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

Safonov, O.G.; Bindi, L.; Vinograd, V.L.

Авторы: Safonov O.G., Bindi L., Vinograd V.L.
Журнал: Mineralogical Magazine
Том: 75
Номер: 4
Год издания: 2011
Издательство: Mineralogical Society
Местоположение издательства: United Kingdom
Первая страница: 2467
Последняя страница: 2484
DOI: 10.1180/minmag.2011.075.4.2467
Аннотация: The available experimental data on chemical composition and crystal structure of K-bearing clinopyroxenes are compiled with the results of atomistic simulations and thermodynamic calculations of mineral equilibria. It is shown that the limited solubility of K2O in clinopyroxenes from crustal rocks cannot be ascribed to the strong non-ideality of mixing between diopside (CaMgSi2O6) and K-jadeite (KAlSi2O6) components. The more likely reason is the instability of the potassic end-member with respect to other K-bearing phases. As the volume effects of typical K-jadeite-forming reactions are negative, the incorporation of potassium in the clinopyroxene structure becomes less difficult at higher pressures. Atomistic simulations predict that the thermodynamic mixing properties of the diopside - K-jadeite solid solution at high temperatures approach those of a regular mixture with a relatively small positive excess enthalpy. The standard enthalpy of formation (ΔfH0 = -2932.7 kJ/mol), the standard volume (V0 = 6.479 J/mol/bar) and the isothermal bulk modulus (K0 = 145 GPa-1) of K-jadeite were calculated from first principles, while the standard entropy (S0 = 141.24 J/mol/K) and the thermal expansion coefficient (α = 3.3 K-1) of the K-jadeite end-member were estimated using quasi-harmonic lattice dynamics 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 potassium at the upper-mantle conditions.
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Potassium-bearing clinopyroxene: a review of experimental, crystal chemical and thermodynamic data with petrological applicationsстатья

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