THE PHASE-BOUNDARY BETWEEN ALPHA-MG2SIO4 AND BETA-MG2SIO4 DETERMINED BY IN-SITU X-RAY-OBSERVATION

The stability of Mg2SiO4, a major constituent in the Earth's mantle, h as been investigated experimentally by in situ observation with synchr otron radiation. A cubic-type high-pressure apparatus equipped with si ntered diamond anvils has been used over pressures of 11 to 15 gigapas cals and temperatures of 800 degrees to 1600 degrees C. The phase stab ility of alpha-Mg2SiO4 and beta-Mg2SiO4 was determined by taking accou nt of the kinetic behavior of transition. The phase boundary between a lpha-Mg2SiO4 and beta-Mg2SiO4 is approximated by the linear expression P = (9.3 +/- 0.1) + (0.0036 + 0.0002)T, where P is pressure in gigapa scals and T is temperature in degrees Celsius.