SILICA MINERALS I 5 



i. Quartz (Si0 2 ) is anhydrous silica in a crystalline state; it 

 belongs in the hexagonal system, and crystallizes in hexagonal 

 prisms capped by six-sided pyramids, or in double six-sided 

 pyramids, or in modifications of these forms. It is insoluble in 

 any acid except hydrofluoric, and only very slowly soluble in boil- 

 ing caustic alkalies. When dissolved, as may be done by some- 

 what complicated processes, silica shows a distinct acid reaction. 



Quartz has no cleavage and is very hard (H = 7), scratching 

 glass readily, while it cannot be scratched with a knife ; the spe- 

 cific gravity (sp. gr.) is 2.6. 



When pure and symmetrically crystallized, quartz is transparent, 

 colourless, and lustrous (rock crystal), but it more commonly is 

 found in dull masses. Different varieties are coloured by metallic 

 oxides : thus, amethyst is quartz stained purple by the oxide of 

 manganese ; smoky quartz, or cairngorm, owes its brownish or yel- 

 lowish colour to the oxide of iron ; and there are many other kinds. 



In its various forms quartz is the most abundant of minerals, 

 and plays the most important part in the formation of the different 

 classes of rocks. 



2. Chalcedony occurs in spheroidal or stalactitic masses, com- 

 posed of more or less concentric shells. The structure is crystal- 

 line, and displays radiating fibres, which are perpendicular to the 

 shells. The chemical composition and behaviour of this mineral 

 are the same as in quartz, but the specific gravity is somewhat 

 lower (2.59-2.64), and the optical properties different. Chal- 

 cedony has a waxy appearance, and is translucent or semi-opaque, 

 and of various pale colours. ' 



3. Opal, Hyalite. Hydrated silica (Si0 2 , ^H 2 0).— These 

 minerals are amorphous and have no crystalline form. Opal is 

 either translucent or opaque and of various colours. Precious opal 

 (a gem) and common opal differ in colour, and in the fact that 

 the former is iridescent, the latter not. Hyalite is colourless 

 and transparent. Hydrated silica is lighter than the anhydrous 

 (sp. gr. = 2.2) and more readily soluble in hot alkaline waters. 

 These minerals are of much less importance as constituents of 

 rock than the forms of quartz. 



