1 8 THE ROCK-FORMING MINERALS 



physical properties. They have a much more restricted distribu- 

 tion than the felspars, but have, nevertheless, an important bear- 

 ing upon the classification of certain groups of rocks in which they 

 occur. 



Nepheline is a silicate of potash, soda, and alumina ((Na, ^) 2 0, 

 A1 2 3 , 2 Si0 2 ) . It crystallizes in transparent and colourless six- 

 sided prisms, of the hexagonal system. H = 5.5-6 ; sp. gr. 2.6. 

 The mineral is soluble in hydrochloric acid, gelatinous hydrated 

 silica separating out. It is an important constituent of certain 

 lavas. 



Leucite is composed as follows : K 2 0, A1 2 3 , 4 Si0 2 , with some 

 of the potash replaced by soda. It crystallizes in twenty-four- 

 sided figures (trapezohedrons), which belong to the tetragonal 

 system, but can be distinguished from the isometric only by very 

 careful measurement. 11 = 5.5-5.6; sp. gr. = 2.44-2.56. It is 

 slowly attacked by hydrochloric acid. 



Leucite cannot be called a common mineral, but its significance 

 will be better seen when we come to take up the study of rocks. 



Analcite. — This mineral is usually regarded as a decomposition 

 product, and placed among the zeolites (see below, p. 21) ; but 

 recent investigations make it very probable that in some cases, at 

 least, analcite is a mineral of primary origin. Its composition is : 

 Na 2 0, AI9O3, 4 Si0 2 , 2 H 2 0. Crystallizes in the isometric system, 

 and is colourless in transmitted light. It is soluble in mineral 

 acids, with separation of gelatinous silica. Sp. gr. = 2.15-2.28. 



III. The Mica Group 



These minerals have a complex chemical composition, and are 

 so variable that it is difficult to give formulae for them ; they are 

 silicates of alumina, together with potash, lithia, magnesia, iron, or 

 manganese. There is a difference of opinion regarding the 

 crystalline system to which the micas should be referred. When 

 crystallized, they all form six-sided prisms, but there are reasons 

 for believing that this is a false symmetry. Usually certain micas 

 are referred to the hexagonal, and others to the orthorhombic 



