AMPHIBOLE AND PYROXENE GROUPS 1 9 



systems, but some authorities regard them all as monoclinic. All 

 varieties have a remarkably perfect cleavage, and split into thin, 

 elastic, and flexible leaves, by which they may be readily recog- 

 nized. They are quite soft, and most of them may be scratched 

 with the finger-nail. 



1. Muscovite may be selected as the most important and 

 wide-spread of the numerous alkaline micas, it being a hydrated 

 potash-mica, with the general formula, K 2 0, 3 A1 2 3 , 6 Si0 2 , 

 2 H 2 0. It is a lustrous, silvery-white mineral, usually transparent 

 and colourless in thin leaves ; it has a specific gravity of 2.76-3.1, 

 and a hardness of 2.1-3. 



Sericite is a silvery or pale green form of muscovite, which is 

 an alteration product and often is derived from a felspar. 



2. Lepidolite is a mica in which part of the potash has been 

 replaced by lithia. 



3. Biotite is the most important and widely disseminated of the 

 numerous dark-coloured, ferromagnesian micas. This mineral is 

 black or dark green in mass, and smoky even in thin leaves ; 

 chemically it is a hydrated silicate of potash, alumina, iron, 

 and magnesia. In hardness and specific gravity it differs little 

 from muscovite. 



IV. The Amphibole and Pyroxene Groups 



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 cleav- 

 age. 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 rock- 

 forming minerals, but the pyroxenes of this form are widely dis- 

 tributed, though less so than the monoclinic. 



