The range of laboratory experimentation has been extended recently to pressures of 5.5 Mbar (0.55 TPa) in diamond anvil studies by J.A. Xu, H.-K. Mao, and P.M. Bell at the Geophysical Laboratory of the Carnegie Institution of Washington. This pressure range encompasses the entire earth from surface to core (central core pressures are estimated to be 3.5 Mbar), as well as pressures equivalent to the upper mantles of the giant planets. This enhanced ability to synthesize mineral phases, measure their properties, and observe their behavior will result in improved evaluation of models of earth and planetary interiors.
The high-pressure experiments to 5.5 Mbar involved implementation of new design concepts for the diamond anvil apparatus. In a previous experiment the maximum pressure achieved was approximately 2.8 Mbar, but the pressure calibration had to be done indirectly from load calculations. The shift of the fluorescent line of ruby crystals placed in the sample, which is normally employed as an internal pressure standard, could not be used in the earlier experiment because of strong interference from diamond anvil fluorescence at pressures above 2.7 Mbar. In the new experiments the overlapping diamond emission was found to disappear at pressures above 3 Mbar, and the ruby pressure calibration scale could be employed once again. The apparatus is suitable for experiments with silicates, metals, and solidified gases. These new methods will facilitate the study of mineral physics under experimental conditions that duplicate the natural conditions of the earth's interior.