(Mg,Fe,Al)
Monoclinic -- prismatic
Environment
Commonly a secondary, and often pervasive, mineral like serpentine, but usually affecting localized spots of primary iron, magnesium, aluminum silicates in the rock rather than the entire mass, as is the behavior of serpentines.
Crystal description
Two types form distinct crystals: penninite, though monoclinic, is pseudorhombohedral and forms thick (often triangular) crystals; clinochlore usually grows in broader thinner crystals and is hexagonal in outline. Also fine-grained, in masses, blades, and fibers, or in little rounded knobs and green surfaces defacing nicer crystals (adularia).
Physical properties
Green, black, also brown, redpurple, yellow, and even white.
Luster
glassy to pearly;
hardness
2-2Ɖ;
specific gravity
2.6-3.0;
cleavage
perfect micaceous. Foliaceous and flexible, but not elastic like mica; transparent to opaque.
Composition
Chlorite actually is a group name, but it is not practical for the amateur to distinguish between varieties. At best, one can usually assign names only on the basis of appearance, when other tests cannot be given. The chlorites are basic iron, magnesium, aluminum silicates with about 36.1% MgO, 18.4% Al
2
O
3
, 32.5% SiO
2
, and 13.0% H
2
O. The reddish varieties contain chromium in place of the aluminum, and the reddish brown varieties contain manganese.
Tests
Whitens, but fuses only with great difficulty; gives water in the closed tube.
Distinguishing characteristics
Very flaky. Usually the larger chlorites can easily be distinguished from the micas by their color and the lack of elasticity in the cleavage flakes, and from talc by its greater hardness.
Occurrence
Most commonly as a spot of green alteration of an earlier pyroxene or amphibole in rock; also in chloriterich to almost pure chlorite schists, which may enclose pyrite and/or magnetite crystals. Occasionally crystallized in triangular wedges (penninite) in cavities, as in the alpine crevices or in rocks altered by hot-water solutions. Also in good crystals in Lancaster Co., Pennsylvania. Some of the best crystal (to 2 in.; 5 cm) clinochlore plates were found with magnetite and chondrodite on the surfaces of a serpentinized rock at the old Tilly Foster Mine, Brewster, New York. Also found with talc in Chester Co., Pennsylvania, and in fissures in California serpentine formations with melanite garnet and brown-black, admantine ƈ-in. (2-3 mm) crystals of perovskite (CaTiO
3
) near the benitoite locality. The red-purple chromiferous variety (kaemmererite) is well developed in small crystals at Texas, Lancaster Co., Pennsylvania, in some of the chromite mines in California, and in showy Ɖ-in. (1 cm) crystals in Erzincan, East Anatolia, Turkey.