OCCURRENCE OF QUARTZ. 217 



THE OXIDES. 



The more important oxides occurring as rock-building- constituents are 

 those of silicon, iron, and titanium. The oxides of silicon are quartz, 

 tridymite, and opal. The important oxides of iron are hematite, magnetite, 

 and limonite. The important oxides of titanium are rutile, octahedrite, 

 and brookite. One oxide of iron and titanium, or else a ferrous titanate, 

 has a widespread occurrence; this is ilmenite. 



QUARTZ. 

 Quartz: 

 Si0 2 . 



Rhonibohedral. 

 Sp. gr. 2.653-2.654. 



— Quartz is second in abundance only to the minerals of the 

 leldspar group. According to Clarke, a quartz comprises 12 per cent of 

 the lithosphere. It is very abundant as an original pyrogenic constituent 

 of the igneous rocks, as an allogenic constituent of the clastic rocks, and 

 as an autogenic mineral in all classes of metamorphosed rocks. The 

 material for secondary quartz may be derived from the alterations of many 

 minerals in situ, or from the decomposition of minerals at some distance. 

 The most widespread of all the alterations which furnish silica to the 

 solutions is that of the decomposition of the silicates by carbonic acid in 

 the belt of weathering, with the simultaneous production of carbonates and 

 quartz, or a solution of colloidal silicic acid from which opal, chert, or 

 quartz may later separate. Such quartz may be extensively deposited from 

 the solutions in the porous rocks of the belt of cementation. It there fills 

 the minute spaces between the individual grains of sedimentary rocks. 

 It occupies spaces in porous tuffs or in vesicular igneous rocks. It fills 

 openings between laminae, and joint, fault, and breccia openings. The 

 quantity of quartz thus deposited is far greater than that of any other 

 mineral, and not improbably greater than that of all other minerals com- 

 bined. By this process the rocks are cemented. (See pp. 617-621.) Not 

 only may the openings be occupied by quartz, but at the time of the 

 deposition of the quartz other minerals may dissolve and their places be 

 taken by the quartz. This process of deposition of silica as quartz is called 

 silicification. (See p. 205.) 



«01arke, F. W., Analyses of rocks from the laboratory of the United States Geological Survey, 

 1880-1899: Bull. U. S. Geol." Survey No. 168, 1900, p. 16. 



