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CATION DETERMINATIVE CURVES for Mg Fe Mn OLIVINES from VIBRATIONAL SPECTRA1

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CATION DETERMINATIVE CURVES for Mg Fe Mn OLIVINES from VIBRATIONAL SPECTRA1 Ameican Mineralooist Vol. 57, pp. 962-9850972\ CATION DETERMINATIVE CURVES FoR Mg_Fe_Mn OLIVINES FROM VIBRATIONAL SPECTRA1 RocnrnG. BunNs ANDtr'RANK E. HucerNs,Department ol Earth and Planetarg Sciences, M ass achus ett s I nstitute oI T echnolo gg, Cambridge, M assachusetts02 I Sg Agsrnegr INrnooucrrox cation systemsonly. 'Paper presented at the lgz0 Annual Meetings of the Geologioar society of America, Milwaukee, Abstr., p. 5ll-Bl?. 967 968 BUNNSAND HUGGINS Early studiesof the infrared spectraof olivines (Lehmann,Dutz' that and Koltermann,1961 ; Duke and'stephens,1964) have-shown peak maxima are compositiondependent' Duke and Stephens(1964) suggestedfrom measurementsdown to 400 cm-l that infrared spectra .orrld b. usedas determinativecurves for olivines.we have extended the spectralrange to 200 cm-1and have derived equationsof cation compositiondeterminative curves for the Mg-Fe, Fe-Mn, and Mg-Mn olivine binary solid-solutionseries, and have constructeda determi- native grid for ternary Mg-Fe-Mn olivines' ExronrlrpNter, MPruoos The chemical analyses of the minerals used in the present study are sum- and marized in Table 1. specimens 1-11 belong to the forsterite-fayalite series, the specimens 11-15 conform closely to the fa.yalite-tephroite series, while and 1' tephroite-forsterite series is represented only by specimens 15, 16' 17' (specimen19), Data are also included for roepperite (specimen 18), monticellite (specimens1-18) and willemite (specimen20). The compositionsof the olivines are plotted oo u triuogrrlar diagram for the MpSiOa Fe"SiOn, and Mn'SiO' pseudobinary components in Figure 1-. This figure demonstrates the restricted nature of olivine minerals. Most of the spectra were measured over the range 2000-200cm-' oi a Perkin- Elmer model 225 recording spectrophotometer using powdered olivine specimens that in pressed cesium iodide discs. The precision of this instrument is zuch -F preparation of certain peak maxima may be read to 0'5 cm-' unit' Sample (Burns and silicat'e minerals for irrfrared, measurements is described elsewhere Prentice. 1968:Bancroft and Bums, 196,9)' lrtg 2 5 io4 T1 tL 7 : Mn25i04 Fc25lC)4 Frq. 1. Compositions of the olivines used fOr infrared measurements. DETERMINATION OF OLIVINES FROM SPECTRA 969 Relationships between olivine composition (X) and frequency (y) of eash peak maximum in the infrared spectrum (in cm-t units) were computed by a least squares method, using the CURFIT 2 ALGOL program on file at the Oxford University Computing Laboratory, England. This progra^rn enables a series of polynomials up to degree n ta be computed, provided there are (n * 2) or more points, together with two statistical error estimates: (l) sum of squares of residuals 2(lz.t" - o",t"l') for all points of coordinates (X, r) for each poly- nomial having the form v = c(n,O)* c(n,l)X + ... * c(n,n) Y and (2) standard deviations d(n, q) for the correspondingcoefficients c(n; q). The best-fit polynomial relating peak maxima to olivine composition was taken to be that for which