Hydromuscovite

Rieder et al. (1988) considered this name to be synonymous with illite. They use the name illite vaguely, as a compositional space for any interlayer-deficient dioctahedral mica. However, they also suggested that all hydromicas (=interlayer-deficient micas) could be considered to be mixed layer structure minerals, and thus illite may be an interlayered mica and smectite.

The name hydromuscovite is typically used to refer to fine grained, often pale green, interlayer deficient muscovite (i.e. potassium deficient), but the literature is full of inappropriately named examples. Hydromuscovite probably does not exactly equal “illite" in all cases; some may be just fine grained muscovite and some may be mixed layer phyllosilicates.

Probably equivalent terms are Damourite, Bachelorite, Gilbertite, Sericite and Pinite.

The name is probably best avoided.