There are many compostional variations of chlorite where substitution of Mg, Al, Fe²⁺ and Fe³⁺ in the octahedral sheets and substitution of Al³⁺ and Si⁴⁺ in the tetrahedral sheets can occur. Common variations include clinochlore, pennine, prochlorite, and chamosite.

perfect cleavage on {001}

colorless-pale green;
Pleochroic

May be positive or negative based on composition and variety

length fast (postive)
length slow (negative)

Refractive Indices

alpha=1.55-1.67
beta =1.55-1.69
gamma =1.55-1.69

Chlorite is an important constituent of many contact and regional metamorphic rocks of low to medium grade, usually with temperatures of to 400 degrees C and pressures up to a few Kb. It also found in amygdules, fractures in altered volcanic rocks, hydrothermal vein deposits and soils.

Chlorite is often found with biotite, garnet, staurolite, andalusite, muscovite, chloritoid, and cordierite in pelitic rocks. In mafic rocks, it occurs with talc, serpentine, actinolite, hornblende, epidote, and garnet. It can also be found with feldspars, quartz, calcite, dolomite, olivine, plagioclase, rutile, ilmenite, titanite, magnetite, chromite, sulfides, zircon, and zeolites.

Chlorite forms by the alteration of mafic minerals such as pyroxenes, amphiboles, biotite, staurolite, cordierite, garnet, and chloritoid. Chlorite can also occur as a result of hydrothermal alteration of any rock type, where recrystallization of clay minerals or alteration of mafic minerals produce chlorite.

Nesse, William D., 1986, Introduction to Optical Mineralogy: New York, Oxford University Press, 325 p.

UCLA Mineral Library