the ratio residual (n): residual (n * l) did not exceed2:1. Rnsur,rs The spectraof minerals with the olivine structure are remarkably similar in the region 1000-550cm-1. This is illustrated by Figure 2, in which the spectraof the olivines forsterite,fayalite, tephroite,and monticelliteare comparedwith that of willemite which doesnot have the olivine structure.Five peaksmay be distinguishedin this region, the positionsof which changewith cation content of the olivine. An additional seven or eight peaks may be recognizedin the region below 550 cm-1,the relative intensitiesand positionsof which may vary with olivine composition. The spectra of the forsterite-fayalite series (except specimen8, which containsappreciable Mn) are illustrated in Figure 3 and the positionsof the peak maxima are summarizedin Table 2. Thirteen peaks or prominent shoulderscould be identified in the spectrum of forsterite (specimen1). Most of the peaks could be traced across the forsterite-fayalite series.Such sets of peaks are here labelled bands1 to 13. All of the bands on the forsterite-fayaliteseries show a shift of peak maxima to lower frequencieswith increasingiron content.How- ever,the compositionalvariations of the individual bandsdiffer, with the result that there are alsonotable changesof relative intensity and breadthof the band envelopesacross the seriescaused by the merging and divergenceof neighboringbands. For example,bands 6 and 7 appearto divergewith increasingiron content,so that band 6, which starts as a prominentshoulder in the spectrumof specimen1 becomes a distinct peak in the spectraof specimens5 to 11. The spectraof the fayalite-tephroiteseries are illustrated in Figure 4. The sameseries of bandsfound for the fosterite-fayaliteseries are remarkably uniform acrossthe fayalite-tephroite series.The peak maxima data summarizedin Table 3 show that the compositional 'D6 +d€do6rNddo tsccqooNhEo ON 6dEdo<+NtdF 6ccqooooqF NO r.oN90 66dEq@Fd! rl cEqEiNboFll N+Ntl N. !dd!6HFv .l ll 6ECEdEo+6ll ..Fdao MddEO@FFE -t I 6cEFilhooFtq) d{mHt e€€EdobbE 6EEENFooqla dd@ol N9F €NNOOEON qeNooco odEd6e@ild .l aE6tro@do4t9 6{eol do Egodoo .l I E@Noool9 .6s660 Ei6 .l c60 lo o6doo d666io .l Chodoolo do OoF q@imoolo dih o Eio6oo = dd---- o hoEE@o oo o r@ro6o bb$io do+ o qso@ooro o .: ,t o do< .t sooq$oro oE. E:3' d@N60N .t e _9? o Eoooooro oa ..:. o 6; o io@itso@ 6@iH@i o J- rl o cooFoo NE: sii@ * 35 o o- oooooo !HAJ ;c: e NNoo06a6a@a o o N No o o o o ! ! .4iHOOCOdd!oObcd-i @H<hhEZAZO?hEEOF DETERMINATION OF OLIVINES FROM SPECTRA 97I Tanr,n 1, coNr. 1. Forsterite; Webster, North Carolina; analysts: J. Carpenter, D. G. W. Smith (microprobe). 2. Chrysolite; Jan Mayan Island, Arctic Ocean (Berkeley 12489); analyst: D. G. W. Smith (microprobe). 3. Chrysolite; Skaergaard intrusion, East Greenland (Oxford 45Zi); analyst: D. G. W. Smith (microprobe). 4. Hyalosiderite; Skaergaard intrusion, East Greenland (Oxford 5107); analyst: D. G. W. Smith (microprobe). 5. Eyalosiderite; Skaergaard intrusion, East Greenland (Oxford 5111); analyst: D. G. W. Smith (microprobe). 6. Hyalosiderite; Skaergaard intrusion, East Greenland (Oxford 4077); analyst: J. V. Srnith (1966). 7. Hyalosiderite; Lydenburg, Transvaal, South Africa (Cambridge 54087); analyst: D. G. W. Smitb (miooprobe). 