DRUSE PYROXENES IN D´ORBIGNY: A MÖSSBAUER SPECTROSCOPY STUDY

R.B. SCORZELLI, I. SOUZA ACEVEDO, S. J. STEWART, M. E. VARELA, G. CURAT,

USA

Congreso; 67th Meteoritical Society Meeting; 2004

D’Orbigny is the sixth and by far the largest angrite known. It is peculiar with respect to three features: the abundance of hollow shells, the abundant presence of glasses and the presence of abundant open druses with perfectly crystallized augites of prismatic habit [2-3]. Here we report on the distribution of Fe2+ among the non-equivalent sites in the druse pyroxenes of the D’Orbigny meteorite as determined by Mössbauer spectroscopy. Sample: Our specimens consisted of 200 mg of euhedral augites, ranging in size from 70 to 400 mm . Electron microprobe analyses showed that augites are chemically homogeneous with a slight compositional change at the outermost ~ 5 µm (rim): SiO2 - 46.4 (45) wt%, TiO2 - 1.53 (1.91) wt%, Al2O3 - 8.1 (9.4) wt%, FeO - 12.7 (13) wt%, MgO - 9.1 (8.3) wt%, CaO - 22.7 (22.9) wt%. Results and Discussion: The study of cation distributions over non-equivalent lattice sites in minerals can provide valuable information on its pT history. In pyroxene crystals that have been cooled slowly to temperatures lower that 500 ºC, the Fe2+ ions populate primarily the M2 position whereas the Mg2+ ions occupy predominantly the M1 position. In crystals that have been rapidly cooled, a more disordered Mg, Fe distribution over the M1 sites is observed. The Mössbauer spectra of D’Orbigny druse Augites obtained at room temperature consist of an intense inner doublet due to Fe2+ at the M2 sites and a less intense outer doublet due to Fe2+ at the M1 sites. Most Fe is present as Fe2+ but traces of Fe3+ have also been observed