Crystal-structure determination of chromites from NWA6077 and NWA725 achondrites

Chromite is a minor but regular constituent of ordinary chondrites and achondrites, its chemistry being studied since the 60es. Structural studies via X-ray single crystal diffraction have been performed only by Lenaz et al. (2015) on some chromites from the Kernouvè H6 chondrite, from ~ 470 Ma old fossil micrometeorites and from an acapulcoite. The here studied chromites are from the NWA6077 and the NWA725 achondrites. According to Garvie (2012) the NWA 6077 is classified as an achondrite meteorite (ungrouped, brachinite-like) with an olivine-rich assemblage with protogranular (possibly cumulate) texture exhibiting triple-junction grain boundaries. Additional minerals include orthopyroxene, clinopyroxene, altered kamacite, chromite, chlorapatite, Ni-bearing troilite and/or pyrrhotite. NWA725 meteorite has been classified as acapulcoite with olivine, orthopyroxene and clinopyroxene by Grossman & Zipfel (2001), but Greenwood et al. (2012) on the basis of its oxygen isotope composition ascribed it to winonaites. Crystal chemistry on six different grains of chromite (3 for each meteorite) have been studied by single crystal X-ray diffraction. The obtained results show that the cell edges are very similar within each meteorite and also among the two meteorites [8.3331-8.3352 for NWA6077 and 8.3362-8.3433 for NWA725]. On the contrary the oxygen positional parameters of the two meteorites are rather different being about 0.2629 for NWA6077 and 0.2622 for NWA725. The H6 chondritic chromites show cell edges ranging from 8.3480 to 8.3501 Å and oxygen positional parameter comprised between 0.2627 and 0.2629 while it is between 0.2625 and 0.2627 in L chondritic chromites (similar to the acapulcoite) (Lenaz et al., 2015). NWA6077 is a brachinite-like meteorite and brachinites are the result of metamorphosed chondrite or partial melting residue. Their chromites differ from those of the H6 chondrite for having the same oxygen positional parameter but a lower cell edge due to a major amount of Al2O3. Their structural parameters recall those of two spinels from the anorthosites-chromitites layering of the Bushveld complex. As regards the NWA725 chromites, their structures show similarities with the previously analysed acapulcoite. Their structural parameters recall those of komatiitic chromites.