First Post-Spinel Mineral Found, Named

This news will make mineral geeks do a double-take. Several important minerals collapse under high pressures into a crystal structure matching that of the mineral spinel. Olivine, for example, turns to the spinel mineral ringwoodite. For decades researchers have looked for a "post-spinel" structure at greater pressures. Most spinel minerals just break down—for instance, ringwoodite decomposes at depths of about 660 kilometers. But chromite can be crushed intact into a post-spinel phase at the equivalent of 500 km depth. Now that very same phase has been found in nature and its name officially approved by the International Mineralogical Association.

Where do you find post-spinel chromite, a mineral that only exists 500 kilometers down? In the same places where ringwoodite comes from: pieces of broken planets—meteorites. The new mineral, with an orthorhombic crystal structure, was first reported in Geochimica et Cosmochimica Acta in 2003, but the formal publication of its name will be in the November 2008 Chinese Science Bulletin. It is named xieite, for the researcher Xiande Xie.

It's not beyond all possibility for xieite-bearing Earth rocks to surface. At the Denver GSA meeting last year, I was astonished to learn that the post-garnet mineral majorite is found in some ultrahigh-pressure rocks, signifying depths of 300 km and more.