sign in sign up
<<
Home , Andradite

MINERALOGICAL MAGAZINE, OCTOBER 1997, VOL. 61, PP 728-730

Morimotoite, a new titanian ? - discussion

Irene T. Rass Institute of of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, 35 Staromonetny, Moscow 109017, Russia

THEvalence state and site distribution of in from , Iivaara, Finland, Deer et al., remains a matter of much discussion. In a 1982. N24 after Zedlitz, 1935) shown for compar- recent paper, Henmi et al. (1995) described a new ison. morimotoite, Ca3TiFe2+Si30I2' found in Apparently, both analyses are closely similar. Note contaminated rocks at Fuka, Bitchu-Cho, Okayama that the wet-chemical analysis from Zedlitz (1935) Prefecture, Japan. The morimotoite chemical compo- contains no Fe2+, whereas the analysis of morimo- sition (mean of seven microprobe analyses) and its toite includes Fe3+ and Fe2+, calculated on the basis cell dimensions are presented in Table I, together of 8 cations per 12 oxygens. Hence, the latter mineral with an earlier known titanian andradite (schorlomite may be either the first find at Fuka or in these rocks, SHORT COMMUNICA nONS 729

TABLE I. Chemical analyses Tarte et at. (1979). But Tarte et at. (1979) emphasize that according to their data, Ti should be distributed over tetrahedral and octahedral sites. Morimotoite SchorIomite To decide between the possibilities encountered in nature, or for the sake of formula calculation, 26.93 26.88 SiOz additional evidence is required. Rass (1984) 18.51 17.3 TiOz suggested a hypothetical garnet end-member zrOz 1.48 n.a. Alz03 0.97 2.83 Ca3Td~ij+OI2 on the strength of the following arguments: (1) mineralogical - the substitution of Fez03 11.42* 19.42 FeO 7.78* (Si+Fe) by Ti, a fact inferred from electron microp- MnO 0.23 0.01 robe analysis of zoned ga,rlets (Rass, 1986; Howie MgO 0.87 0.05 and Woolley, 1968); (2) crystallochemical - the CaD 31.35 32.35 above possibilities; (3) an advantage of this hypothetical end-member over the earlier-offered Total 99.54 100.09 Ca3Fe2Ti3012, (Huchenholz, 1969) and a (A) 12.162 12.167 Ca3ThFe2Si012 (Ito and Frondel, 1967): when the required Ca atom content is subtracted, the remainder is close to the theoretical andradite formula; (4) the n.a. not analysed existence of a linear dependence of cell dimension, * calculated on the basis of 8 cations per 12 oxygens and on Ca3Ti2Ti3012 mole fraction. This hypothetical garnet end-member does not or it may be regarded as just a new way of calculating exist in nature, because its stability field occurs under the cation proportion. Henmi et at. (1995) propose higher temperatures and lower oxygen fugacities than this procedure for formula calculation, using a that of , which is stable in mantle hypothetical end-member Ca3Ti4+Fe2+Si30I2' and environments. The conditions of Ca3Ti2Ti3012 presuming that Ti4+ and Fe2+ may occupy the stability are: to, -15 to -35 at 1300-900°C, at octahedral site in the garnet structure. Although this log aCaO-3 in melt, and log aSiO, < -0.622 (Rass possibility does exist (Amthauer et at., 1977; and Dubrovinskii, 1997). Gongbao and Baolei, 1986), it is only one variant If even the synthesized compound of many other possibilities: the presence of two 3Ca:lTi:lFe:3Si:120 (Kiihberger et at., 1989) different valence states of Ti (Howie and Woolley, consists of two distinct end-members and is 1968; Schwartz et at., 1980; Gongbao and Baolei, described by the formulae Ca3Ti4+Fe2+Si30I2 and 1986; Rass, 1986; Waychunas, 1987; Fehr et at., Ca3Ti3+Fe3+Si3012(Fehr and Amthauer, 1996), it is 1990;Malitesta et at., 1995); the occurrence of Ti in reasonable to expect that natural titanian andradite the tetrahedral site (Huckenholz, 1969; Dowty, 1971; (melanite and schorIomite) can also consist of the Amthauer et at.,1977; Tarte et at., 1979; two end-members (with a 'melanitic' silicon Povarennykh and Shabalin, 1983); the occurrence deficiency) together with Ca3Fd+Ti3012' of octahedral titanium (Amthauer et at., 1977; Ca3Fd+TiuSiu0I2, and Ca3Td+Tij+0I2 (in the Huggins .et at., 1977; povarennykh and Shabalin, case of schorlomite). 1983;Waychunas, 1987; Flohr and Rose, 1989); the In any case, a matter of discussion should now be occurrence of octahedrally coordinated trivalent the method of calculation or the predominance of one titanium (Ktihberger et at., 1989); the presence of end-member. A new mineral with its assigned name both octahedral and tetrahedral Fe3+ (Amthauer et might be discussed if the type specimen is re- at., 1977; Gongbao and Baolei, 1986); other examined, and true valence states of Ti and Fe are speculation concerning the positions of Fe2+, Zr4+ established (not calculated), and the presence of more and Ae+ in the garnet structure. than 50% Ca3Ti4+Fe2+Si30I2 in its composition is It is worth noting that most of the methods used proved. (Infrared spectra, Mossbauer study, XANES spectro- scopy) provide only possibilities rather than the Acknowledgements actual mechanisms. And an XPS study qualitatively confirms two different states of Ti (and Fe), although I am indebted to Prof. L.P. Bershov and Dr R.M. their quantitative proportions may remain uncertain Mineeva, whose courtesy made possible the use of the because of the possible impurity of the samples for various analytical methods discussed in this paper. XPS analysis (e.g. some perovskite in schorIomite grains, as may often be seen under the microscope). References According to Henmi et at. (1995), the IR spectra of morimotoite are very similar to those reported by Amthauer, G., Annersten, H. and Hafner, S.S. (1977) 730 SHORT COMMUNICA TrONS

