234 



DESCRIPTIONS OF MINERALS. 



also coarse and fine granular ; also compact, either amor- 

 phous, or presenting stalactitic and mammillary shapes. 



Crystals often as pellucid as glass, and colorless ; some- 

 times topaz-yellow, amethystine, rose, smoky, or other tints. 

 Also of all degrees of transparency to opaque, and of various 

 shades of yellow, red, green, blue and brown colors to black. 

 In some varieties the colors are in bands, stripes, or clouds. 

 H.=7. G. =2-5-2-8. 



1. 



5. 



Composition. Si 2 = Oxygen 53*33, silicon 46 -67 =100. 

 Opaque varieties often contain oxide of iron, clay, chlorite, 

 or some other mineral disseminated through them. B.B. 

 infusible. With soda, fuses with effervescence. 



Biff. Quartz is exceedingly various in color and form, 

 but may be distinguished, by (1) absence of true cleavage ; 

 (2) its hardness ; (3) its infusibility before the blowpipe ; 

 (4) its insolubility with either of the common acids; (5) its 

 effervescence when heated B.B. with soda ; and (G) when 

 crystallized, by the forms of its crystals, which are almost 

 always six-sided prisms terminating in six-sided pyramids. 



The varieties of quartz owe their peculiarities either to 

 crystallization, mode of formation, or impurities, and they 

 fall naturally into three series. 



I. The vitreous varieties, distinguished by their glassy 

 fracture. 



II. The chalceclonic varieties, having a subvitreous or a 

 waxy lustre, and generally translucent. 



III. The jaspery cr yptocry stall ine varieties, having barely 

 a glimmering lustre or none, and opaque. 



I. VITREOUS VARIETIES. 



Rock Crystal, Pure pellucid quartz. 



This is the mineral to which the word crystal was first 

 applied by the ancients ; it is derived from the Greek krus- 

 tallos, meaning ice. The pure specimens are often cut and 

 used in jewelry, under the name of " white stone." 



It is often used for optical instruments and spectacle 



