Examination of the Siderite-Magnesite Mineral Series by Fourier Transform

American Mineralogist, Volume 74, pages 187-190, 1989

Examination of the siderite-magnesitemineral seriesby Fourier transform infrared spectroscopy

J. Y. DunuwsKl, A-L. CneNNoN BHP Central ResearchLaboratories, P.O. Box 188, Wallsend, NSW 2287, Australia S. Sr. J. WanNn Department of Geology, University of Newcastle,Newcastle, NSW 2308, Australia

AssrRAcr Minerals belonging to the siderite-magnesiteseries have been studied using Fourier transform infrared spectroscopy.Pronounced shifts were observed in several of the carbonate vibrational bands. Shifts in the 1430-cm-, and 880-cm-' bands and a shift of about 60 cm ' acrossthe seriesfor the 350-cm ' band indicate that FrrR can be usedto distinguish members of this solid-solution series.Plots of mole fraction Fe * Mn against band frequency yielded straight lines for the Mg-rich minerals that deviated from linearity as the siderite end of the serieswas approached.The observed shifts reflect the differing mass and ionic radius of substituting cations and possible compressionof the carbonateion.

INrnonucrroN wereobtained for the pelletsand normalized to a concentration of I mg.cm-'?inthe pellet. Siderite is usually not found as pure FeCO, in nature; Peakpositions of the bandswere determined as the point of Both Mn'z* and Mg2* can substitute for Fe2*in apprecia- minimumchange in percenttransmission with variationin wave ble amounts. A complete solid-solution seriesexists be- numberacross the peakprofile. These were obtained both by tween siderite and magnesite,and between siderite and manipulationofthe screencursor ofthe data-processingtermi- rhodochrosite (Deer et al., 1962).This paper investigates nal uponthe expandedspectra and by noninteractivecomputer the former series. analysis.The diferencebetween the two methodswas tlpically ' Mineral carbonatescontaining Fe and Mg have been tl cm for thebroad bands. the subject of study by a variety of different techniques, Rnsur,rs AND DlscussloN such as X-ray diffraction (Goldsmith et al., 1962; Howie and Broadhurst, 1958),differential thermal analysis(Kulp Typical spectra of several members of the series are et al., 1951),and reflectivitymeasurements (Beran, 1978). shown in Figure I for the range 3200-200 cm-'. Com- Infrared spectroscopyhas also been applied to a wide parison of the spectra reveals that the major bands are range including magnesite affected by substitution. Significant shifts occur in the of carbonates siderite and ' ' (Huang and Kerr, 1960;Adler and Kerr, 1963a,1963b; bands near 1430 cm (zr), 880 cm (zr),and particularly in the band near 350 cm-' (zu).The largest shifts are ob- White, 1974;Farmer and Warne,1978;-Wenshi et al., '), 1985). More recently, Raman microprobe analysis has servedwith the 350-cm-' band (about 60 cm whereas a shift ofabout 30 cm 'and 20 cm-r occursfor vrandvr, been used to discriminate among carbonate minerals ' (Herman et al., 1987). respectively. The band at 740 cm (zo)shows the least shift (about l0 cm-'). ExpnnrnnnNTAl DETAILS From Figure 1, it is apparent that the spectraare char- Elevencarbonate minerals, spanning the compositionalrange acteized essentiallyby two very sharp (vtand zo)and two from sideriteto magnesite,were analyzed using standard atomic broad bands (2, and zu).The asymmetric internal stretch- absorptionmethods (Table 1). Fourier transform infrared (rrn) ing mode z. is characteristicallybroad for many carbon- spectrawere obtained on a Nicoletr"rx-rr spectrometer equipped ates. The measuredvalue representsa composite of the with a Nicolet 12005data-processing terminal. Samples were transverseand longitudinal componentsof this vibration preparedby addinga portionofthe finelyground material (<40 and, as seenin Figure l, showsa shift acrossthe mineral pm, about2 mg accuratelyweighed) to a knownamount of ce- series. Therefore, although the peak position was mea- (about sium iodide 450 mg) in the agatecapsule of a smallvi- sured precisely,it is not necessarilyan accurateestimate bratorymill. The mixturewas milled for 3 min after which a of the asymmetric stretching frequency itself. The broad pelletweighing 200 + I mg waspressed in a 13-mmdie for 1 vibration, whereasvr&rtdvo arise from min undera loadof9000 kg. ru band is a lattice Five hundredforty scans(interferograms) were signal-aver- the out-of-plane bending mode and in-plane bending agedto producea primaryspectrum in the wave-numberrange mode of the carbonateion, respectively. 4800-225cm-r, from whicha referencespectrum ofcesium io- The use of these vibrational frequenciesto determine didewas subtracted to yieldthe spectrumofthe sample.Spectra mineral composition was investigated. From Figure 2- 0003-004x/89/0l 02-o I 87$02.00 t87 r88 DUBRAWSKI ET AL.: SIDERITE-MAGNESITE BY FTIR

