This paper presents the results of an experimental study of muscovite solid solutions in the system K₂O-M²⁺O-Al₂O₃-SiO₂-H₂O-(HF), with M²⁺ = Mg²⁺ or Fe²⁺ in the temperature range 300-700°C under 2 kbar P_H2O. Muscovite solid solutions can be described, in this system, as the result of two substitutions. One is the phengitic substitution (x), which preserves the pure dioctahedral character of the mica; the second is the biotitic substitution (y), which leads to trioctahedral micas and does not change the composition of the tetrahedral layer Si₃Al. The general formula of muscovite in this system is K(Al2−x−2y∕3Mx+y2+□1−y∕3)(Si3+xAl1−x)O10(OH,F)2. Both substitutions x and y are more extensive at lower temperatures. The extent of solid solution decreases drastically with increasing temperature.

For T > 600°C, the phengitic substitution (x) becomes negligible, but some biotitic substitution (y) persists. This unsymmetrical decrease of the solid solution of muscovite with increasing temperature is similar to that previously observed in phlogopite, the micas with a tetrahedral layer composition of Si₃Al being the most stable. The behaviour of muscovite solid solutions in the ferrous system is qualitatively identical to that observed in the magnesian one, but the extent of solid solution is smaller than with Mg²⁺. Fluorine neither changes the size nor the shape of the solid solution fields but increases their stability by about 50°C.

A comparison of these experimental results with data on natural muscovites is presented. Most natural primary (magmatic) granitic muscovites lie very close to the muscovite end member, in agreement with their high-temperature origin. Low-temperature muscovites (300–400°C), typically muscovites from hydrothermally altered granitic rocks, can have high x and y values. The rate of the biotitic substitution y can reach 0.6, which corresponds to an octahedral occupancy of 2.2 atoms per formula unit (based on 11 oxygens), consistent with the experimental data.