Chlorite clay octahedral sheet, composition

The hydrothermal dioctahedral chlorites have considerably less tetrahedral substitution than those formed in sediments. The former would appear to be a stable phase and the latter a metastable phase. The tetrahedral composition of the hydro-thermal specimens is similar to that for the other dioctahedral clays and represents a reasonable fit between the tetrahedral and octahedral sheets. 

If the silica sheets in a 2 1 Fe-rich clay had enough A1 substitution to adjust to the size of the octahedral sheet, this would most likely provide sufficient layer charge to cause contraction and the formation of a biotite mica, or the bonding of a positively charged brucite sheet and the formation of a chlorite. Low-temperature micas of this composition may well exist, but have not yet been recognized. It is not known if low-temperature chlorites of this composition can form. 

The chlorite clays appear to have octahedral sheets with compositions that are largely intermediate between the 2 1 and 1 1 octahedral sheets. The chloritic structure allows for a wider range of substitution than the other clays. In part this is because most data on the octahedral composition are an average of two octahedral sheets, each of which could have relatively restricted compositions. 

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