been formed as a canary-yellow coating at the surface. On the whole, however, tungsten minerals are very resistant to weathering, and in all their deposits secondary concentration by chemical action at the surface has not played any appreciable part. The disappearance of tungsten minerals from alluvial materials which are undergoing laterization, which has been described in Burma,2 seems to indicate that the tungsten is dissolved in surface waters to some extent; but in the main it is probably carried completely out of the vicinity and not reprecipitated below.
MOLYBDENUM ORES
Economic Features
The main use of molybdenum is in the manufacture of high-speed tool steels, in which it has been used as a partial or complete substitute for tungsten. Its steel-hardening qualities are more effective than those of tungsten, but it is more difficult to control metallurgically. It has been used in piston rods and crank shafts for American airplanes. Its use in tool steel is mainly confined to Europe, where its metallurgical application is in a more advanced stage than in the United States. Molybdenum is added to steel either as powdered molybdenum or in the form of ferromolybdenum, an alloy containing 60 to 70 per cent of the metal. Molybdenum chemicals are essential reagents in iron and steel analysis and other analytical work; they are also used as pigments. Molybdenum metal has been used to a small extent in incandescent lamps and as a substitute for platinum in electric contacts and resistances.
Molybdenum ores range from considerably less than 1 per cent to about 5 per cent in molybdenum.
The world's principal sources of molybdenum ores in approximate order of importance are the United States, Canada, Norway, Australia, Korea, Austria, Peru,