Page:The Journal of geology (IA journalofgeology21894univers).pdf/242
On page 16 the author says that "the group of silicates is extremely large, including many of the important rock-forming minerals, but as ores they are of little importance." He has evidently overlooked the fact that calamine, a hydrous silicate of zinc, is an important ore, and that garnierite, a hydrous silicate of nickel and magnesia, is the source of a large part of the nickel of commerce. The latter is the ore mined at New Caledonia, off the coast of Australia, one of the two largest nickel producing regions in the world. It also occurs in the United States. The author again overlooks this silicate when, on page 26, in enumerating the ores of nickel, he says, "nickel is obtainen from the two sulphides, millerite, niccoliferous pyrrhotite, and the arsenide niccolite." In other deposits also silicates form a minor but an important part of the ore, as in the case of chrysocolla in the copper ores of Arizona.
On page 18 the author states that, "Sometimes, though not commonly, gold occurs in iron pyrites in invisible grains." It is almost unnecessary to say that one of the most common modes of occurrence of gold is in intimate association with iron pyrites, so that this statement is extremely misleading.
On page 20 the author says: "Gold occurs in the earth in only two mineralogical forms, so far as known, one in association with tellurium, the other native, the latter being its typical occurrence and the one from which the gold in use is obtained." It is true that native gold is the source of most of the gold in use, but the telluride ores, far from producing no commercial gold, are in many mines an important source of that metal. At Cripple Creek, in Colorado, the tellurides form an important part of many of the ores, and this district produced between $2,500,000 and $3,000,000 in gold in 1893. In Boulder county, Colorado, tellurides are also of importance, and have been so for many years past, while tellurides frequently occur in still other places.
On page 22 the author, in speaking of copper, says: "Its most common occurrence, however, is as the sulphide, chalcopyrite (CuFeS2), or copper pyrites, which is in reality a sulphide of iron and coppes combined, the proportion varying from an exceedingly cupriferous variety (chalcopyrite) to pure iron pyrite." The sulphide of copper known as copper pyrites is a definite chemical compound, with proportions of iron and copper in a definitely fixed ratio, so that the mineral cannot vary from an exceedingly cupriferous variety to pure iron pyrites. The same may also be said of other sulphides of copper.