Occurrence and mineral chemistry of chromite and xieite in the Suizhou L6 chondrite

Abstract

The occurrence and mineral chemistry of chromite and its high-pressure phase xieite in the Suizhou meteorite were studied by different modern micromineralogical techniques. Three types of occurrences for chromite were observed in the Suizhou L6 chondrite: coarse chromite grains, cluster of chromite fragments in molten plagioclase, and exsolution lamellar chromite in olivine. All the chromite grains of the first two types are remarkably similar in chemical compositions, but the composition of exsolution chromite is inhomogeneous and variable in Al2O3 content. Xieite is a post-spinel CT-phase of chromite firstly found in the Suizhou meteorite. Three types of occurrences of xieite have also been revealed in this meteorite: coarse xieite grains, complex three-zone-grains consisting of the inner xieite, the intermediate lamellae-like CF-phase and the outer chromite phase, and two-phase-grains consisting of xieite and one of the high-pressure silicate minerals lingunite, ringwoodite or majorite. The curved boundary between xieite and the silicate half in two-phase grains is indicative of some partial or even full melting of the silicate phase. EPMA and EDS results show that the compositions of xieite inside/contacting the shock veins are also identical to that of chromite outside the veins. However, some element diffusion appeared in between the xieite and the silicate half in the two-phase grains, namely, some of Al3+ from lingunite, or Fe2+ from ringwoodite migrated to xieite, and some of Cr3+ migrated from xieite to lingunite or ringwoodite. Majorite in two-phase grains shows remarkable decrease of SiO2 and MgO, and notable increase of Al2O3 and CaO, indicating that its host mineral pyroxene was fully molten and mixed with the surrounding silicate melt of the vein matrix. The complexity in mineral chemistry of these two-phase grains in shock veins can be explained by the much higher shock peak temperature in shock veins (1800–2000°C) than in unmelted main body (∼1000°C), and by the much lower density of the silicate minerals (2.6–3.3 g/cm3) than that of chromite (4.43 g/cm3). Being a refractory and a relatively high-impedance material, chromite is chemically more stable and easier to reflect shock wave into the silicate half causing the partial or even full melting of silicate phases, upon which some diffusion of elements between the two phases themselves, or even mixing of molten pyroxene and the surrounding silicate melt.