Uranium is widely distributed in its ores but is not found uncombined in nature. It is a fairly abundant element in the earth's crust, being about 40 times as abundant as silver. Several hundred uranium-containing minerals have been found but only a few are commercially significant. The most common are uraninite (essentially uranium dioxide) and pitchblende; other commercially important uranium-containing minerals include carnotite (a potassium uranate-vanadate) and brannerite (a uranium titanate). Ores with as little as 0.1% uranium are mined and processed. Most ores are processed by chemical methods including leaching and solvent extraction. Leaching produces the material known as yellowcake, which is largely triuranium octoxide (U3O8); it is then processed with nitric acid. The uranium is obtained as pure uranyl nitrate, UO2(NO3)2·6H2O, which is typically decomposed to the trioxide, UO3, by heating and reduced to the dioxide, UO2, with hydrogen. The dioxide is chemically and physically stable at high temperatures, and is the form most often used as nuclear reactor fuel. The dioxide may be converted to the tetrafluoride, UF4, by treatment with hydrogen fluoride gas, HF. The pure metal is obtained by electrolysis or chemical reduction of the tetrafluoride, or by chemical reduction of the dioxide. Uranium is further processed, or enriched, to increase the percentage of uranium-235 so that the uranium can be used in a reactor or, with much greater enrichment, a weapon. In enrichment, uranium-238 is separated from uranium-235 by a diffusion or centrifuge process using the gaseous hexafluoride, UF6, which is produced when the metal reacts with fluorine.