Reaction reversals to within 0.2 kbar locate the quartz ⇆ coesite equilibrium at 24.6, 24.9, 25.5, 26.2, 27.0, 27.9, 28.8, and 29.7 kbar at 350, 400, 500, 600, 700, 800, 900, and 1000°C, respectively, using well‐calibrated (frictionless), 2.54‐cm diameter NaCl furnace assemblies in piston‐cylinder apparatus. The equilibrium is unaffected by the presence of H2O or jadeite, contrary to previous proposals. The positive dP/dT of the reaction at T < 500°C is inconsistent with existing thermochemical data for coesite. Numerous tight reversals provide a sound basis for use of the quartz ⇆ coesite equilibrium as a calibration of pressure in high‐pressure apparatus.

No abstract

No abstract

No abstract

Experiments in a piston-cylinder apparatus have been carried out at 700-1050°C, 10-16 kbar to determine the stability of ferrosilite (FeSiOa) relative to fayalite + quartz. Reaction reversals within 0.1-kbar intervals locate the