An oxide is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of -2. Most of the Earth's crust consists of solid oxides, the result of elements being oxidized by the oxygen in air or in water.  Hydrocarbon combustion affords the two principal carbon oxides: carbon monoxide and carbon dioxide. Even materials considered pure elements often develop an oxide coating. For example, aluminium foil develops a thin skin of Al₂O₃ (called a passivation layer) that protects the foil from further corrosion. Different oxides of the same element are distinguished by Roman numerals denoting their oxidation number, e.g. iron(II) oxide versus iron(III) oxide. The two most common oxides in nature are silicon dioxide and water.