The phase boundary between Alpha- and Beta-Mg2SiO4 determined by in situ X-ray observation

The stability of [Mg.sub.2][SiO.sub.4], a major constituent in the Earth's mantle, has been investig experimentally by in situ observation with synchrotron radiation. A cubic-type high-pressure apparatus equipped with sintered diamond anvils has been used over pressures of 11 to 15 gigapascals and temperatures of 800°C to 1600°C. The phase stability of α-[Mg.sub.2][SiO.sub.4] and β-[Mg.sub.2][SiO.sub.4] was determined by taking account of t transition. The phase boundary between α-[Mg.sub.2][SiO.sub.4] and β-[Mg.sub.2][SiO.sub.4 the linear expression P = (9.3 ±0.1) + (0.0036 ± 0.0002)T, where P is pressure in gigapascals and T is temperature in degrees Celsius.
The mineral [([Mg.sib.0.9],[Fe.sub.0.1]).sub.2][SiO.sub.4] olivine is a major component of the Earth's upper mantle, and its pressure-induced phase transition into modified spinel structure (β-phase) is believed to be responsible for the [...]