SILICA. 



257 



CLASS VI. 



EARTHY MINERALS. 



Composed chiefly or in part of one or more of the earths proper ; they f?-equentli/ contain 

 some alkali or alkaline earth, acid or metallic oxide. 



ORDER I. SILICA. 



L Minerals in ivhich the silica is nearly pure. 



1. Quartz, and its varieties. 



IL Minerals in ivhich the silica plays the part of acid. 



2. Tabular Spar. 



3. Nenialite. 



4. Serpentine. 



5. Magnesitc. 



6. Cliundrodite. 



7. Boltonitc. 



8. Talc. 



y. Pyroxene. 



10. Hornblende. 



11. Arfwedsonite. 



12. Hypersthene. 



13. Schiller Spar. 



14. Anthophyllite. 



15. Hydrous Anthophyllite. 



QUARTZ. 



Quartz, Clcavdand. — (iuartz, Calccdony, .Tasyicr and Hornstonc. Phillips. — Ciuarz. IIu in/ und Bcu<l-i!it. — 

 Rliombohcdral (iuarlz. Jamenni. — Rhombocdrischor (iuarz. Mukf. 



Fig. 129. 



Fig. 130. 



Description. Colour white, grey, less 

 frequently bhirk, liluc, green, yellow, red 

 and brown. It occurs regularly crystallized ; 

 also massive, disseminateil in plates, slalar- 

 litic, renil'orm, botryoidal, globular, specu- 

 lar, vesicular and cellular. Primary lorm 

 an obtuse rhombohedron. Fig. 129. P' on 

 P' 94° 24' {Hai/y) ; 94° 15' {Phillips). 

 The common form is a six-sided prism ter- 

 minated by six-sided pyramids, prising of 

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

 MiN. — Part H. 33 



