COMMON MINERALS AND VALUABLE ORES. 



IL— QUARTZ AND THE SILICATES. 



THEO. F. BROOKINS, B.S. 

 Principal Au Sable Academy, N. Y. 



COMPARATIVELY few persons 

 associate the gem opal, with its 

 brilliant internal colored reflec- 

 tions, with that material form- 

 ing so large a part of the soil, sand. 

 Yet the two are almost identical in 

 composition. The mineral constituent 

 of sand and of opal is quartz, though 

 the latter often contains in addition 

 some water. 



Quartz is composed of the two 

 elements occurring the most abun- 

 dantly in the earth's crust, silicon and 

 oxygen, both non-metals. As already 

 indicated, the most common represen- 

 tative of the mineral substance is the 

 sand of the soil. The sand grains are 

 generally so eroded by the atmosphere 

 and surface waters as to show little of 

 the true quartz structure. A typical 

 specimen of quartz, commonly known 

 as " rock crystal," clear, transparent 

 and approximately perfect in form, is 

 * not difficult to obtain for study. If 

 not occurring in the particular locality 

 it may be obtained from a dealer in 

 minerals at slight expense. As studied 

 by means of the rock crystal, quartz is 

 remarkable for its transparency, its 

 regular crystal form, and its great de- 

 gree of hardness. Its transparency is 

 such that printing may be read through 

 the crystal. Its crystalline form 

 affords an unfailing means to the 

 mineralogist of recognizing the sub- 

 stance as quartz. If our specimen be 

 large and perfect, we note that it is 

 bounded by planes in such manner 

 that we have a hexagonal prism termi- 

 nated at either end by a hexagonal 

 pyramid. With convenient apparatus 

 for measurement, we learn the all- 

 important fact to the mineralogist, that 

 the angle between any prism face and 

 an adjacent pyramid face is 141° 47'. 

 The mineralogist obtains his accurate 

 measurements by m.eans of an instru- 

 ment known as the goniometer. We 

 may obtain cruder results by bending 



a readily flexible wire over the two 

 faces, perpendicular to the edge of 

 their . intersection, until it is tight 

 against either face. Then placing one 

 arm of the bent wire along the base of 

 a protractor, the point of flexure at the 

 center of the base, the number of de- 

 grees between the two arms may be 

 read, thus giving roughly the angle be- 

 tween the prism and pyramid faces of 

 the quartz crystal. The great hard- 

 ness of quartz is apparent in that it can- 

 not be scratched with the point of a 

 knife and that it will cut glass. Often 

 clear parts of quartz crystals occur 

 studding the surface of a rock structure, 

 in the form known as a crystal aggre- 

 gate. One property of quartz rock 

 (any sandstone or quartzite) we must 

 not fail to notice is the irregular frac- 

 ture. This is recognized in the state- 

 ment that quartz has no cleavage. 



The study of the rock crystal should 

 not lead us into the false conclusion 

 that quartz is commonly transparent. 

 Instead it occurs in various shades and 

 colors from smoky white through yel- 

 low, red, purple, and brown to black. 

 The cause of the abundance of sand on 

 the soil surface is also liable to misin- 

 terpretation. While sand is naturally 

 of great abundance, yet its common- 

 ness at the surface of the soil is due 

 largely to its great resistant powers to 

 the agencies of weathering. 



Quartz has an economic value di- 

 rectly in glass sand and of course as a 

 soil constituent. In the latter capacity, 

 it is taken up by many plants, and is 

 the silica that studs the saw edges of 

 the blades of sedges and grasses. The 

 precious stones, agate, amethyst, and 

 jasper are varieties of quartz. 



The silicon that is so important a 

 constituent of quartz, composes with 

 aluminum a large part of various 

 minerals comprised under the name 

 feldspar. This substance is slightly 

 less hard than quartz and has many 



