The syntheses of randomly stacked one-layer monoclinic (IMd), one-layer monoclinic (1M), and twolayer monoclinic (2M) muscovite, have been accomplished. The upper stability limit of muscovite is represented by a curve passing through the points 625°C, 5000 p.s.i. water pressure; 665°C, 15,000 p.s.i.; and 715°C, 30,000 p.s.i. Immediately above this curve sanidine + corundum + vapour are the stable phases. The stability ranges of the muscovite polymorphs named could not be fixed accurately because of the sluggish nature of the transformations; however, the transformation of 1Md → 1M → 2M was effected. It is believed that this sequence obtains in the progressive metamorphism of a sediment. The first transformation is probably dependent on factors affecting reaction rate. The second transformation may be related to a univariant curve of equilibrium (i.e., an isograd). The growth of a three-layer trigonal muscovite (3T) was suspected in only one run with 2M muscovite, and its synthesis is, therefore, not substantiated. Study of natural moscovites and muscovite-like materials indicates that the hitherto unrecognized 1Md and 1M muscovites are common. Their relation to materials called illite, high-silica sericite, hydromuscovite as well as other dioctahedral minerals, is described. The complete destruction of the muscovite structure by grinding is demonstrated by selected area electron diffraction studies. The alleged resynthesis of muscovite by further grinding is shown to be suspect. A comparison of the maximum upper stability curve of muscovite with the minimum melting curve of the “granite” system suggests that muscovite may form in granitic magmas above approximately 1500 atmospheres water pressure, and in the solid state below that pressure in granitic rocks when the water pressure equals the total pressure. These relations may account for the apparent two generations of muscovite in some granites. A limitation on the amount of muscovite in igneous granites is believed to be imposed by the character of the liquidus surfaces. The upper stability curve of muscovite marks the maximum possible conditions of the second sillimanite or orthoclase isograd in metamorphic rocks. The nature of the processes of weathering, abrasion, and diagenesis of muscovite in sediments is outlined.