Hydroxides, Natural

a large group of minerals; compounds of metals (aluminum, manganese, iron, magnesium, uranium, tungsten, vanadium, and so on) with hydroxyl (OH)¹” or OH¹” and oxygen (so-called hydroxyhydrates) that are stable at the earth’s surface. In addition to hydroxyl (OH)^1-, the oxygen-hydrogen groups in natural hydroxides are often also represented by H₂O, which is present in them as a solid solution or water of the crystal hydrate type. In most natural hydroxides the cations are crystallochemically linked to the O²” and (OH)^1- anions in symmetrical octahedrons. The latter when joined form lamellar, short-chain and, less frequently, framework crystal structure motifs.

According to their chemical structure, natural hydroxides are subdivided into simple hydroxides, such as goethite, FeOOH, and hydrargillite Al(OH)₃; and complex hydroxides, such as becquerelite Ca[(U0₂)₆O₄(OH)₆]8H₂O. Upon heating, natural hydroxides lose water in stages, becoming stable, often very refractory simple oxides (A1₂O₃, MgO, Fe₂O₃, MnO₂, and so on). With the exception of the hydroxides of manganese, aluminum, and iron, natural hydroxides are readily soluble in inorganic acids. They have a vitreous, oily, or semimetallic luster. Most natural hydroxides are transparent or translucent in thin fragments. Their color depends on the chromophoric properties of the atoms of which they are composed—for example, Mn³⁺ and Mn⁴⁺ are black, Fe³⁺ is reddish brown, and U⁶⁺ is yellow. Their hardness on the mineralogical scale ranges from 2.5 (brucite, uranium hydroxides, and so on) to 7.2 (diaspore and psilomelane). The density depends on the atomic weight of the cation, the presence of water molecules, and the structural packing of the atoms in the crystal lattice and varies from 2,400 to 7,300 kg/m³. The most widespread minerals are diaspore, goethite, manganite, psilomelane, boehmite, lepidocrocite, hydrotungstite, heterogenite, hydrargillite, brucite, and becquerelite.

Natural hydroxides form in processes of hypergenesis through hydrochemical destruction and redeposition of substances of primary minerals or rock and ores on the earth’s surface, often with the participation of living organisms. Natural hydroxides are the most important constituent of soils and of mineral formations of the so-called weathering crust, of oxidation zones of deposits, and of marine and continental lacustrine precipitates and those of flowing waters. Many natural hydroxides form huge commercial mineral deposits (bauxite, bog iron ore, oxide and hydroxide manganese ores, and uranium and vanadium ores).