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]

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