SUPERFICIAL ALTERATION OF ORE DEPOSITS. 313 



as kermesite. Valentinite and senarmontite have the same 

 chemical composition but differ in their crystalline forms. Native 

 antimony sometimes occurs, and this also, by alteration, gives 

 rise to the oxides. 



Alteration in bisTnuth deposits. — The allied metal bismuth occurs 

 most commonly as native bismuth, though the sulphide (bis- 

 muthinite), the selenide (guanajuatite), the telluride (tetrady- 

 mite), etc.- also occur. Native bismuth, by alteration, forms the 

 carbonate (bismutite) and probabty also the oxide (bismite) and 

 the silicate (eulytite). 



Alteration in mercury deposits. — In the case of mercury the 

 metal commonly occurs as the sulphide (cinnabar), though other 

 mercury minerals also occur. By the alteration of cinnabar and 

 some of the other mercury minerals, metallic mercury is set free 

 and occurs as globules or filling cavities in the ore. 



Alteration in molybdenum deposits.- — Another case of surface 

 alteration in metalliferous deposits is that seen in molybdenite. 

 This mineral is the sulphide of the metal molybdenum, and often 

 occurs in quartz or calcite veins in the crystalline rocks of parts 

 of Canada, and in many ore deposits of the Rocky Mountains 

 and elsewhere. By surface oxidation, molybdenite passes into a 

 brilliant yellow oxide of molybdenum, commonly known as 

 molybdite or molybdic ocher, which, in the Canadian region, 

 occurs as a powdery coating on the cleavage planes of the 

 molybdenite. 



Alteration i?i other deposits. — Superficial alteration like that 

 already described in various deposits, occurs also in many others 

 not yet mentioned, as in aluminum, nickel, cobalt, chromium, 

 tungsten, and many rarer deposits, but the changes already 

 described show the general features of the subject. It may be 

 said, however, that one of the important ores of aluminum, known 

 as bauxite, is probably derived from the alteration of feldspar 

 under certain conditions ; and its source, therefore, is not alto- 

 gether unlike that of the hydrous sesquioxide of 'iron derived 

 from the alteration of certain silicates. The conditions during 

 formation, however, were probably quite different. 



