Frey, Friedrich; Weidner, Eveline; Pedersen, Bjoern; Boysen, Hans; Burghammer, Manfred; Hoelzel, Markus
Pyroxene from martian meteorite NWA856: Structural investigations by X-ray and neutron diffraction.
In: Zeitschrift fur Kristallographie, Vol. 225, Nr. 7: S. 287-297
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Despite high mosaicity and complex micro structures in pyroxenes fromMars average occupancies and average atomic positions of pigeonite weredetermined from X-ray single crystal and neutron and X-ray powderdiffraction data. The chemical composition of selected single crystalsvaried between En(39)Fs(55)Wo(6) and En(60)Fs(34)Wo(6). The averagecomposition of the whole sample was determined from powder diffractionto be about En(43)Fs(51)Wo(6). The ordering of the cations on the M1 andM2 positions, expressed by a partitioning or distribution coefficientK(D), in the pigeonite structure was determined and a closuretemperature of about 540 degrees C was estimated for crystals stemmingfrom the core of our specimen General trends of the crystal chemistrywith K(D) are compared with literature data. From in-situ hightemperature neutron powder data, however, it is deduced that a simpleintraphase-ordering process is not adequate, since diffusion processesbetween intimately intergrown augite and pigeonite phases attemperatures above 600 degrees C also take place. This makes the usualinterpretation of structural data towards the prior history of themartian sample material doubtful.