General comments on the identification of chlorites in thin sections

Abstract

Although a ‘chlorite’ is usually simple to recognize in thin section because of its characteristic colour, form, and often very low or anomalous interference colours, the identification of the various species is more difficult and requires both chemical and X-ray work for accurate determination. Roy and others have demonstrated that chlorites are polymorphous and occur in two structural groups, the normal 14 Å chlorites and the 7Å septechlorites (Nelson and Roy, 1958). Many chlorite varieties may be found in both structural states, low-temperature end-members and compositionally extreme end-members favouring the septechlorite structure, especially antigorite, amesite, chamosite, greenalite, and cronstedtite. Phillips (1964) subsequently proposed that six variables, Y, R³⁺, R²⁺ , Σ, H, plus the appropriate structural designation, must be fixed to define completely a variety of the chlorite group and from this he developed a numerical system of classification. Unfortunately these specific chemical and structural properties cannot be directly recognized by normal optical methods.