CHLORITES are hydrous silicates of aluminium, magnesium, iron, and rarely chromium, and occur as metamorphic minerals in crystalline schists. They are also found extensively as hydrothermal minerals in igneous rocks, particularly in association with metallic ores in veins or replacement bodies, or in alteration zones around ore bodies. Mother materials are usually mafic minerals or basic glass, but felsic minerals or rhyolite glass have also altered to chlorites. Furthermore, chlorites are found extensively in sedimentary rocks such as shale, mudstone, and Recent marine sediments. Weathering processes and diagenetic changes have played important roles in formation of chlorites in sediments, but it is also considered that in some cases chlorites in sediments are fragments of chlorites transported from mother rocks.
Size of chlorite crystals ranges from macroscopic flakes, as exemplified by chlorites in crystalline schists or in hydrothermal rocks, to extremely fine-grained powders such as chlorites in metamorphic rocks or sediments.
Chlorites normally have a trioctahedral structure, in which talc-like silicate layers alternate with brucite-like layers (see, e.g. Brindley, 1961). Variations are, however, possible, in which the layers are either wholly or partially dioctahedral, or in which the octahedral cations are unequally distributed between the brucite and talc-like layers. Brindley and Gillery