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The Crystal Chemistry and Petrogenesis of a Magnesian Rhodonite'

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The Crystal Chemistry and Petrogenesis of a Magnesian Rhodonite' American Mineralogist, Volume 63, pages ll37-1142, 1978 Thecrystal chemistry and petrogenesis of a magnesianrhodonite' DoNem R. PrecoR,Enrc J. EssENE,PHrr-lp E. BnowNnNo Ge,Rv A. WrNrrn Departmentof Geologyand Mineralogy, The Uniuersityof Michigan Ann A rbor,M ichigan48 I a9 Abstract Rhodonite with unusually high magnesium and low iron contents, Mnr 6 Mgo rsCaourFeo.o.SiuOru, occurs in a metamorphosed sedimentary evaporite sequenceat Balmat, New York. lt coexistswith an unusualpyroxene having averagecomposition close to MnMgSirOu,talc, calcite,and a number of other minerals.Refinement of the crystalstructure using single-crystalX-ray diffraction data showsthat Mg has a marked preferencefor the M4 site (Mg'{?Mno.ur),and that Ml, M2, and M3 have nearly equal Mg occupanciesof slightly more than 0. l. Relationswith coexistingphases indicate that both the Mg content and degree of ordering of Mg in M4 are near the maximum to be expectedfor naturally occurring rhodonite. Introduction spectrometer analyseswere therefore obtained for rhodonite from both samples, and. the results are Two samplesof rhodonite were found by miners in presentedin Table l. Single-crystalX-ray diffraction different areasof the Balmat Mine No. 4, and saved confirmed that the analyzedmaterial had the rhodo- only because of their attractive appearance and nite structure. Becausethis rhodonite has a Mg con- uniqueness.Portions of each sample were kindly tent greater than that of most other rhodonites,and made availableto us by Dr. Dill of the mine staff,and becausethe iron content is exceptionally low, we we refer to them simply as samples I and 2, respec- concludedthat a structure refinementwould provide tively. The rhodonite of sample I occurs with a py- definitive data regarding the occupancy of Mg over roxene of approximate composition MnMgSirO. the octahedral sites. which is currently under investigation, and Iine- grained talc and quartz. The secondsample also has X-ray diffraction data both rhodonite and the pyroxene, but also contains Unit-cell parameters were determined by least- barite, anhydrite, manganoan calcite, apalite, quartz, squaresrefinement, with data for nineteenreflections hauerite, and a phase tentatively identified as dato- obtained from a powder diffractometer pattern as lite. These two sampleswere recoveredfrom inter- corrected with quartz as an internal standard. The layeredmetasediments (evaporites) consisting largely parameters are a : 9.791(3), b : 10.497(3),c = of talc, tremolite, calcite, and anhydrite, which are 12.185(4)A, a 108.55(4),P : 103.02(4),t : the host rocks for the ore body. The deposithas been 82.49(4)". The spacegroup is C I . We usethis setting, regionallymetamorphosed with estimatedconditions recommendedby Ohashi and Finger (1975), as well of P : 6.5 + 0.5 kbar, T : 625 L 25"C (Brown et al., as the atom nomenclatureofthose authors,because it 1978).The conditionsfor equilibrationof rhodonite provides a more direct meansof comparing pyroxe- and its coexisting phasesare thus reasonablywell- noid structures.Powder diffractometerdata are listed defined. in Table 2. Preliminary energy-dispersiveanalyses using the Intensity data were obtained from a cleavagefrag- electron microprobe revealedthat the rhodonite was ment measuringppproximately 0.15 X 0.20 X 0.40 unusually rich in Mg and poor in Fe. Quantitative mm, mounted for rotation about the c axis on the Weissenberg-geometrydiffractometer. Mor(o radia- 'Conlribution tion, monochromatedwith a flat graphitecrystal and No. 341 from the MineralogicalLaboratory, De- partment.of Ceology and Mineralogy, The University of Mich- detectedwith a scintillation counter, was used. The lgan. Supper-Paceautomated systemwas used,employing 0003-004x/78llI l2-l 137s02.00 I 138 PEACOR ET AL,: MAGNESIAN RHODONITE Table l. Electron microprobe analysis of magnesian rhodonitel Refinement Refinement was carried out using the program oxide wr. Z Molar ratio/5 Si RuNr2 of Finger and Prince (1975),employing scat- sio2 47 '66 5.00 tering factorsof Doyle and Turner (1968),the weight- ing schemeof Cruickshank(1965, p. ll4), and be- AL|O3 0.06 0.00 ginning with the structure parametersof Ohashi and l'lno 42.62 3.73 Finger (1975). Reflectionshaving individual discrep- MgO 4.72 o.73 ancy factors greater than 0.5 were rejected.During Feo O.37 0.03 the first stages of refinement all of the Mg was as- sumed to be in the M4 siteand the Ca to be in the M5 Cao 4.58 0.51 site,with additional Mn completingthe occupancyof Mn exclusivelyassigned