QUARTZ-COESITE TRANSITION REVISITED - REVERSED EXPERIMENTAL-DETERMINATION AT 500-1200-DEGREES-C AND RETRIEVED THERMOCHEMICAL PROPERTIES

We have determined the quartz-coesite transition by reversed experimen ts in a piston-cylinder apparatus in the range 500-1200 degrees C. The difference between the sample pressure and apparent pressure was cali brated by (1) studying the friction decay in the hysteresis loop defin ed by the relationship between apparent (''nominal'') pressure and pis ton position in the compression and decompression cycles, and (2) dete rmining the melting temperature of LiCl by DTA in pressure cells simil ar to those used in the reversal experiments and comparing the results with those determined in the gas apparatus. The equilibrium transitio n boundary can be expressed as P (kbar) = 21.945 (+/-0.1855) + 0.00690 1 (+/-0.0003)T(K). It is subparallel to, but similar to 1.5 kbar highe r than, the transition boundary determined by Bohlen and Boettcher (19 82). We have also retrieved the entropy [39.56 +/- 0.2 J/(mol . K)] an d enthalpy of formation (-907.25 +/- 0.007 kJ/mol) from elements of co esite at 1 bar, 298 K, from our phase-equilibrium data and selected th ermochemical data from the literature. From the characteristics of the hysteresis loop we conclude that the often-used practice of maintaini ng a constant nominal pressure by repeated pressure adjustment during an experiment leads to variation of pressure on the sample.