General comments on the identification of chlorites in thin sections

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 A chlorites and the 7/k 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, R 3+, R z§ , Z, 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. C H L O R I T E S are Mg, Fe, A1 phyllosilicates with a layered structure similar to the micas, of tabular habit, occurring in flaky and scaly aggregates. In thin section they exhibit mottled extinction and range from colourless to green, Mn and Cr varieties displaying orange, orange-brown, pink, or lavender colours. Members of the chlorite family are monoclinic or triclinic and therefore biaxial, yet because the refractive indices of the two rays in (001) sections are so close, the optic axial angle (2V) is very small. For most purposes chlorites may, therefore, be considered to be uniaxial, With positive and negative varieties. The septechlorite, antigorite, however, has 2V 20-60 ~ They may occur in all environments and their composition is thus closely related to that of the original rocks or minerals from which they have formed. Chlorite is characteristic of low-grade metamorphosed pelitic sediments and basic igneous rocks or as an alteration product of other ferromagnesian minerals. In igneous rocks it occurs most commonly as a late-stage crystallization product especially in amygdales, whereas in sedimentary rocks it is a common detrital component, often associated with clay minerals in a mixed-layer structure. Varietal names are related to chemical composition and structure, the scheme presented by Hey (1954) recognizing the importance of the Fe/(Fe + Mg) ratio as well as the substitution of Si for A1 in tetrahedral coordination. Many attempts have been made to relate the chemical composition of chlorites to their optical properties, e.g. Winchell (1936), Hey (1954), and Albee (1962). Hey (1954) presented a two-dimensional scheme (see fig. 1) showing the relationships between refractive index, birefringence, density and composition, and separating oxidized chlorites from orthochlorites on the basis of the Fe20 3 content, which exceeds 4 ~ in the former. A third axis to Hey's diagram to indicate Fe 3+ substitution in tetrahedral and octahedral sites (cf. Nelson and Roy, 1958, p. 721) would demonstrate the relationship between the four main compositional series listed by Phillips and Griffen (1981) viz.: Amesite Antigorite series Pseudothuringite Brunsvigite series Klementite Delessite series Thuringite Strigovite series. Albee preferred to discard varietal names and to base routine petrographic identifications on optic sign and interference colours relating these to Fe/(Fe + Mg) ratios (Table I). He recognized, however, that chlorites in ultramafic rocks, and amygdales and veins may depart from his generalizations. Deer et al. (1966) further commented that pleochroism is generally exhibited more strongly by chlorites with higher iron content. A consequence of Hey's scheme is that the increase in Fe/(Fe + Mg) ratio and oxidation state is accompanied by an increase in refractive index and a general increase in absorption (pleochroism) from colourless (Mg-Al-rich varieties) to green (Fe-rich members); those displaying anomalous interference colours being those with extremely low birefringence where e -~ o. Aluminous chlorites can also be identified in terms of the scheme. Albee's scheme is useful for general purposes when dealing with marie and ultramafic assemblages and though Copyright the Mineralogical Society