20 THE ROCK-FORMING MINERALS 



a. Orthorhombic Pyroxenes are silicates of magnesia and iron 

 (Mg, Fe)0, Si0 2 . 



i. 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. Monoclinic Pyroxenes. 



1. Augite. — This very abundant and important mineral is a 

 silicate of lime, magnesia, iron, and alumina (Ca, Mg, Fe)0, 

 (Al, Fe) 2 3 , 4 Si0 2 . 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 chemi- 

 cal 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. = 2.9- 

 3.5 ; H = 5-6. 



2. Tremolite is a silicate of magnesia and lime (CaO, MgO), 

 Si0 2 . This mineral is pale green or white and occurs in laminae or 

 long, blade-like crystals. 



3. Actinolite resembles tremolite in composition, with the addi- 

 tion of iron (CaO, MgO, FeO) Si0 2 . Colour, 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 fibres are 

 often like flexible threads and may be woven into cloth. 



V. The Olivine Group 



Olivine is the only mineral of this group of sufficient impor- 

 tance to require mention ; it is a silicate of magnesia and iron 

 2 (MgO, FeO)Si0 2 , though the percentage of iron varies greatly. 



