Quartz-coesite transition revisited: reversed experimental determination at 500-1200°C and retrieved thermochemical properties

The quartz-coesite transition was determined by reversed experiments in a piston-cylinder apparatus in the range 500-1200°C. The equilibrium transition boundary can be expressed as P (kbar) = 21.945 (±0.1855) + 0.006901 (± 0.0003)T (K). It is subparallel to, but ~1.5 kbar higher than, the transition boundary determined by Bohlen and Boettcher (1982). The entropy [39.56 ± 0.2 J/(mol.K)] and enthalpy of formation (-907.25 ± 0.007 kJ/mol) were also retrieved from elements of coesite at 1 bar, 298 K, from phase-equilibrium data and selected thermochemical data from the literature. From the characteristics of the hysteresis loop it is concluded that the practice of maintaining a constant nominal pressure by repeated pressure adjustment during an experiment leads to variation of pressure on the sample. -from Authors