SILICA. 



257 



CLASS VI. 



EARTHY MINERALS. 



Composed chiefly or in part of one or more of the earths proper ; they frequently contain 

 some alkali or alkaline earth, acid or metallic oxide. 



ORDER I. SILICA. 



I. Minerals in which the silica is nearly pure. 



1. Quartz, and its varieties. 



II. Minerals in which the silica plays the part of acid. 



Cluartz. Cleavdand. — Cluartz, Calcedony, Jasper and Hornstone. PMUips. — Q.uarz. Haay and Beudant. 

 Rhombohedral duartz. Jameson, — Rhomboedrischer duarz. Mohs. 



Fig. 129. 



Fig. 130. 



Description. Colour white, grey, less 

 frequently black, blue, green, yellow, red 

 and brown. It occurs regularly crystallized ; 

 also massive, disseminated in plates, stalac- 

 titic, reniform, botryoidal, globular, specu- 

 lar, vesicular and cellular. Primary form 

 an obtuse rhombohedron. Fig. 129. P' on 

 P' 94° 24' {Haiiy); 94° 15' {Phillips). 

 The common form is a six-sided prism ter- 

 minated by six-sided pyramids, prismk of 

 Haiiy. Fig. 130. P on r, P' on r', r on «', or r- on « 141° 40' ; r on r' 120°. Fracture 

 MiN. — Part II. 33 



! i 



