OXIDKS 



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magma. It may therefore include within its mass all those min- 

 erals which have separated previous to it, as the oxides of iron, 

 nitilc, apatite, zircon, mica, amphibole, and pyroxene; and in 

 some instances it forms a ground mass in which the individual 

 crystals of other minerals are imbedded. 



In the final solidification of acid igneous rocks, quartz usually 

 preserves a balanced equilibrium with orthoclase, separating as a 

 cutcctic,- which is well 

 illustrated in the struc- 

 ture of the micropeg- 

 matites, where the 

 crystals are so fine and 

 intimately mixed as to 

 be revealed only by 

 the microscope. 



In thin sections 

 quartz is colorless and 

 transparent. Its index 

 of refraction is so near 

 that of Canada balsam 

 that there is scarcely 

 any relief, and the 

 quartz grains when free 

 of inclusions have the 

 appearance of holes in 



the rock sections. In sections less than .04 mm. in thickness the 

 interference is gray or yellow of the first order. It is often filled 

 with small inclusions of liquids or gas. 



As a secondary mineral quartz may be formed by the solvent 

 action of percolating waters containing carbon dioxide, decompos- 

 ing complex silicates by carrying out the bases in solution, forming 

 carbonates of the bases and quartz. The SiO 2 may dissolve also, 

 to be later deposited in veins, cracks, and cavities, either in the 

 form of quartz or as hydra ted silica (opal). Such quartz is asso- 

 ciated with many ore deposits as the gangue mineral or vein filler ; 

 it has a peculiar greasy luster and splintery fracture and is often 

 termed vein quartz. 



The quartz grains of granites resist atmospheric weathering ; and 

 when changes and decomposition of the other minerals are in prog- 

 ress at the surface, these grains remain unchanged in the residue> 

 forming the sands of the soil ; or when washed clean and redeposited 



Fio. 427. Enlarged Micropegniatite between 

 Crossed Nicols. The Light Areas are Quartz, the 

 Dark Orthoclase. 



