254 MATERIALS AND THEIR ARRANGEMENT 



iron oxides. These minerals are described briefly below. Some of 

 them occur in sedimentary and metamorphic woasks, as well as in 

 igneous rocks. 



Quartz. SiOz; H. 7; Sp. gr. 2.65. A mineral of very widespread occurrence. 

 It is found not only in igneous rocks, but in veins and cavities in other sorts of rock, 

 as nodules and concretions in limestones, and is the most abundant constituent 

 of sands, sandstones, and quartzites. 



Quartz is a very hard mineral; that is, it cannot be scratched with steel and it 

 will scratch glass. It is said to have conchoidal fracture; that is, it breaks like 

 glass, without any distinct tendency to break along parallel planes. In igneous 

 rocks, quartz usually looks rather dark and glassy by contrast with the lighter 

 colored, less transparent minerals with which it is commonly associated. Some 

 quartz has a sort of greasy or oily look because of its comparatively high luster. 

 In veins and cavity fillings it may occur, (i) as 6-sided crystals capped by pyra- 

 mid-like forms. The crystals may be so closely spaced that only the pyramid-like 

 forms can be seen; (2) as a sort of hummocky crust with a waxy luster (chalcedony), 

 or (3) as a series of bands of variegated color (agate). As concretions in limestone 

 it may have a variety of colors, but is usually between white and dark grey, in some 

 cases nearly black. Concretions are usually irregular in form, and contain a 

 considerable proportion of impurities, but can be recognized by the hardness of a 

 freshly broken surface. 



Some of the less common varieties are used as semi-precious stones; for ex- 

 ample, amethyst, cairngorm, rose quartz, jasper, prase, cat's-eye, and agate. True 

 onyx is also a variety of quartz. 



The feldspars. H. 6; Sp. gr. 2.5-2.6. 



Orthoclase, Potassium aluminum silicate 



pj . , ( Sodium aluminum silicate 

 (Calcium aluminum silicate 



Feldspars are abundant in igneous rocks and their metamorphic products, 

 but are not found abundantly in other rocks. Feldspars are not so hard as quartz, 

 but cannot be scratched by any but the very hardest steel. They have good 

 cleavage; that is, they have a strong tendency to break along parallel plane sur- 

 faces. This can be detected by holding a freshly broken surface to the light so 

 that a reflection is seen. If the whole surface of a crystal seems to reflect the light 

 when the fragment is held in a given position, it is usually due to the cleavage of 

 the mineral. Feldspars are commonly the dominant light colored constituents of 

 igneous rocks, but they range in color through white, buff, pink, red, and grey, 

 and a comparatively rare variety is green. 



It is not always easy to distinguish or thoclase, KAlSisOg, from plagioclase, a mix- 

 ture of NaAlSisOs and CaAl 2 Si2Os; but the cleavage faces of some plagioclase crys- 

 tals show distinct parallel striations, almost as true as if made by a ruling engine. 

 These are never present in orthoclase. The dark, dull or waxy looking feldspars 

 are more likely to be plagioclase, while buff or pink feldspars are more likely to 

 be orthoclase; but such distinctions are too uncertain to be used with great assur- 

 ance. 



Some of the rarer feldspars are used as semi-precious stones. Among these 

 are Amazonstone, sunstone, moonstone, peristerite and labradorite. 



