1. I. Chou, 955 National Center, U.S. Geological Survey, Reston, VA 20192, USA. J. G. Blank, A. F. Goncharov, H. Mao, R. J. Hemley, Geophysical Laboratory and Center for High Pressure Research, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015, USA.