The Mossbauer spectrum of 57Fe in titanium-bearing titanium . Amer. Mineral., 52,773-81. andradites. Phys. Chem. , 1,399-413. Ktihberger, A., Fehr, K.T., Huckenholz, H.G. and Deer, W.A., Howie, R.A. and Zussman, J. (1982) Rock- Amthauer, G. (1989) Crystal chemistry of a natural forming Minerals, Vol. 1 Ortho- and -silicates, schorlomite and Ti-andradites synthesized at differ- London. ent oxygen fugacities. Phys. Chem. Minerals, 16, Dowty, E. (1971) Crystal chemistry of titanian and 734-40. zirconian garnet: 1. Review and spectral studies. Malitesta, c., Losito, 1., Scordari, F. and Schingaro, E. Amer. Mineral., 56, 1983-2009. (1995) ZPS investigation of titanium in melanites Fehr, K.T. and Amthauer, G. (1996) Comment on from Mont Vulture (Italy). Eur. J. Mineral., 7, 'Morimotoite, Ca3TiFe2+Si3012, a new titanian 847-58. garnet from Fuka, Okayama Prefecture, Japan' by Povarennykh, A.S. and Shabalin, B.G. (1983) The Henmi et al. (1995). Mineral. Mag., 60, 842-5. structural role of titanium and iron in synthetic Fehr, K.T., Ktihberger, A. and Huckenholz, H.G. (1990) zirconium- and titanium-containing garnets. Ceol. J., Stability relations and crystal chemistry of Ti- 43(1), 45-50 (in Russian). andradites. Terra Abst., 2, 72 Rass, LT. (1984) On new end-member, Ca3Ti2Ti3012, in Flohr, MJ.K. and Ross, M. (1989) Alkaline igneous titanian andradite group. Doklady, Acad. Sci. USSR, rocks of magnet Cove, Arkansas: metasomatized 277, 196-9 (in Russian). xenoliths from Jo quarry. Amer. Rass, LT. (1986) Paragenetic Analysis (~f Zoned Mineral., 74, 113-31. Minerals. Moscow, Nauka, 143 pp. (in Russian). Gongbao, W. and Baolei, M. (1986) The crystal Rass, LT. and Dubrovinskii, L.S. (1997) The schorlo- chemistry and Mossbauer study of schorlomite. mite thermodynamic parameters and stability field. Phys. Chem. Minerals, 13, 198-205. Doklady RAS, 354(6), 191-8. Henmi, C., Kusachi, 1. and Henmi, K. (1995) Schwartz, K.B., Nolet, D.A. and Burns, R.G. (1980) Morimotoite, Ca3TiFe2+Si30 12,a new titanian garnet Mossbauer spectroscopy and crystal chemistry of from Fuka, Okayama Prefecture, Japan. Mineral. natural Fe-Ti garnets. Amer. Mineral., 65, 142-53. Mag., 59, 115-20. Tarte, P. Cahay, R. and Garcia, A. (1979) Infrared Howie, R.A. and Wooley, A.R. (1968) The role of spectrum and structural role titanium in synthetic Ti- titanium and the effect of Ti02 on the cell-size, garnets. Phys. Chem. Minerals, 4, 55-63. refractive index, and specific gravity in the andra- Waychunas, G.A. (1987) Synchrotron radiation XANES dite-melanite-schorlomite series. Mineral. Mag.,36, spectroscopy of Ti in minerals: effects of Ti bonding 775-90. distances, Ti valence, and site geometry on absorp- Huckenholz, H.G. (1969) Synthesis and stability of Ti- tion edge structure. Amer. Mineral., 72, 89-101. andradite. Amer. J. Sci., Schairer vol., 267A, Zedlitz, O. (1935) Ober titanhaltige Kalkeiengranate. II. 209-32. Zentralbl. Min.., A, p. 68. Huggins, F.E., Virgo, D. and Huckenholz, H.G. (1977) Titanium-containing silicate garnets: 11. The crystal [Manu.vcript received 9 September 1996: chemistry of me1anites and schorlomites. Amer. revised 7 February 1997] Mineral., 62, 646-65. Ito, 1. and Frondel, C. (1967) Synthetic zirconium and (!;) Copyright the Mineralogical Society

KEYWORDs: morimotoite, andradite, garnet, titanium.