GUIDE TO THE MIXEKALOGIC COLLECTIONS 



69 



Opal occurs in cavities and fissures in igneous rocks, as con- 

 cretions in limestones, as sinter in the vicinity of geysers, hot 

 springs, etc. The precious variety is found in Hungary, Aus- 

 tralia, Mexico and in Washington and Idaho. 



Precious opal is highly valued as a gem. The chalky variety 

 is used for polishing and washing purposes, in the manufacture 

 of dynamite and in the preparation of a soluble glass. 



OXIDS OF METALS 



Cuprite (red copper ore) Cu 2 



Cuprite is the oxid of copper and contains 11.2$ oxygen and 

 SS.Sft copper. 



Crystals of cuprite (fig. 1S4, 185) are isometric, the prevailing 



Fig. 1S4 Fig. 185 



Cuprite 



forms being the cube, octahedron and dodecahedron, often 

 highly modified; in the variety chalcotrichite the cube crystals 

 are distorted to such an extent as to produce hairlike forms 

 i pi. 20 2 ). Massive, granular and earthy forms are common. The 

 luster of cuprite is submetallic or adamantine to earthy and 

 the color varies from a dark red which is nearly black to a 

 vermilion or scarlet, seen in some of the massive varieties. It 

 is transparent to opaque. 



Cuprite results from the oxidation of the sulfids of copper and 

 is found associated with other copper minerals and with 

 limonite. It occurs in fine crystals in the Cornwall mines and 

 is found in considerable deposits in Chile, Bolivia, Peru, the 

 Lake Superior region and Arizona. 



It is a useful copper ore. 



