The α-β quartz transition was observed using laser interferometry at simultaneous high temperatures and high pressures up to 850 °C and 1100 MPa in situ in a diamond-anvil cell. The α to β transition manifested itself as an abrupt collective motion in the interference fringes produced by the laser light reflected from the top and bottom surfaces of a doubly polished quartz platelet. The β to α transition was observed also as an abrupt shift of the interference fringes, but in the opposite direction. By using the method described in Shen et al. (1992a) and employing the equation of state of H2O formulated by Haar et al. (1984), we were able to calculate transition pressures accurately. The transition temperatures were fitted to a quadratic function of pressure, and the following result is obtained:
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