Mineral phases containing only oxide or hydroxide anions in their structures. By volume, oxide and hydroxide minerals comprise only a small fraction of the Earth's crust. However, their geochemical and petrologic importance cannot be overstated. Oxide and hydroxide minerals are important ores of metals such as iron, aluminum, titanium, uranium, and manganese. Oxide and hydroxide minerals occur in all geological environments. Some form as primary minerals in igneous rocks, while others form as secondary phases during the weathering and alteration of silicate and sulfide minerals. Some oxide and hydroxide minerals are biogenic; for example, iron(III) and manganese(IV) hydroxides and oxides often result from bacterial oxidation of dissolved Fe²⁺ and Mn²⁺ in low-temperature aqueous solutions.