Druse Pyroxenes in D’Orbigny: A Mössbauer Spectroscopy Study

Abstract

D’Orbigny meteorite, the largest angrite known, is peculiar in its abundance of hollow shells, abundant presence of glasses, and open druses with perfectly crystallized augites of prismatic habit. We report on the distribution of Fe2+ among the nonequivalent sites in the druse pyroxenes of D’Orbigny using Mössbauer spectroscopy measurements on 200 mg of euhedral augites (70–400 mm). Electron microprobe analyses showed augites chemically homogeneous with a slight compositional change at the outermost ∼5 μ (rim). Cation distribution over nonequivalent lattice sites in minerals can give information on its pT history. When pyroxene crystals cool slowly to temperatures below 500°C, the Fe2+ ions populate primarily the M2 position, whereas the Mg2+ ions occupy predominantly the M1 position. Crystals cooling rapidly have a more disordered Mg,Fe distribution over the M1 sites. Mössbauer spectra of our sample obtained at RT 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, whose relative areas are A2 = 70% and A1 = 27%, respectively. Most Fe is present as Fe2+, but traces of Fe3+ (∼3%) have also been observed and can be due to superparamagnetic particles of Fe oxide. Using Mössbauer analysis of relative areas A1 and A2, we determined the population of the Fe2+ in M1 and M2 crystallographic sites of the augite. The Fe2+ occupancies at M1 and M2 in these two nonequivalent sites are given by XFe(M1) = 2yA1 = (A1+ A2) and XFe(M2) = 2yA2 = (A1+ A2), with y = Fe= (Fe + Mg + Ca + Al). Considering the disordering reaction due to intracrystalline Mg2+ and Fe2 exchange among the M1 and M2 sites, the site populations of Fe2+ and Mg2+ can be related to the disordering coefficient r, defined by r = X1(1 − X2) = X2(1 − X1), where X1 = XFe(M1) and X2 = XFe(M2). Accounting for chemical composition data, our results yield a disordering parameter r = 0.31. Comparing this result with r values from heating experiments with orthopyroxenes under controlled conditions, we suggest that D’Orbigny augite exhibits a cation distribution corresponding to equilibrium temperatures of at least 1000°C to 1200°C. This is in good agreement with late phases equilibrium, indicating formation of kirschsteinite and ferroaugite at similar temperatures.