TABLE1. Comoositionof membersof the siderite-maonesite mineralseries

Fe Mn Mg Ratio Ratio Sample. (%) (%) (./"1 1f 2I

1. (FeossMnoo42MgoooJCO. 45.95 2.00 0.15 0955 0997 2. (FeosTMnoo$Mgoor.)COg 42.68 2.79 1.60 0 907 0.966 3. (FeosMno@Mgo,.)CO" 41.45 2.15 2.74 0.894 0.941 4. (FeouMnooloMgorr)CO. 39.s3 0.s0 4.89 0.880 0.890 5 (Feo74Mnooa6Mgo,")CO. 3640 861 1 85 0.777 0.961 6 (Feo66Mno21Mgols)CO3 3301 10.30 288 0.715 0.938 7. (Feoo"Mno*Mgouo)CO" zo.oY t.+o 12.03 0.664 0.701 8. (FeolsMnoseMgoru)CO" 11.44 2.93 20.22 0.331 0.415 9. (Feo15Mno o@Mgo*)CO" 9.20 0.49 23.03 0.281 0.296 '10. (Feo@rMnooGMgo"u)CO" 2.75 0.59 26.91 0.091 0.110 11 MgCOs 0.00 0 00 28.83 0.00 0.00 . 1, Denmark;2, BrokenHill, NSW; 3, locationunknown; 4, Kalgoorlie, WesternAustralia; 5, locationunknown; 6, Exmoor,UK; 7, Traversella; 8, Mount Bischoff,Tasmania; 9, Kambalda,Western Australia; 10, Shel- tand Lindsey,UK; 11, Rockhampton,Queensland. z tRatio 1:Fe/(Fe+ Mn + Mg).Ratio2: (Fe + Mn)/(Fe+ Mn + Mg). o U) a o z shown in Figure 3 in which all points fall on a smooth (E curve. F The plots of mole fraction + I Fe Mn against v3 lnd v6 are shown in Figures 4 and 5, respectively.Overall, the plots obtained for v, and vr are quite similar. An initial linear dependenceat the magnesiteend ofthe seriesgrad- ually deviates at the siderite end. In both Figures 3 and 4, this deviation appearsat about 0.7 mole fraction Fe * Mn. The effect is most pronounced for the lattice mode zr, however, where a significant decreasein frequency is evident beyond 0.5 mole fraction. Both magnesiteand siderite possessthe calcite structure, and sites occupied by Mg't are replaced by Fe2*or Mn2* during substitution. This replacementpossibly oc- curs in a fashion analogousto dolomite (CaMg(COr)r), which is known to be derived from the calcite structure (White, 1974). Similarity in the structures of dolomite- group and calcite-group minerals is reflectedin their infrared spectra(Huang and Kerr, 1960). In dolomite the cation sites of the calcite structure are 3200 2000 1400 800 200 occupied by two different cations. One cation layer is WAVENUMBERS occupiedby Ca2'ions, and the alternate layer by smaller Fig. 1. FrrRspectra of membersof the siderite-magnesitese- cations such as Mg2* or Fe'z*.This l:l ordering could ries.The numberedspectra follow the sequenceshown in Ta- apply to the magnesite-sideriteseries and involve layers blel. of Mg'?*alternating with those of Fe2*and Mn2r. It appearsthat the carbonateinternal modes ofvibra- tion are not greatly perturbed by the presenceofFe2t or the plot of /2 against mole fraction Fe in the mineral Mn2* until a level of about 0.7 mole fraction is reached. samples-it is apparent that the trend toward lower fre- However, zuis more readily influenced at lower levels of quenciesfollows the increasein substitution by Fe. How- substitution.A possibleexplanation may lie in the changes ever, two points fall noticeably below the line. They cor- that occur to the compressionof the carbonate ion as it respond to samples 5 and 6 (Table l) that contain 8.6 is packed among the different cations. This would espe- and 10.30/oMn, respectively.These amounts are signifi- cially influence/6, which is a lattice mode and is therefore cantly higher than the levels encounteredin the remain- sensitive to the behavior ofcation neighbors around the ing carbonates,which otherwisecontain about 0.5 to 2.9o/o carbonateion. Moreover, as suggestedby White (1974), Mn. It is more useful,therefore, to considerthe combined the compression would be larger for small cations and mole fraction Fe + Mn, sinceMn, becauseof its closeness would decreaseas cation size increases,resulting in a de- in mass and radius to Fe, is expectedto exert a similar creasein frequency. influence upon the observed frequency. Such a result is Finally, theseplots serve to distinguish between mem- DUBRAWSKI ET AL.: SIDERITE-MAGNESITE BY FTIR 189

