Potassium-bearing clinopyroxene: a review of experimental, crystal chemical and thermodynamic data with petrological applications

摘要

Available experimental data on chemical composition and crystal structure of K-bearing clinopyroxenes are compiled together with the results of atomistic simulations and thermodynamic calculations of mineral equilibria. It is shown that the limited solubility of K_2O in clinopyroxene from crustal rocks cannot be ascribed to the strong non-ideality of mixing between diopside (CaMgSi_2O_6) and K-jadeite (KAlSi_2O_6) components. The more likely reason is the instability of the potassic endmember with respect to other K-bearing phases. As the volume effects of typical K-jadeite-forming reactions are negative, the incorporation of K in the clinopyroxene structure becomes less difficult at higher pressure. Atomistic simulations predict that the thermodynamic mixing properties of diopside-K-jadeite solid-solutions at high temperature approach those of a regular mixture with a relatively small positive excess enthalpy. The standard enthalpy of formation (?_fH~0 = -2932.7 kJ/mol), the standard volume (V~0 = 6.479 J mol~(-1) bar~(-1)) and the isothermal bulk modulus (K_0 = 145 GPa) of K-jadeite were calculated from first principles, and the standard entropy (S~0 = 141.24 J mol~(-1) K~(-1)) and thermal-expansion coefficient (á = 3.3 x 10~(-5) K~(-1)) of the K-jadeite endmember were estimated using quasi-harmonic lattice-dynamic calculations based on a force-field model. The estimated thermodynamic data are used to compute compositions of K-bearing clinopyroxenes in diverse mineral assemblages within a wide P-T interval. The review substantiates the conclusion that clinopyroxene can serve as an effective container for K at upper-mantle conditions.