Chlorites are 2:1:1 phyllosilicates minerals. They have a structure that resembles that of micas or vermiculites, but with an additional layer of octahedrally-coordinated Mg, Al or Fe in the interlayer space. The 2:1 layers are formed by the stacking of 2 sheets of tetrahedrally-coordinated silica, and one sheet of octahedraly coordinated Mg, Fe, or Al between the two tetrahedral sheets, as in micas or vermiculites. Substitutions of Al for Si in the tetrahedral sheets and Fe2+ and/or Mg2+ for Al3+ in the octahedral sheets generates a net negative layer charge which is balanced by the octahedral sheet in the interlayer space. Consequently, the interlayer trioctahedral sheet usually has a net positive charge. Trioctahedral magnesian chlorites exhibit the general simplified formula:
[(Mg,FeII)(OH)2].[Si4-xAlx].[Mg3-y(Al,Fe)y]O10(OH)2

where [(Mg,FeII)(OH)2] represents the charge balancing octahedral sheet in the interlayer space, x is the fraction of Al substitution for Si in the tetrahedral sheet, and y is the fraction of Al and/or Fe substitution for Mg in the octahedral sheet. Chlorites have different Space groups, primarily: C2/m, Cm and C1. They exhibit various structural arrangements (polytypes), which differ by the orientation of the interlayer sheet relative to the 2:1 layer structure. The crystals commonly have a platy habit. They may have any combination of dioctahedral or trioctahedral occupancy between the interlayer sheet and the layer sheet of the structure.

Distribution

Chlorite minerals are often fine-grained aggregates, green colored as their name implies: Greek chloros = green. They are commonly found in low- to medium-grade metamorphic rocks and in altered igneous rocks. They occur as products of hydrothermal alteration of minerals such as biotites, amphiboles, pyroxenes, olivines, and other ferromagnesian minerals. Chlorites of detrital and diagenetic origins are also found in significant amounts in sedimentary rocks. Chlorites are a characteristic mineral of the green schist facies. In basalts, chlorites are common in amygdalus cavities and fracture surfaces.

Dixon, J.B. 1989. Kaolin and Serpentine Group Minerals. 467-526. In J.B. Dixon and S.B. Weed eds. Minerals in soil Environments. 2nd Ed. SSSA Book Series 1. Soil Sci. Soc. Amer. Madison, Wisconsin, USA.

Brindley, G.W. and G. Brown. 1980. Crystal Structure of Clay Minerals and their X-ray identification. Mineralogical Society, London, UK.