Nickel. This ore is never found in metallic form, but always in combination. Pyrrhotite, or magnetic pyrites, is the source of about all the nickel of commerce. This ore has been already noticed under iron. Rare but valuable ores of nickel are millerite, nickelite, glance, and nickel bloom.
| Per cent. nickel. | ||
| Millerite | NiS | 64.4 |
| Niccolite | NiAs | 44.0 |
Some of the nickel of commerce is derived from nickelliferous pyrrhotite.
Petroleum. Crude petroleum is never found in metamorphic or igneous rocks. The stratified rocks of the Devonian, Carboniferous and Cretaceous ages are most likely to hold it. The crude oil is almost black, and consists of about 85 per cent. of carbon, and 15 per cent. of hydrogen. A long iron-shod stick is all the prospector requires to take with him in his search for surface indications of oil. The warmer the day the easier the search, as the oil rises to the surface of the streams, and is found in greater quantities than on cold days.
Oil existing in the lower rocks ascends through them until it accumulates under some layer that will not let it pass through. In this condition deep boring finds it, the rod usually tapping gas first. Petroleum may be noticed oozing out of gravel banks, or floating as a scum on the surface, whenever abundant. It has been found in rocks of widely different age, from extremely ancient formations to some that did not precede man by so very long, geologically speaking.
Platinum. This metal is only found native. Its gravity is very high, from 16 to 22. Hardness, 4 to 4.5. Luster, metallic. Opaque. Whitish-gray. Smooth. Ductile. Cleavage, none. Fracture, hackly. Texture, granular, fine. Platinum is unaffected by acids, but if alloyed with 10 per cent. of silver it dissolves in nitric acid. Almost infusible. Platinum occurs with placer gold in the beds of streams. Usually it is in small grains, but one or two large nuggets are on record from Brazil and Siberia. Serpentine rocks are believed to have originally held the platinum found in the beds of rivers, but none has been found in veins. The entire product of the United States was 300 ounces in 1898; valued at $3,837. In 1899 there was none produced.
Silver. Silver is generally found in serpentine, trap, sandstone, limestone, shale, or porphyry rocks, the gangue being quartz, calc, fluor, or heavy spar. All silver ores are heavy, and many of them are sectile, i.e., may be cut with the knife. Western men test for silver by heating the ore and dipping it into water. Some metal comes to the surface in a greasy scum, should silver be present. Native silver is found occasionally. Owing to the fall in value of this metal its future is not assured. It has fallen, during the past year, once to forty-nine cents an ounce, and this has had a most disastrous effect upon many silver mines, forcing them to suspend operations. Should the fall continue, as seems likely, and the price of silver go down to forty cents an ounce, little will be produced except as a by-product in the treatment of argentiferous lead ores.
As silver enters into chemical combination with sulphur easily, as is seen by the black film that forms on silver articles in a room where gas is burnt, most silver ores are sulphides. The very abundance of silver has caused its great fall in value, and it does not appear that it is ever likely to remain for long at a price exceeding fifty cents an ounce, owing to the ease with which it may be produced, and the large quantities that must find their way to market through it being a by-product in lead smelting. From 1859 to 1891 the Comstock lode in Nevada produced $325,000,000. This lode is a belt of quartz, 10,000 feet long and several hundred wide, and is a contact vein between diorite and diabase. In America galena is the principal source of silver; the chlorides and oxides rank next; while, lastly, some silver is parted from gold when it reaches the mint, as gold always contains more or less of that metal. No precise statement as to the manner of its occurrence may be made since it is found in many different positions, and is associated with all sorts of minerals. It is never found in placer deposits, as it breaks up under the influence of water, air, etc. Its original source is doubtless the igneous rocks, where it occurs in association with augite, hornblende and mica. Silver may be expected in mountainous regions of recent origin. Between 1875 and 1891 the world's product rose from $82,000,000 to $185,599,600. Three quarters of this came from the western hemisphere.
The commercial ores of silver are:
| Silver. | ||
| Argentite | Ag2S | 87.1 per cent. |
| Proustite | 3Ag2SAs2S3 | 65.5 per cent. |
| Prysagyrite | 3Ag2SSb2S3 | 59.9 per cent. |
| Stephanite | 5Ag2SSb2S3 | 68.5 per cent. |
| Cesargerite | AgCl | 75.3 per cent. |