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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.

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