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Witherite Composition, Physical Properties, and Genesis

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Witherite Composition, Physical Properties, and Genesis American Mineralogist, Volume 64, pages 742-747, 1979 Witherite composition,physical properties,and genesis ARTHUR BaTDRSARI' AND J. ALnxaNonn SpBen Orogenic Studies Laboratory, Department of Geological Sciences Virginia Polytechnic Institute and State University B lacksburg, Virginia 2406 I Abstract Microprobe analysesof 17 natural witherite specimensfrom various localities show sub- stitution of strontium (up to I I mole percent),lesser amounts of calcium ( < I mole percent) and no detectiblelead. Most witheritescontain lessthan 4 mole percentSr and 0.5 mole per- cent Ca. Lattice parametersand density vary regularly with compositionin the entire range. The equation,equivalent mole percentSrCO, (+l.l): -1102.58(drro,A) + 2515.74,can be used to determinethe approximatecomposition of witherite-strontianite solid solutions for X(SrCO3)< I L A plot of mean ionic radius ys.cell volume for natural witheriteslies above the plane connectingthe pure end-membersBaCOr-SrCOr-CaCOr, suggesting a small posi- tive excessvolume of mixing. Calcium substitution 1slimited to minor amounts becauseof the miscibility gap between witherite and orthorhombic CaBa(COr)r,alstonite. The limited Sr substitutionand negligible Pb substitution,however, are believed to depend upon the composition of the pre-existing sulfate (barite) from which witherite forms and the disequilibrium behavior of low-temper- ature solutions(<200'C) that crystallizeorthorhombic carbonates. Introduction mated ARL SEMQmicroprobe at 15 kV and l0 This work is part of a systematic study of the nanoamps,employing Bence-Albeemethods of data chemistry and physical properties of the ortho- reduction. Standardsincluded synthetic BaCOr(Ba) rhombic carbonates.The probable conditions and and SrCOr(Sr)and natural calcite (Ca) and cerussite mechanismsof witherite genesisare examinedin or- (Pb). The carbon content was obtained by calcu- der to explain the limited chemical composition of lating the number of carbon atoms accordingto the : natural witherites. relation C Sr + Ca + Ba + Pb. No elementsother than Ba, Sr, and Ca could be detectedwith a Kevex Experimental procedures X-ray energy-dispersiveunit. With standardsand un- The 17 witherite samplesexamined in this study knowns of similar compositions,Ba analysesshould are listed in Table l.'? In addition to the literature be accurateto about 2 percentof the amount present. summarizedby Palacheet al. (1951) and Deer et al. The relative errors for Sr and Ca arc higher because (1962), other data used in the following discussions of counting statisticsand the problem of estirnating of witherite compositions are from Frank-Kame- backgroundcounts for minor elements. netskii(1948), Gvakhariya (1953), De Villiers (1971), Unit-cell parameterswere determinedfrom smear and Sidorenko(1947). mounts of powderedwitherite using a Picker powder The witherite sampleswere analyzed on an auto- di-ffractometer.Scans at the rate of 0.5olmin were made with monochromatized CuKa radiation using BaF, (a : 6.19784) as an internal standard. All rPresent address:Department of Geology, University of Geor- peakswere measuredat one-half peak height and in- gia, Athens, Georgia 30602. dexed accordingto the data ofSwanson et al. (1954). 2To obtain a copy Table of l, order Document AM-79-100 from program the BusinessOffice, Mineralogical Society of America, 2000 Flor- The least-squares of Appleman and Evans ida Ave., NW, Washington,DC 20009.Please remit $1.00in ad- (1973)was used to refine the cell parameters. vance for the microfiche. Densitiesof severalsamples were determinedwith 0m.3-.wx/ 79 /0708-0742$02.00 742 BALDASARI AND SPEER: WITHERITE 743 a 2 nI pycnometer,using toluene (correctedfor tem- Equ ivo I ent Mol o/o Sr perature) as the displaoementmedium. Several mea- to 5 surementswere made for eachsample and were aver- aged. Sample (2), Table l, was synthesizedfrom J. T. Bakerreagent grade BaCO,Lot418067 at 550oCand I kbar. This sample served as a standard for micro- a probe analysis. E anr o Results Resultsof the study are given in Table l. Because o ""' of the compositional similarity of the majority of samples,Table I has been abridgedin Table 2 to tn- clude representativecompositions. The microprobe analysesshow substitutionsof up to I I mole percent SrCOr, lessthan I mole percentCaCOr, and no mea- t45 t46 t47 surable substitution of lead. Sample 19 is the most Meon lonic Rodius (A) strontium-rich of the witheritesstudied or reportedin Fig. l. Plot of cell volume and mean ionic radius for witherite the literature. Higher calcium contentshave been re- samplesin Table l. Becausethe calcium contentsof the witherites ported (Sidorenko, 1947)but were attributed to the are small, the meanionic radius is alsogiven in termsof the equiv- presenceof