Pyroxene from martian meteorite NWA856: Structural investigations by X-ray and neutron diffraction

Abstract

Despite high mosaicity and complex micro structures in pyroxenes from Mars average occupancies and average atomic positions of pigeonite were determined from X-ray single crystal and neutron and X-ray powder diffraction data. The chemical composition of selected single crystals varied between En₃₉Fs₅₅Wo₆ and En₆₀Fs₃₄Wo₆. The average composition of the whole sample was determined from powder diffraction to be about En₄₃Fs₅₁Wo₆. The ordering of the cations on the M1 and M2 positions, expressed by a partitioning or distribution coefficient K_D, in the pigeonite structure was determined and a closure temperature of about 540 °C was estimated for crystals stemming from the core of our specimen. General trends of the crystal chemistry with K_D are compared with literature data. From in-situ high temperature neutron powder data, however, it is deduced that a simple intra-phase-ordering process is not adequate, since diffusion processes between intimately intergrown augite and pigeonite phases at temperatures above 600 °C also take place. This makes the usual interpretation of structural data towards the prior history of the martian sample material doubtful.