**Summary:** Only two, or at most three, variables can be considered on a two-dimensional composition diagram; six variables must be fixed to completely define a variety of the chlorite or septechlorite groups. The positions of tetrahedral coordination may be described by a single variable and positions of sixfold coordination require three independent variables for complete definition. The number of OH^{−} ions is probably very near the theoretical value of eight, however, it may vary somewhat from this value, and, in so doing, influences the ratio of trivalent and bivalent cations in octahedral coordination.

The normal 14 Å chlorites and 7 Å septechlorites have traditionally been associated together and, although they represent two distinct structural groups, are considered together for classification purposes, since they share many identical variety compositions; a sixth variable denotes structural type, as determined by X-ray powder diffraction or D.T.A.

Each chlorite or septechlorite variety can be described by a numerical notation (chlorite number) consisting of six units and, by dividing the chlorite numbers into units according to accepted nomenclature, each possible chlorite composition can be assigned a variety name, and all variety names can be assigned to a definite composition range.

*Mineralogical Magazine*; December 1964 v. 33; no. 267; p. 1114-1124; DOI: 10.1180/minmag.1964.033.267.07

© 1964, The Mineralogical Society

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