Natural occurrence and synthesis of two new postspinel polymorphs of chromite

Abstract

A high-pressure polymorph of chromite, the first natural sample with the calcium ferrite structure, has been discovered in the shock veins of the Suizhou meteorite. Synchrotron x-ray diffraction analyses reveal an orthorhombic CaFe 2O4-type (CF) structure. The unit-cell parameters are a ‫؍‬ 8.954(7) Å, b ‫؍‬ 2.986(2) Å, c ‫؍‬ 9.891(7) Å, V ‫؍‬ 264.5(4) Å 3 (Z ‫؍‬ 4) with space group Pnma. The new phase has a density of 5.62 g͞cm 3 , which is 9.4% denser than chromitespinel. We performed laser-heated diamond anvil cell experiments to establish that chromite-spinel transforms to CF at 12.5 GPa and then to the recently discovered CaTi 2O4-type (CT) structure above 20 GPa. With the ubiquitous presence of chromite, the CF and CT phases may be among the important index minerals for natural transition sequence and pressure and temperature conditions in mantle rocks, shock-metamorphosed terrestrial rocks, and meteorites. F orty years ago, in search of denser polymorphs of the then newly discovered silicate spinel (ringwoodite) and modified spinel (wadsleyite) that are stable at the pressure and temperature (P-T) conditions of the Earth's transition zone, Ringwood (1, 2) proposed orthorhombic CaFe 2 O 4 -type (CF) and CaTi 2 O 4 -type (CT) structures as the top candidates for ''postspinel'' transitions in the Earth's mantle (Fig. 1). Although ferromagnesian silicate spinels were later found to break down to simple oxides (3) or stishovite plus perovskite (4), several postspinel oxides convert to a single phase with the CF or CT structure (1, 2, 5-12). Neither dense postspinel polymorphs nor silicate perovskites, however, have been discovered in nature and confirmed as new minerals. Many natural high-pressure minerals presumably predominating in the Earth's mantle have been found mainly in the shock-metamorphosed meteorites, which include ringwoodite, wadsleyite, silicate ilmenite, magnesiowüs-tite, (Na,K)AlSi 3 O 8 -hollandite, and poststishovite polymorphs (13-19). Special P-T conditions developed in the impacted meteorites, for instance, a long duration of high-pressure and -temperature up to several seconds and quenching under pressure, play an important role in the formation and preservation of high-pressure phases (16). It makes the naturally shockmetamorphosed meteorites a window for discovery of new high-pressure minerals and for investigation of the constituents in the deep Earth. Preliminary examination of the Suizhou meteorite has recently revealed a shock-metamorphosed CT polymorph of chromite composition (20). In this article, we report the identification of a previously uncharacterized CF polymorph of chromite in the same meteorite. High P-T experiments demonstrate that both CF and CT are indeed quenchable polymorphs of chromite formed above 12.5 and 20 GPa, respectively.The Suizhou meteorite is a moderately shock-metamorphosed L6-chondrite, and a shock stage S5 was estimated for the meteorite according to the classification of shock metamorphism (21). The meteorite contains shock-produced melt veins ranging...

High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite

“…The knowledge that chromite bulk chemistry can be stable to Ͼ20 GPa through at least two high-pressure polymorphs (26), and the experimental demonstration that the SiO 2 solubility in spinel-structured phases increases greatly at very high pressures (27), are consistent with the hypothesis that at least part of the massive chromite ore at Luobusa has a very deep origin and its refractory nature has shielded relics of such depths from obliteration by shallow processes.…”

“…2 A and B). It also contains inclusions of the ultrahigh-pressure (UHP) phases TiO 2 II (described herein) and exsolution lamellae of coesite and diopside (9), suggesting that a precursor of such chromite could be a high-pressure polymorph of chromite (26). Given the totality of these observations, we suggest that the resolution of conflicting fO 2 indicators is that in the deep mantle (Ͼ300 km) the fO 2 is low and the elevated abundance of Fe 3ϩ is a consequence of crystallographic constraints imposed by high-pressure host phases.…”

“…Kyanite is stable only to Ϸ15 GPa, where it breaks down to corundum ϩ stishovite (30), but the reverse reaction could have accompanied decompression events, similar to stishovite inversion to coesite. Chromite is stable only to 12 GPa, but chromite stoichiometry is stable to at least 20 GPa (26). Thus, the origin of these materials can have been either from the deep upper mantle or the lower mantle.…”

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

“…Our previous studies reported two types of occurrences of xieite in the Suizhou meteorite, namely, the coarse xieite grains and the complex three-zone-grains consisting of the inner xieite, the intermediate lamellae-like CF-phase and the outer chromite phase [21][22][23]. More recently, we observed the third type of xieite occurrence in the Suizhou shock veins.…”

“…The first is xieite, the CT-phase with CaTi 2 O 4 -type structure, and the second is CF-phase with CaFe 2 O 4 -type structure [20][21][22]. Both they are of orthorhombic symmetry, but with different crystal structures.…”