It's time to revise solid-state physics textbooks, according to a report in this week's issue of the Physical Review Letters. Scientists from the Brookhaven National Laboratory and the State University of New York at Stony Brook have discovered a third mechanism by which materials change phase. In addition to diffusional transformationsthe sort that turn graphite into diamondsand diffusionless transformations, the books will now have to include what have been christened pseudomartensitic transitions.

Stony Brook professor Jiuhua Chen led the team that made the discovery, using a state-of-the-art high-pressure x-ray diffraction system they developed over the past few years. The group studied how olivine, the most abundant mineral in the earth's upper mantle, transforms under pressure to become a more dense form called spinel. They found that substructures in the material could transform by a diffusionless mechanism, while the rest of the atoms underwent short-range diffusion transformations. Because such shifts in olivine change the structure of earth's subducted lithosphere, the new phase transition mechanism could help explain the origins of some deep earthquakes.