K. Bose et J. Ganguly, QUARTZ-COESITE TRANSITION REVISITED - REVERSED EXPERIMENTAL-DETERMINATION AT 500-1200-DEGREES-C AND RETRIEVED THERMOCHEMICAL PROPERTIES, The American mineralogist, 80(3-4), 1995, pp. 231-238
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.