1. 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 usually of a green colour. They crystallize in the monoclinic system, 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.

2. Talc

Talc is a hydrated silicate of magnesia, 3MgO, 4 Si02, H20; the water varies in amount to as much as 7 per cent. Sp. gr. = 2.56-2.8; H=1. It is of a white or pale green colour, with a pearly lustre and a greasy, soapy feeling to the touch. Talc is rarely found crystallized; 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.

3. Steatite, Or Soapstone

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

4. Serpentine

Serpentine is a hydrated silicate of magnesia and iron: 3(Mg, Fe)0, 2 Si02, 2 H20. It does not crystallize, but is rather common in pseudomorphs. Sp. gr. = 2.5-2.65; 11 = 2.5-4. Its proper colour 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

Kaolinite is the hydrated silicate of alumina, A1203, 2 Si02, 2 H20. 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

Glauconite is a hydrated silicate of iron and potassium, with small quantities of alumina, lime, magnesia, and soda. It is of a green colour, soft and friable.