitting the nominal composition, but the average composition synthetic pyroxenes, a Gaussian component was is probably within one-half percent of the nominal added in some cases,in order to take better account value. of several types of nonideality of line shape. Line broadening caused by essentially random factors, Mdssbauer such as inhornogeneities in composition, chemical Samples were fine-ground and mounted in one- disorder over some sites, temperature variations, inch diameter disks of Buehler transoptic plastic. crystalline defects, e/c, should lead to a Gaussian Spectra were taken on a 200 channel, constant- "smearing" of the line. The true line shape in this acceleration"flyback" spectrometer (DeVoe, 1967). case would probably be closely approximated by a Velocity increment per.channel was usually about combination of Gaussian and Lorentzian shapes.A 0.04 mm/sec. Concentrationsof iron in the absorbers line shape of this kind, called the
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