These two groups contain parallel series of minerals of similar chemical composition, but differing in their crystallization and physical properties. In composition they range from silicates of magnesia to those of lime and lime-alumina, while iron is present in most of them. They belong to the orthorhombic and monoclinic systems, and can be distinguished by their cleavage. The pyroxenes have a prismatic cleavage of nearly 90°, while in the amphiboles the angles are 1240 30' and 550 30'. The ortho-rhombic amphiboles are rare and unimportant as rock-forming minerals, but the pyroxenes of this form are widely distributed, though less so than the monoclinic.

A. Orthorhombic Pyroxenes Are Silicates Of Magnesia And Iron (Mg, Fe)0, Si02

1. Enstatite has less than 5 % of FeO.

2. Bronzite has 5-14 % of FeO.

3. Hypersthene has more than 14 % of FeO.

The colour becomes darker and the optical properties change with the increase in the percentage of iron.

B. Monodinic Pyroxenes

1. Augite - This very abundant and important mineral is a silicate of lime, magnesia, iron, and alumina (Ca, Mg, Fe)0, (Al, Fe)203, 4 Si02. Sp. gr. =3.3-3.5; H = 5~6. It crystallizes in oblique rhombic prisms, and in colour is green to black and opaque.

2. Diallage is a variety of augite, usually of a green colour, which is distinguished by its laminated structure, with lustrous faces.

C. Monoclinic Amphiboles

1. Hornblende, like augite, which it closely resembles in chemical composition, is among the most important of rock-forming minerals. In colour it is usually green, brown, or black, and it crystallizes in modified oblique rhombic prisms. Sp. gr. = 2.o-3.5; H. = 5-6.

2. Tremolite is a silicate of magnesia and lime (Ca, Mg)0, Si02. This mineral is pale green or white and occurs in laminae or long, blade-like crystals.

3. Actinolite resembles tremolite in composition, with the addition of iron (Ca, Mg, Fe)0, Si02. Colour, green; sp. gr.= 3-3.2; usually occurs in long and thin crystals. A fibrous variety of tremolite or actinolite, in which the fibres are often like flexible threads and may be woven into cloth, is called asbestus.