to the sun 99.99 thesesites, and being Ml, M2, and M3 sites.Following the convergenceof -Standards positional parameter refinement, site occupancies used were synthetic tephroite for l'ln, ANU rhodonite for Si, Irving kaersutite for Al, were allowed to vary with both Mn and Mg assigned Ca, Fe, Mg. Drift, atomic nunber, fluorescence, absorption and background corrections were to Ml through M4, and Ca and Mn assignedto M5. applied to the data. No bulk-chemistry restraintswere placedon the oc- cupancy factors. Refinement was completed with a scanacross each reflection with backgroundmea- refinement of occupancy values, atom coordinates, suredon eachside. A total of 2473intensities were and isotropic temperaturefactors. The final R value measured,of which152 were unobserved, up to a sind is 4.8 percent for all data and 4.2 percentexcluding limit of 0.46and an / indexlimit of 13.All intensitiesrejecteddata. Structure factors are listedin Table 3,2 werecorrected for Lorentz-polarizationand absorp- atom coordinates and isotropic temperaturefactors tion effects(p : 51.5cm-'). A modifiedversion of a in Table 4, and site occupanciesin Table 6. Selected programAssnp written by C. W. Burnhamwas used interatomic distancesas calculatedusing the program for the absorptioncorrection. Onrrn (Busing et al., 1964)are listed in Table 5 with standard errors calculated from standard errors of unit-cell parametersand the variance-covariancema- Table 2. X-ray power diffraction datar trix from the final cycle of refinement. hkl d(calc) d(obs) I hkl d(calc) d(obs) I Discussionof cation ordering 110* 7.105 7.7 50 334 2.260 2.259 3 110* 6.666 6.67 t5 330 2 222 The rhodonite crystal structure has been described 200* 4.763 4.76 50 205 2.221 ozr* q.rtal,,^ 2.220 24 o 403* 2.220 in detail elsewhere(e.9. Peacor and Niizeki, 1963) zot 4.1I3_l 223 2 220 11.2 3.811| 33r 2 207I 2.208 B ''or rJ and will not be further describedhere. We are princi- 2?1 3.802f 241* 2 -178 2 178 53 023 3.562 | 331 2.154 ,.tt d) 2.152 pally concernedwith the occupanciesof the Ml-M5 220* 3.552-l-- 3T4 2 r52t- 6 t31* 3.401 3.40 11 z. tla Niizeki concluded, 2zr 3.254I 2 113 sites. Peacorand on the basis of t''" tt 3t3r 2 I13F zz3 3.231-f- 424* 2.083 2 034 M-O interatomic distancesand peaksin a difference l3t* 3.I34 3.13 76 151* 2.061 221 3.os8 I 2.061 ,.u6 ruu 042 2.059 electron-densitysynthesis, that the Fe and Mg prob- 3I0 3.082l-- 332 1.950 3Io* 2.970- 2.969 loo 512 1.949 1.949 ably occupiedthe M4 site and that Ca was confined 113* 2.927 2.9?5 63 s11 1.949 131 z.Btzf 513 1.394 to the M5 site. The formula of rhodonite from the 31_1 2.809F2.809 r0 510 1.894 313 2.807) 443 l.890 locality serving as a source for the crystal used in 311* 2.782 2 781 26 134 1.889 224 2.748 2.748 46 315 1.888 that diffraction study was (Mnn.orCao.rrMgo.ruFeo.rr) 023* 2 648 ?-648 18 006 I 888 o4I* 2.593 2.593 34 (Si4.rrAlo.02)O,u.,. polyhedra qrc 1.864 The coordination about 112 r I 025 1.861 lat , t11 | 316 1.861 Ml, M2, and M3 are only slightly distorted octa- 35i" ,'.;6;' 2.505 65 3s2 1.361 242 z.42qf 441 1.860 hedra,having averageM-O distancesof 2.219,2.215, 4oI z.qzal-z qzt lo 403 1 857 115 z.qzsl 1.832 and 2.229A respectively,in good agreementfor occu- 400 2.382 2.382 8 I 832 33Q 2.368 2.369 16 444 1.830 I. B3] pancy only by Mn. The M4 coordination is highly 422 2.312 2.3t2 4 405 1.830 irregular, however, approaching five-coordination lobserved v^alueswere obtained using a phi llips Diffractometer, becauseone of the six M-O distancesis 2.88,4,.Two employingCuKoradiation rcnochromaaedwith a curved graphite crystal. Quartz was used as an internal standard. indjces were obtained for the C-centered unit cell. *These 2To receivea copy of this material,order documentAM-78-076 reflections were used for the refinenent of latt.ice parareters. Whereanbiguity exists in the choice of hkl, from the BusinessOlfice, Mineralogical Society of America, Suite oecrsrons were madeon the bdsis of intensities as determined 1000lower level, 1909K Slreet,N.W., Washington,D.C., 20006. using the single-crystal diffractoreter. Pleaseremit $1.00 for the microfiche. PEACOR ET AL.: MAGNESIAN RHODONITE I 139 M4-O distances( 1.98and 2.04A) are the smallestof Table 5. Cation-anion interatomic distances(A) (standard errors all M-O distances,and this was good evidencethat in parentheses) the smaller Mg and Fe preferentiallyoccupy this site. The more irregular seven-coordinateM5 polyhedron M1-OA1 2. L82(3) 142-OA2 2.2L3(3) 0A1 2.333(4) oA3 2.242(3) has an averageM-O distanceof 2.42A, the smallest oA2 2.268(4) oA3 2.28O(3) distance being2.26A, a reflection of the strong prefer- oA4 2.253(3) oA4 2.3s3(4) 0A5 2. L39(3) oR2 2.1s0(4) enceof Ca for this site.
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