THE ALPHA-BETA QUARTZ TRANSITION AT HIGH-TEMPERATURES AND PRESSURES IN A DIAMOND-ANVIL CELL BY LASER INTERFEROMETRY

The alpha-beta quartz transition was observed using laser interferomet ry at simultaneous high temperatures and high pressures up to 850-degr ees-C and 1100 MPa in situ in a diamond-anvil cell. The alpha to beta transition manifested itself as an abrupt collective motion in the int erference fringes produced by the laser light reflected from the top a nd bottom surfaces of a doubly polished quartz platelet. The beta to a transition was observed also as an abrupt shift of the interference f ringes, but in the opposite direction. By using the method described i n Shen et al. (1992a) and employing the equation of state of H2O formu lated by Haar et al. (1984), we were able to calculate transition pres sures accurately. The transition temperatures were fitted to a quadrat ic function of pressure, and the following result is obtained: T (degr ees-C) = 574.3 + 0.2559P (MPa) - 6.406 x 10(-6)P2.