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Knorringite-Uvarovite Garnet and Cr-Eskola Pyroxene in Ureilite Lew 88774

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Knorringite-Uvarovite Garnet and Cr-Eskola Pyroxene in Ureilite Lew 88774 64th Annual Meteoritical Society Meeting (2001) 5016.pdf KNORRINGITE-UVAROVITE GARNET AND CR-ESKOLA PYROXENE IN UREILITE LEW 88774. C.A. Goodrich1 and G.E. Harlow2. 1Max-Planck-Institut für Chemie, PO 3060, D-55020 Mainz, Germany ([email protected]). 2American Museum of Natural History, New York, NY 10024 USA. Introduction: In LEW 88774 chromite grains (~100-800 µm) are enclosed by oikocrysts of orthopyroxene with exsolved augite [1-5]. They are generally associated with patches of graphite and commonly also with olivine. All grains have 5-40 µm-wide rims of glass (~70-75% SiO2, ~13-18% Al2O3), with corroded edges against the glass, and are surrounded by an assemblage of unusual Cr-rich minerals including Fe,Cr-carbide(s), Fe,Cr-sulfide(s), and eskolaite-corundum [1,2]. We describe two new members of this assemblage: knorringite-uvarovite garnet, and what appears to be a Cr-rich pyroxene that would be an Mg-rich Cr-analog of Ca-Eskola pyroxene. The latter should be a new mineral. Cr-rich Garnet and Pyroxene: These minerals form rims or bands (10-20 µm wide) at the contact between glass-rimmed chromite grains and the surrounding silicates. Where the surrounding silicate is olivine, the rims/bands consist of the Cr-rich garnet. Where the surrounding silicate is opx, they consist of the Cr-rich pyroxene. Where the surrounding silicate is augite, they are absent. The garnet contains 27% Cr2O3 and has the structural formula: (Mg3.7-3.8Fe0.2-0.4Ca1.7-1.8)5.9-6.1(Cr3.2-3.5Al0.4-0.7Ti0.1)3.8-4.0Si6O24. It is nearly a pure solid solution of 65-70% knorringite (Mg6Cr4Si6O24) and 30-35% uvarovite (Ca6Cr4Si6O24). The Cr-rich pyroxene contains 29% Cr2O3 and has the structural formula: !0.5(Mg0.29-0.45Fe0.06-0.19Ca0.01)0.5(Cr0.78-0.90Al0.08-0.21Ti0.01)0.99(Si1.98Al0.02)2O6. Its closest known analog may be Ca-Eskola pyroxene: !0.5Ca0.5AlSi2O6 [6,7]. We suggest that it be referred to as Cr-Eskola pyroxene: !0.5(Mg,Fe,Ca)0.5CrSi2O6. In reflected light, this pyroxene shows anomalous blue-green scattering (similar to that of kosmochlor - NaCrSi2O6 [8]), and the garnet shows yellow-pink or blue-green scattering. The garnet commonly shows partial breakdown: along the contact with glass it is clouded with tiny dendrites that appear to be chromite, and analyses yield a non- stoichimetric composition with excess Cr. Formation: Textural relations suggest that Cr-enriched silicate liquid surrounding the chromites reacted with olivine to form knorringite-uvarovite garnet and with opx to form Cr-Eskola pyroxene. The chromites have been affected by a late, low-P carbon-reduction reaction (like that experienced by all ureilites [9]): they show various degrees of Fe-loss (fe# ~0.50-0.04), and at high degrees of Fe-loss (fe# <0.10) also Cr-loss (Cr/[Cr+Al] ~0.7-0.3). It has been suggested [1,2] that the carbides, sulfides and eskolaite formed from Cr liberated in this reaction. However, the 15 chromite grains around which we have observed garnet and/or Cr-Eskola pyroxene have fe# ~0.35-0.15 and show no loss of Cr. Therefore, the silicate liquid surrounding them must have been enriched in Cr before reduction. This liquid is interpreted [3] as a residuum of the liquid from which the oikocrysts grew. The latter was in reaction relationship with chromite [3], and could have become highly enriched in Cr in trapped pockets around grains that were being resorbed. P,T Conditions: It seems likely that this Cr-rich garnet and pyroxene formed at high pressure and temperature. The Cr/(Cr+Al) ratio of garnet increases with both pressure and temperature [10], and knorringite-rich garnets are known only from diamond-bearing kimberlites [11-13]. The geothermobarometer of [10] for coexisting Mg-garnet and spinel suggests T >2000°C and P ~4-4.5 GPa for the assemblage in LEW 88774. Likewise, the vacancy-containing Ca-Eskola pyroxene molecule is stabilized by pressures >3 GPa [14,15], and at lower pressures breaks down to CaTs + Qtz [7,14]. A similar pressure-sensitivity would be expected for the vacancy-containing Cr-Eskola pyroxene. If these minerals formed at P >3 GPa, then they must be shock-produced. References: [1] Warren P.H. & Kallemeyn G.W. (1994) LPSC 25, 1465. [2] Prinz et al. (1994) LPSC 25, 1107. [3] Goodrich C.A., this volume. [4] Goodrich C.A. (1999) MAPS 34, A44. [5] Goodrich C.A. & Keller L.P. (2000) MAPS 35, A60. [6] Gasparik T. & Lindsley D.H. (1980) In Pyroxenes (ed. C.T. Prewitt) RIM 7, 309. [7] Katayama I. et al. (2000) Am. Min. 85, 1368. [8] Harlow G.E. & Olds P.E. (1987) Am. Min. 72, 126. [9] Mittlefehldt D.W. et al. (1998) In Planetary Materials (ed. J. Papike) RIM 36. [10] Girnis A.V. and Brey G.P. (1999) Eur. J. Mineral. 11, 619. [11] Nixon P.H. & Hornung G. (1968) Am. Min. 53, 1833. [12] Mukherjee A. et al. (1997) J. Geol. Soc. India 50, 441. [13] Daniels L.R.M. & Gurney J.J. (1989) In Kimberlites and Related Rocks 2 (eds. J. Ross et al.), 1012. [14] Smyth J.R. (1980) Am. Min. 65, 1185. [15] Mao H.K. (1971) Carnegie Inst. Yrbk. 69, 60..
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