Experiments using laser-heated diamond anvil cells show that methane (CH4) breaks down to form diamond at pressures between 10 and 50 gigapascals and temperatures of about 2000 to 3000 kelvin. Infrared absorption and Raman spectroscopy, along with x-ray diffraction, indicate the presence of polymeric hydrocarbons in addition to the diamond, which is in agreement with theoretical predictions. Dissociation of CH4 at high pressures and temperatures can influence the energy budgets of planets containing substantial amounts of CH4, water, and ammonia, such as Uranus and Neptune.

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Compression measurements on synthetic MgAl 2 O 4 spinel, obtained by quasihydrostatic x-ray powder diffraction at room temperature, are in good agreement with ultrasonically determined values of the bulk modulus and its pressure derivative at zero pressure. Contrary to previous extrapolations of the ultrasonic data, from which an elastic ͑volume͒ instability had been predicted at ϳ25 GPa, our results show that the spinel structure is ͑meta͒stable from 0 to 65 GPa. The zero-pressure values of the bulk modulus and its pressure derivatives which best fit all the available data are K 0 ϭ196(Ϯ1) GPa, K 0 Јϭ4.7(Ϯ0.3), and K 0 K 0 ЉϷϪ6.