Geology

These two groups contain parallel series of minerals of similar chemical composition, but differing in their crystalline form and physical properties. In composition they are silicates of various protoxide bases, and range from silicates of magnesia to those of lime and lime-alumina, while silicate of iron is present in most of them. In crystalline form 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 124° 30′ and 55° 30′. The orthorhombic amphiboles are rare and unimportant as rockforming minerals, but the pyroxenes of this form are widely distributed, though less so than the monoclinic.

1. Orthorhombic Pyroxenes are silicates of magnesia and iron (Mg, Fe)O, Si02.
   1. Enstatite has less than 5% of FeO.
   2. HBronzite has 5-14% of FeO.
   3. HHypersthene has more than 14% of FeO.

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

2. Monoclinic Pyroxenes
1. Augite - this very abundant and important mineral is a silicate of lime, magnesia, iron, and alumina (Ca, Mg, Fe)O, (AI, 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 color, which is distinguished by its laminated structure, with lustrous faces.
3. Monoclinic Amphiboles
1. Hornblende, like augite, which it closely resembles in chemical composition, is among the most important of rock-forming minerals. In color it is usually green, brown, or black, and it crystallizes in modified oblique rhombic prisms. Sp. gr. = 2.9-3.5; H = 5-6.
2. Tremolite is a silicate of magnesia and lime (CaO, MgO), Si02. This mineral is pale green or white and occurs in laminse or long, blade-like crystals.
3. HActinolite resembles tremolite in composition, with the addition of iron (CaO, MgO, FeO) Si02. Color, green; sp. gr. = 3-3.2; usually occurs in long and thin crystals. Asbestus is a fibrous variety of tremolite or actinolite, in which the fibers are often like flexible threads and may be woven into cloth.