APPENDIX 689 



Lead Minerals 



Galenite (galena), PbS. — The color is lead gray. Its softness 

 (H = 2.5), its high specific gravity which is greater than that of iron, 

 and its strong cubical cleavage make it one of the most easily recog- 

 nizable minerals. It occurs in masses and as cubical crystals. Galena 

 is valuable as an ore of lead, as well as for the silver which it usually 

 carries. 



Silica Minerals 



Quartz and its Varieties, Si0 2 . — When pure, quartz is colorless or 

 white, but in no other mineral do the colors vary so widely ; red, pink, 

 yellow, brown, green, blue, lavender, and black, in fact almost every 

 conceivable color is found in quartz. Quartz is harder than steel and 

 scratches glass (H = 7). It is the commonest of minerals. " It 

 makes up most of the sand of the seashore; it occurs as a rock in the 

 forms of sandstone and quartzite, and is a prominent part of many 

 other important rocks, as granite and gneiss." It is readily dis- 

 tinguished from other minerals by its hardness and its lack of cleavage. 

 The crystals are six-sided (hexagonal). The principal varieties are: 

 rock crystal, as the clear quartz crystals are called, which is used 

 for making "pebble lenses," "Japanese balls," and other objects; 

 amethyst, purple crystalline quartz which is cut for gem stones ; rose 

 quartz, which is light pink or rose color; milky, smoky, and yellow 

 quartz, named because of their color. Chalcedony is a translucent 

 variety with a waxy luster which varies greatly in color. Agate is a 

 banded chalcedony in which the bands are variously colored. Flint 

 (p. 524) and chert are gray to black translucent or opaque quartz 

 masses which occur in chalk and limestone. Jasper is similar to flint 

 in appearance, but is usually red, black, white, or yellow. 



Silicate Minerals 



Orthoclase Feldspar (potash feldspar), KAlSisOs- — The color is 



usually white, gray, or flesh. The hardness is about that of steel (H = 

 6). The mineral cleaves readily, the cleavage planes being at right 

 angles to each other. Orthoclase feldspar is an important constitu- 

 ent of granite and sometimes occurs in large crystals. Pure feldspar 

 is used to make the glaze on porcelain. 



Labradorite Feldspar (lime feldspar). — The color is dark gray, often 

 with blue, green, and red iridescence. It is slightly harder than steel 



