Olivine is the only mineral of this group of sufficient importance to require mention; it is a silicate of magnesia and iron, 2(Mg, Fe)0 Si02, though the percentage of iron varies greatly. Sp. gr. =3.2-3.5; 11 = 6.5-7. Olivine crystallizes in the ortho-rhombic system, and occurs in prisms, flat tables, or irregular grains. The colour varies from olive-green to yellow, or it may be colourless, and usually the irregular grains look like fragments of bottle glass.

VI. The Epidote Group

Epidote is a silicate of alumina, with iron and lime, the different varieties being named according to the preponderance of one or other base. Iron-epidote (Pistazite) forms monoclinic crystals, with a specific gravity of 3.2-3.5, and a hardness of 6-7. Lime-epidote (Zoisite), which has little or no iron, is orthorhombic.

VII. The Garnet Group

The Garnets are highly complex silicates of alumina, iron, lime, magnesia, chromium, and manganese, though in most cases only two or three of the bases are present in any considerable quantity, and the different varieties have received names in accordance with the predominating bases. They usually crystallize as dodecahedrons, or twelve-sided figures. Sp. gr. =3.4-4.3; 11=6.5-7.5. Clear and brilliantly coloured garnets are considerably used in jewellery.

The commonest variety (Almandine) is a silicate of alumina and iron, and is usually red.