Geology

Chlorite

Under this name are grouped a number of closely allied minerals, which are hydrated silicates of alumina, magnesia, and iron. They are soft minerals, with a hardness of 1-1.5 and a specific gravity of 2.6-2.96, and are of a green color. The crystalline form is somewhat uncertain, but is now generally regarded as monoclinic, with a pseudo-hexagonal symmetry. These minerals are laminated and split readily into thin leaves, as do the micas, from which they may be distinguished by the fact that the leaves are not elastic.

The chlorites result from the decomposition of hornblende, augite, or the magnesian micas.

Talc is a hydrated silicate of magnesia, 3 Mg O, 4 SiO₂, H₂O; the water varies in amount to as much as 7%. Sp. gr. = 2.56-2.8, H = 1. It is of a white or pale green color, with a pearly luster and a greasy, soapy feeling to the touch. Talc is rarely found crystallized j the crystals have a false hexagonal symmetry, and it is doubtful whether they should be referred to the orthorhombic or monoclinic systems. Usually it occurs in flakes or foliated masses, which split into thin, non-elastic leaves. Talc results from the alteration of magnesian minerals.

Steatite, or Soapstone, has the same composition as talc, but is not foliated, and may be much harder, as much as 2.5.

Serpentine is a hydrated silicate of magnesia and iron: 3 (MgO, FeO) 2 SiO₂, 2 H₂O. It does not crystallize, except sometimes in pseudomorphs. Sp. gr.= 2.5-2.65, H = 2.5-4. Its proper color is green, but it is usually mottled with red or yellow by iron stains. Serpentine is generally formed from the decay of olivine, less commonly from augite, or hornblende.

Kaolinite is the hydrated silicate of alumina, Al₂0₂, 2 SiO₂, 2 H₂O. It is usually soft and plastic, but orthorhombic crystals of pseudo-hexagonal symmetry may be sometimes detected with the microscope. Kaolinite arises from the decomposition of the felspars and especially of orthoclase. Glauconite is a hydrated silicate of alumina and iron, with small quantities of lime, magnesia, potash, and soda. It is of a green color, soft and friable, and results largely from the decay of augite.