OXIDES 



nucleus. Nodular mann:ine>e ox'nles <cciir in clays and oilier 

 sedimentary formations. 



1'silomolane occurs at Brandon, Vermont, and at several points. 



Fia. 433. Psilomelane from Langenberg, Saxony. 



in Arkansas. Asbolite is found at Silver Bluff, South Carolina. 



Lampadite, as well as wad, occurs in the Copper Queen mine, 

 Arizona. 



OPAL 



Opal. Si0 2 . n (H 2 0) ; Amorphous ; The amount of water 

 varies according to the dehydration; Brittle, fracture con- 

 choidal; H. =5.5-6.5; G. = 1.9-2.3; Color, all colors; Luster, 

 vitreous to resinous ; Streak, white ; Transparent to nearly opaque ; 

 n = 1.436-1.450. In thin sections isotropic and colorless, fill- 

 ing small cracks and as crusts in amygdaloidal cavities. 



B.B. Like quartz, but yields water in the closed tube; 

 in some specimens this may be small in amount. Soluble in boil- 

 ing alkali solution. 



General description. Opal is gelatinous or colloidal silica, 

 deposited from solutions. Percolating waters decompose many 

 silicates, especially the orthosilicates, the silica of which may be 

 again deposited either as quartz or opal. If there are alkalies pres- 

 ent in the solution, quartz is usually the mineral formed ; if alka- 

 lies are absent, opal is the form in which the silica is deposited. 

 Thus opal is connected as a secondary component of lavas and sedi- 



2B 



