Page:The Journal of geology (IA journalofgeology21894univers).pdf/329
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 bismuth deposits.—The allied metal bismuth occurs most commonly as native bismuth, though the sulphide (bismuthinite), the selenide (guanajuatite), the telluride (tetradymite), etc. also occur. Native bismuth, by alteration, forms the carbonate (bismutite) and probably 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 molybdenite or molybdic ocher, which, in the Canadian region, occurs as a powdery coating on the cleavage planes of the molybdenite.
Alteration in 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 altogether unlike that of the hydrous sesquioxide of iron derived from the alteration of certain silicates. The conditions during formation, however, were probably quite different.