Abstract: The principal end members of the micas believed to be common in sediments have been synthesized and some of their stability relations determined. The polymorphs of muscovite and paragonite, the principal dioctahedral end members, obtained were 1Md, 1M and 2M1, and those of phlogopite and annite, the principal trioctahedral end members, 1Md and 1M or 3T. The range of stability of each of the polymorphs could not be fixed accurately because of the slow rate of transformation ; however, the transformations 1Md → 1M → 2M1 were effected for muscovite and paragonite and 1M → 1 Md or 3T and 2M1 → 1M or 3T for phlogopite. The growth characteristics of these micas in the laboratory are believed to be analogous to the formation of micas in sediments.
Knowledge of the synthetic micas contributes greatly to an understanding of the natural materials called illite, hydromica, and high-silica sericite. The dioctahedral members of these materials and related minerals may be delineated accurately in the system muscovite—Al-celadonite—pyrophyllite and their iron analogues. The trioctahedral members of some of the same materials may be outlined in the system phlogopite—eastonite—talc and their iron analogues. The postulated substitution schemes in these systems are mainly MgSi ⇌ AlVIAlIV, KAl ⇌ Si, and H3O ⇌ K. In materials intermediate between these systems, such as most biotites and vermiculites, the substitution of 3Mg ⇌ 2AlVI is of major importance. The mixed-layer structures involving micas are elucidated.