8. Hortonolite; location unknown (Berkeley 12494); analyst: D. G. W. Smith (microprobe). 9. Ferrohortonolite; Skaergaard intrusion, East Greenland (Oxford 5181); analyst: D. G. W. Smith (microprobe). 10. Ferrohorionolite; Skaergaard intmsion, East Greenland (Oxford 4147); analyst: D. G. W'. Smith (microprobe). 11. Fayalite; Rockport, Massachusetts (USNM R9517); analyst: D. G. W. Smith (microprobe). 12. Knebelite; Schysshyttan,Sweden (Berkeley l%lll); analyst: D. C. Harris (microprobe). 13. Knebelite; Dannemora, Sweden; analyst: G. E. Brown, personal communi- . cation (microprobe). 14. Manganknebelite; Broken llill, Australia; analyst: D. C. Harris (micro- probe). 15. Tephroite; Clark's Peninsula, Wilkes Land, Antarctica; Mason (1959). 16. Picrotephroite; Sterling, New Jersey, U.S.A. (Ilarva.rd 105490); analyst: Hurlbut (1961). 17. Picrotephroite; Franklin, New Jersey, U.S.A. (Harvard 85551); analyst: Hurlbut (1961). 18. Roepperite; Franklin, New Jersey, U,S.A. (Harvard Bauer Collection); analyst: D. C. Harris (microprobe). 19. Monticellite; Crestmore, California, U.S.A. (BM 1960, 334); ana.lyst: Moehlman and Gonyer (1934). 20. Willemite; Belgium (Oxford Museum); unanalysed. variations of all bands for the fayalite-tephroiteseries are smaller than those for the forsterite-fayaliteseries (Table 2). In the forsterite-tephroiteseries the poor representationof speci- menshaving compositionsbetween 10 and 50 percentMnzSiO+ makes it difficult to correlatebands unambiguouslyacross the seriesat low frequencies.This region of the infrared spectrais illustrated in Fig- ure 5 and the peak maximadata are summarizedin Table 3. The spec- 972 BURNS AND HUGGINS mg2sl04 (rl, to25l04 (*||] z = t123l04 I (rlt, a 2 a C.tla!lOa (/19) ln25l O4 (*20) ,lrrrrl looo aoo 600 aoo 200 Gn'r Frc. 2. Comparative infrared spectra of willemite and the olivines forsterite, faYaliLe, tephroite, aud monticellite. DETERMINATION OF OLIVINES FROM SPECTRA 973 -T--r--l-,-Trooo 90() 80() 400 300 #4 *6 #7 o E o : E c E lrl'l l'lrl rooo 90() 800 6()() 50() 4()(, 3()() cm-l Fra. 3. rnfrared spectra of Mg-Fe olivines of the forsterite-fayalite series. 974 BURNS AND HUGGINS 60() 500 400 300 20() #lt #.12 o I c o lb t E UI tr o L F 9N !! cc oo !! lrlrlLlrl 600 500 40(, 300 2oo cm-l Frc. 4. Infrared spectra of Fe-Mn olivinee of the faya,lite-tephroite series. DETERMINATION OF OLIVINES FRCIM SPECTRA w5 Table 2. Peak naxima in the infrareil spectra ol olivines of the f,orsterite - fayalite series Specinen No. L23 4567 891011 Banal 1 982 9 84 980 967 913 968 965 961 958 950 945 2 954 951 945 935 940 939 935 927 926 919 9r4 3 885 885 885 882 882 880 881 874 877 A76 472 838 838 836 834 835 834 833 830 830 a29 427 505 604 597 588 588 585 583 573 57r 568 558 5 s s 510 513 508 5r0 510 506 506 504 496 A90 49L 492 487 482 482 450 472 I 469 s 465 ssss s 408 s s 9 4r5 413 406 394 396 392 390 377 377 366 356 10 399s s s 1I Jt6 3t5 S t2 352 350 348 349 334 338 327 304 I3 294 294 288 284 282 28o s 266 s s 253 :es a proninent shoulder or tra appear to conform with trends in the forsterite-fayalite and fayalite-tephroite series, as most bands (in the forsterite-tephroite series) move to lower frequencies with increasing manganese content' Table 3, Peak mdina in the infrared sPectra
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