Fo+Mn

.,r.,. ,., ;;Hr* -or.or]u.,,o'u.l' Fig. 4. Plot of 1430-cm ' band (zr) against mole fraction Fe "rrro-.. Mn. bersof the magnesite-sideriteseries. Wenshi et al. (1985) ticularly sensitive to the high-Mg end of the series.The examined the spectra of a large group of magnesite-sid- shift in the 880-cm-' band is more suitablein distinguish- erite minerals using a grating instrument and, from a plot ing members of the magnesiteseries. The first five points of mole fraction Fe against ur, derived a straight line (R, (samples 7-l I in Table l) yield an acceptableline that : 0.969). They concludedthat a correlation exists be- can be usedfor this purpose[2, : 885.3 - 7.8(Fe+ Mn)/ tween frequenciesand chemical composition. However, (Fe + Mn + Mg), R'z: 0.9861.Less useful is the plot in their data also suggestsome deviation from linearity at Figure 4 that yields a poorer linear fit lv3: 1443 - I 8.8(Fe the high-Fe end. + Mn)/(Fe * Mn * Mg), R, : 0.9041.As mentioned Our results indicate that the plot in Figure 5 is partic- earlier, although the measuredvalue for eachz, frequency ularly useful in distinguishing members of the seriesbe- is precise,each does not necessarilydefine the true fre- cause of the resolution it affords along the sideritic or quency value, which may account for the increasedscat- high-Fe portion ofthe curve. The zufrequency value de- ter. Indeed, this may be true of earlier-reportedvalues of creasesby about 3 cm-' per l0loincrease in total cation the asymmetric stretching frequency (White, 1974). content acrossthe series.However, this curve is not par- It is clear that the inherent precision of rrrn allows the

h (cm-r) 990

MgC03 --a- __ _a _ _a_ _

t't(.

0.2 0.4 0.6 0.8 | 0 0,4 Fe + Mn 0.6 F" * Mn F6+Mn+Mg FE +Mn +Mg

Fig. 3. Plot of 880-cm-' band (z) against mole fraction Fe Fig. 5. Plot of 350-cm-r band (z) against mole fraction Fe t Mn. + Mn. 190 DUBRAWSKI ET AL.: SIDERITE-MAGNESITE BY FTIR substitution efects to be clearly visible for infrared-active Farmer, V C, and Warne, S.StJ (l 978) Infrared spectroscopicevaluation carbonatevibrations. The technique could be applied to ofiron contentsand excesscalcium in minerals ofthe dolomite-ankerite seriesAmerican Mineralogist, 63, 77 9-7 I l. distinguishing even relatively small changesin the chem- Goldsmith,J R, Graf, D.L, Withers,J, and Northrop,D A. (l 962)Stud- ical composition of this seriesdue to cationic substitu- iesin the systemCaCO3-MgCO,-FeCO,. Journal of Geology,70,659- tion. 688. Herman,R.G, Bogdan,CE, Sommer,AJ, and Simpson,D.R (1987) AcxNowr,BocMENT Discrimination among carbonateminerals by Raman spectroscopyus- ing the lasermicroprobe. Applied Spectroscopy,41, 3,437-440 We wish to acknowledgethe support of The Broken Hilll Proprietary Howie, R A , and Broadhurst, F M (l 958) X-ray data for dolomite and CompanyLimited in carryingout this study. ankerite American Mineralogist, 43, l2l0-12 | 4 Huang, C.K., and Kerr, P.K. (1960) Infrared study of the carbonatemin- RpnrnnNcns crrpn erals AmericanMineralogist, 45, 311-324. Kulp, J.L., Kent, P., and Kerr, P F. (1951)Thermal studyof the Ca-Mg- Adler, H H , and Kerr, P.F (1963a)Infrared absorption frequencytrends for anhydrous normal carbonates.American Mineralogist, 48, 124- Fe carbonateminerals American Mineralogist, 36, 643-670. t37 Wenshi,P, Gaokui, L., and Liqin, K. (1985)Infrared spectrastudy of magnesite-sideriteseries. Acta MineralogicaSinica, 5, 229-233. -(1963b) Infrared spectra,symmetry and structurerelarions of some White, W.B (1974)The minerals In V.C. Farmer, Ed., In- carbonateminerals AmericanMineralogist, 48, 839-853. carbonate minerals, p. 227-284. Mineralogical Mono- Beran, A (l 978) Dolomite-ankerite and magnesite-sideritecompositions frared spectraof Society graph 4, London determined by reflectivity measurements.Neues Jahrbuch fiir Miner- alogieMonatshefte, 5 59-564. Deer, W.A , Howie, R A., and Zussman,J (1962) Rock-formingmin- M.lm;scnrm R-EcErvEDOcrospn 13, 1987 erals,vol 5,273 p. Longmans,London. Mlm-rscnrpr AccEmEDOcroser 3. 1988