calcite inclusions.The analysesof with- alent mole percentSrCO3 substitution in witherite. The ionic radii fornine-coordinatedBa2+ (1.4?A), SP* (l.3lA), and Ca2*(l.l8A) are from Shannon(1976). The unit-cell volumesused to draw the ideal mixing lines are BaCO3, 3M.24A3 (Swansonand Fuyat, Table 2. Crystal and chemical data for witherites 1953),CaCO3, 226.85A3(Swanson et al., 1954),and SrCO3, 25935 A3 (Speerand Hensley-Dunn,I 976). t8 Pb0 0.00 0,00 0.00 0.00 erites in this study show a mean of 3.3 mole percent ca0 0.08 0.11 o,24 0.02 sr0 0.68 ]-72 3.26 6.06 Sr and 0.3 mole percentCa, comparableto the natu- Ba0 17.r5 76.58 75.00 72.LO ral witherite compositions in the literature. co2* 22.50 22.78 23.L0 23.28 Total 100.41 101.19 r01.60 101.46 The unit-cell parameters exhibit a small, system- Nunber of lons on the basis of 1 COe atic variation betweenthe extremewitherite compo- sitions (Table 2). Unit-cell volume varies nearly lin- Pb 0.0000 0.0000 0.0000 0.0000 Ca 0.0028 0.0037 0.0081 0.0008 early with mean ionic radius over the compositional Sr o.or29 0.0320 0.0599 0.1104 Ba 0.9844 0.9644 o.9320 0.8888 range (Fig. l). If there were ideal mixing in the Ba- Sr and Ba-Ca solid solutions,the samplesshould lie l,l.r.R, *** r.4672 t.4639 1.4580 L.452L on the lines joining end-member orthorhombic car- bonatesin plots of cell volume vs.ionic radius. How- v, 63 303.93(6) 302.87(9) 301.32(10)300.38(6) ever, the witherite sampleslie abovethe ideal mixing arA s.3L2(.7) 5.305(1) 5.297(2) s.293(1) b,L 8.999(1) 8.890(2) 8.877(3) 8.861(2) lines (Fig. l), suggestinga positive excessvolume of cr L 6.428(r) 6.42t(2) 5.408(2) 6404(1) mixing in this compositional range. The excessvol- ume may result from the large barium atoms pre- d22o calc, 2.2807 2.2779 2.2745 2.2722 d22o ^"^" . 2.2807 2.2777 2.2750 2.2?2L venting the structure from contracting around the smaller Sr and Ca ions when they are present in ocalc. 4.29 4.28 4.27 4.24 omeas. 4.31 4.28 4.26 4.22 small amounts. The approximateSr contentof witherite can be de- termined indirectly, u5ing Figure l, becauseCa is a *CaLculated to gioe C = Ba + ST + Ca + Pb minor constituent. A simpler method is to use the **The estimted standttd errors a?e giuen in puentheses and. xefer to the Laet decinaL place(s). single 4zo spacingof witherite. The equation derived ***Man i.onic radius 4 Anglesarke, Inrcashi"e, Eng1'and I|MNH #103262 by linear regressionis 1 3 s e tlting s tone MLne, Nor tlmb etLand, England 18 Arkhys, Kaukoeus, USSR equiv.mole percent Sr CO3(+l.l) Colmbin NMC#34999 19 Cassiar DtstrLet" British : -l102.58 (d,'o,A)+ 2515.74 BALDASARI AND SPEER: WITHERITE The 220 reflection, rather than the 132 peak used for Equivolenl Mol 7o Sr other orthorhombic carbonates,was chosenbecause ro5'o of its intensity, sharpnessand the lack of interfering peaks. Table 2 contains measured and calculated den- sities, which compare favorably. Density was found to vary linearly with the mean atomic mass of the cationsin the sample(Fig. 2a), fitting a linear regres- sion equation meancation atomic mass: 86.48(p) - 235.01 427 which can be recast as Equiv Mol o/. SrCOJ i | = : -173.9 - r73 9(P) + 748 9 equiv.mole percentSrCO3(il) (p) + 748.9 E o 425 This equation differs from the regressionline for the CD calculateddensity of syntheticorthorhombic carbon- o 4?4 atesof the group IIa cations: a Ca, Sr, Ba, and Ra (Fig. a_ 2b). Although over the range of the end-member group IIa carbonatesthese cations show a linear rela- tionship, in detail they exhibit a larger deviation than expected for the errors in cell volume and atomic mass determination. Other metal cations that form orthorhombic carbonates,Sm, Eu, and Pb, lie well aboveboth lines. Thesepossess suitable sizes and va- lencesto form orthorhombic carbonates,but they are heavieratoms than the group IIa cations. Discussion 60 too t40 t80 220 Natural witherite examined in this study and re- Cofion Atomic Moss (omu) ported in the literature showsonly a limited composi- Fig. 2. (a) Plot of calculateddensities ys. mean atomic massfor tional range; only Sr substitutesfor Ba in significant cations of witherite samples in Table l. Becausethe calcium con- amounts. tents of the witherites are small, the mcan atomic mass is also Barium has been rarely reportedin analysesof ce- given as equivalent mole percent SrCO, substitution in witherite. russite and aragonite above trace amounts (Palache (b) Plot of the calculated densities and atomic mass for cations of et al., 1951,p. 194-196;Deeret al., 1962,p.319-322). the orthorhombic carbonates. Unit-cell volumes used to calculate the densitiesin addition to those used in Figure I are alstonite Speer and Hensley-Dunn(1976) and Speer(1976) (als),267.3A3(Sartori, 1975);EuCQ, 259.10A3(Mayer et al., found that Ba is present in only minor amounts 1964);SmCO:, 260.72A3(Asprey et al., 1964\;RaCO3, 335.3A3 (<3600 ppm) in strontianite.These results combined (Wiegel, 1977,p.
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