Chlorite Group

A group of mostly monoclinic (also triclinic or orthorhombic) micaceous phyllosilicate 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 coordination 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_5-6T₄Z₁₈, 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.

The most common species in the chlorite group are clinochlore and chamosite.

The Árkai index (ÁI; Árkai, 1991) is an index for the estimation of a very low metamorphic grade in rocks, based on the evolution of chlorite. It is often used to complement the Kübler index (KI), earlier called the 'illite crystallinity' for the characterisation of pelites, and can also be applied in metabasites.