08 January 1993
Vol 259, Issue 5092

About The Cover

Six crystals of silicate spinel, a high-pressure mineral abundant in the transition zone of Earth's mantle, confined in a diamond-anvil cell (mount diameter, 0.45 millimeter). High-presure x-ray studies of these crystals reveal that the Mg2SiO4 end member is 13% more compressible than the Fe2SiO4 end member even though its molar volume is 6% less. Such anomalous compressibility suggests that the behavior of Mg2+ and Fe2+ at mantle pressures is divergent. See page 206. [Photo: R. M. Hazen]