AlOOHnH2O

AlOOHnH2O is a general notation for a family of aluminum oxyhydroxide hydrates with the formula AlOOH·nH2O, where n denotes the number of water molecules associated with the AlOOH unit. In common terminology, the hydrate includes diaspore (n = 0) and boehmite (n = 1), with higher hydration levels observed in synthetic or metastable forms. The term is used for both naturally occurring minerals and laboratory-synthesized hydrates.

In mineral form, the two principal natural representatives are diaspore (α-AlOOH) and boehmite (γ-AlOOH·H2O). These phases occur as secondary constituents in aluminum-rich rocks, such as rocks subjected to weathering or diagenesis, and are notable in some bauxite deposits. Boehmite, in particular, is frequently found as fine lamellae within bauxitic materials and can be produced or enriched under hydrothermal or high-pumidity conditions.

Structurally, aluminum in these materials is typically six-coordinate with oxygen and hydroxide ligands, forming layered or framework-like arrangements stabilized by hydrogen bonding. They are white to grayish and are generally insoluble in water. Upon heating, they dehydrate and eventually convert to anhydrous aluminum oxide (alumina, Al2O3). Dehydration behavior and phase transitions depend on temperature and precise composition.

Synthesis of AlOOH·nH2O forms is common in industry and research. They can be prepared by controlled hydrolysis or precipitation of aluminum salts, by dehydration of hydrated aluminum hydroxides, or by hydrothermal treatment of related alumina precursors. Applications include use as precursors to high-surface-area alumina, catalyst supports, adsorbents, desiccants, and flame-retardant fillers in polymers and coatings.