Class of hydroxyl-bearing silicate minerals with a sheet-like structure. They result from aqueous alteration are dominantly serpentine and smectite in meteorites; found in the matrixes of carbonaceous chondrites. Phyllosilicates consist of repeating sequences of sheets of linked tetrahedra (T) and sheets of linked octahedra (O). The T sheet consists of linked silica tetrahedra with a hydroxyl group, (OH), at center of 6-fold tetrahedral rings.

The O sheet is formed of sheets of OH anionic groups with the spaces between OH sheets occupied by divalent cations (Mg2+, Fe2+) and trivalent cations (Al3+, Fe3+). If all three octahedral cation sites are occupied by divalent cations, the sheet is termed trioctahedral. If sites are occupied by trivalent cations, one of three sites remains empty to maintain charge balance, and it the layer is termed dioctahetral.

The T sheet is always attached to at least one O sheet. Phyllosilicates consisting of alternating TO units are termed 1:1 layer silicates; those with alternating TOT units are termed 2:1 layer silicates.

Substitution of Al3+ for Si4+ in T layer yields a net negative charge, which is balanced by positively charged interlayer cations (K+, Na+, Ca2+) in the true micas.

Important phyllosilicate minerals are given in the table below:

Some or all content above used with permission from J. H. Wittke.

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