Abstract

The muscovite–paragonite (Ms–Pg) series [K1–xNaxAl2(Si4–yAly)O10(OH)2] is a group of micas with end-members of Ms (x = 0, y ≈ 1) and Pg (x = 1, y ≈ 1). This mineral series is found in the Earth’s crust and upper mantle. The series shows a wide immiscibility gap between the end-members.

Density functional theory (DFT) is used to show the compression in five models of the 2M1 polytype Ms-Pg series to 6 GPa. Bulk moduli and cell-parameter moduli were obtained from a least-square fitting of pressures and volumes to a third-order Birch-Murnaghan equation of state. Bulk-modulus values of the end-members of the series agree with the range of experimental values. Bond lengths and atomic-group geometries were studied as a function of the pressure and composition of the series by determining the moduli. Compression mechanism has been determined.

The excess volumes, Vex, were higher for the Na-rich members than for the K-rich members. Vex follow a Redlich-Kister behavior. The excess free energy, Gex, was calculated isobarically in a semiempirical way: the DFT excess volume data were calculated in one experimental model (A from Roux and Hovis 1996) in a Redlich-Kister function. The Gex as a function of the composition of the Ms-Pg join of the A model show two minima with constant composition to 0.75 GPa, evolving to richer end-member compositions at greater pressures. Therefore, the solvus should increase the gap of immiscibility at high pressure.

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