1 6 THE ROCK-FORMING MINERALS 



4. Agate is a banded mineral, composed of layers of amorphous 

 and crystalline silica, chalcedony, jasper, amethyst, rock crystal, etc. 



5. Flint and Chert are also believed to be mixtures of hydrated 

 and anhydrous silica. They occur in amorphous masses of neutral 

 or dark colours, and are opaque, or somewhat translucent in thin 

 pieces. 



B. MINERALS COMPOSED OF SILICATES 



Silica is an acid and forms a very extensive series of compounds 

 with various metallic bases. As rock-forming minerals the silicates 

 are second only to the silica minerals in importance. 



I. The Felspar Group 



The felspars are essentially silicates of alumina (A1 2 3 ) to- 

 gether with potash, soda, or lime. Three primary felspars 

 occur: orthoclase, a potash felspar (K 2 0, A1 2 3 , 6 Si0 2 ) ; albite, 

 a soda felspar (Na 2 0, A1 2 3 , 6 Si0 2 ) ; and anorthite, a lime felspar 

 (2 CaO, 2 A1 2 3 , 4 Si0 2 ). From the combination of these primary 

 minerals two series are formed : the lime-soda series, oligoclase, 

 andesine, and labradorite ; and the potash-soda series, as yet 

 imperfectly known. 



The felspars crystallize in either the monoclinic or triclinic sys- 

 tems, but the forms of the crystals are very much alike. With 

 few exceptions, these minerals are of pale colours and, except 

 when decomposing, are very hard. 



1. Monoclinic Felspars 



Orthoclase is a potash felspar (K 2 0, A1 2 3 , 6 Si0 2 = K, Al, 

 Si 3 8 ), though soda may replace part of the potash, and small 

 quantities of lime and iron are usually present. Hardness = 6, 

 sp. gr. = 2.54-2.57. Orthoclase crystallizes in oblique rhombic 

 prisms and is very generally twinned ; there are two sets of cleav- 

 age planes, which intersect at a right angle and have thus given 

 its name to the mineral. Orthoclase is usually dull and turbid, 

 which is due to the presence of various alteration products, and 



