OXIDKS 



358 



'J.ii.') ; Color, when pure, colorless or white, when impure, all shades ; 

 Luster, vitreous, splendent to dull or greasy; Streak, white, or in 

 colored specimens very pale ; Transparent to opaque ; c = 

 1.5532; (0 = 1.5441; -co =.0091; Optically (+); Rotary 

 polarization in thick sections. 



B.B. Infusible (1600) ; yields little or no water in the closed 

 tul>e; when finely ground and fused with two volumes of soda on 

 the platinum wire yields a clear bead when cold. Insoluble in 

 acids except hydrofluoric. 



General description. Crystals are very common and well 

 developed, usually elongated parallel to the 6 axis, combinations 

 of the plus and minus rhombohedrons and the unit prism. The 

 prism faces are often striated horizontally, which serves to identify 

 the prism faces on distorted specimens. The trigonal pyramid s 



FIG. 422. Quartz Var. Rock Crystal. Hot Springs, Arkansas. 



the trapezohedron x are common in certain localities, but 

 are, like the large number of other forms that have been described, 

 rare except for the few favored localities. When the two rhom- 

 bohedrons are equally developed, the crystals have the appearance 

 of being terminated by a hexagonal pyramid, but this is not pos- 



