Hydromicas

micalike minerals of the group of layered aluminum silicates, containing additional water and possibly oxonium (H₃O⁺). Hydromicas are usually intermediate products of the phase transition of various micas to kaolin, montmorillonite, vermiculite, and chlorites. The most widespread hydromicas are illite, (K,H₂O)Al₂[(Al,Si)Si₃O₁₀]-(OH)₂.nH₂O; rectorite, (H₂O,K)Al₂[Al_xSi_4-xO₁₀](OH)₂3H₂O; glauconite, (K,H₂O)(Fe,Mg,Al)₂[(Al,Si)Si₃O₁₀]-(OH)₂; and hydrobiotite, (K,H₂O)(Mg,Fe³⁺)₃[AlSi₃O₁₀]-(OH)₂nH₂O.

The transformation of mica into hydromica is accompanied by the consumption of alkalies and the replacement in the interlayer space by molecular water, probably oxonium, and also the introduction of water associated with cations into special additional layers. Hydromicas increase greatly in volume when heated, as a result of the widening of the intervals between packets by water that is boiling and being driven off. The formation of hydromicas is predominantly associated with the weathering and change of mica minerals in granites, pegmatites, and other rocks. They are also formed as products of the decomposition of aluminum silicates of marine deposits during diagenesis. Less frequently they are formed in low-temperature hydrothermal associations by changes in rock containing ore veins.