Volume behavior of hydrous minerals at high pressure and temperature: II. Compressibilities of lawsonite, zoisite, clinozoisite, and epidote

The pressure dependence of the lattice parameters of natural zoisite [Ca₂Al₃Si₃O₁₂(OH)], clinozoisite [Ca₂Al₃Si₃O₁₂(OH)], and epidote [Ca₂Al₂FeSi₃O₁₂(OH)] as well as synthetic lawsonite [CaAl₂Si₂O₇(OH)₂·H ₂O] have been measured at ambient temperatures by energy-dispersive X-ray diffraction in a diamond-anvil cell. The experimental results for each phase may be summarized concisely in terms of the ambient-temperature isothermal bulk modulus K₂₉₈ (using the Murnaghan equation of state and assuming K′ = 4): Lawsonite: K₂₉₈ = 191 ± 5 GPa; zoisite: K₂₉₈ = 279 ± 9 GPa; clinozoisite: A₂₉₈ = 154 ± 6 GPa; epidote: K₂₉₈ = 162 ± 4 GPa. These new measurements, together with the new thermal expansion data in the companion paper (Pawley et al. 1996), were used to calculate some phase equilibria for lawsonite dehydration to high pressures for comparison with experimental brackets. Important discrepancies between calculated and experimentally determined reactions become evident above 3 GPa.