Synthesis and stability of micas in the system K2O-MgO-SiO2-H2O and their relations to phlogopite

A series of alumina-free micas was synthesized hydrothermally in the potassium-poor portion of the system K₂O-MgO-SiO₂-H₂O. One end member of this series has the composition KMg_2.5Si₄O₁₀](OH)₂, which, because of its octahedral occupancy, is intermediate between the dioctahedral and trioctahedral micas.From this end member a series of mica solid solutions extends towards more Mg-rich compositions. Single phase micas were obtained along the substitution line 2Mg for Si which appears to involve incorporation of part of the Mg in tetrahedral sites. It leads to a theoretical end member with a structural formula KMg₃Si_3.5Mg_0.5O₁₀](OH)₂. Solid solutions containing up to 75 mole % of this theoretical end member could be synthesized. The observed densities, water contents, and a one-dimensional Fourier synthesis are consistent with the assumed substitution.At 1 kb fluid pressure and 620° C the Si-rich end member KMg_2.5Si₄O₁₀](OH)₂ decomposes to a more Mg-rich mica, the roedderite phase K₂Mg₅Si₁₂O₃₀, liquid, and H₂O-rich vapor. With increasing Mg-content the thermal stability of the mica solid solutions increases up to 860°C at a composition of about K₂O·6.2MgO·7.4SiO₂·2H₂O, i.e. KMg_2.8Si_3.7Mg_0.3O₁₀](OH)₂. This mica disintegrates directly into forsterite + liquid + H₂O-rich vapor. The mica phase richest in Mg with a composition of about K₂O·6.5MgO·7.25SiO₂·2H₂O, i.e. KMg_2.875 Si_3.625Mg_0.375O₁₀](OH)₂, breaks down at 765° C into forsterite, a more Si-rich mica, liquid, and H₂O-rich vapor.This binary series of alumina-free micas forms a complete series of ternary solid solutions with normal phlogopite, KMg₃Si₃AlO₁₀](OH)₂. Analyses of some natural phlogopites showing Si in excess of 3.0 (up to 3.18) per formula unit can be explained through this ternary miscibility range.