Chlorite Group

A group of triclinic and monoclinic micaceous minerals with a structure consisting of T-O-T layers with two layers having their silicate tetrahedral apices pointing towards each other, separated by an interlayer that may be simple octahedrally coordinated cations or which may be a brucite-*like* layer of two sheets of closely packed OH groups with the interstices between sheets providing the octahedral cordination site; the T-O-T layers and interlayer are bonded by electrostatic and hydrogen bonding forces; as the "a" and "b" directions of the T-O-T layer may be oriented to the interlayer "a" and "b" directions in twelve different stacking sequences, resulting in twelve different polytype possibilities (not all of which have been found in Nature yet for each species).
The general formula may be stated A_5-6T₄₀Z_10-8, where A = Al, Fe²⁺, Fe³⁺, Li, Mg, Mn, or Ni, while T = Al, Fe³⁺, Si, or a combination of them, and Z = O and/or OH.