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(also hydrargillite), a mineral with the chemical composition Al[OH]3. It contains 65.4 percent alumina (A1203); admixtures of Fe3+ and Ga3+ that replace Al3+ in the structure are known. Gibbsite crystallizes in a monoclinic system. Its crystalline structure is of the layer type and consists of bifoliate (OH) packets, in the midst of which Al3+ ions are distributed. Good cleavage occurs along weak interpacket bonds. Gibbsite forms small laminar crystalline particles, usually colorless with a pearly luster, as well as powdery aggregates and incrustations with a radiated or imbricate microstructure. Its hardness on the mineralogical scale is 2.5-3.5; its density is 2,300-2,400 kg/m3. It usually forms upon weathering of alumina-rich rocks. Together with the other hydroxides of aluminum (diaspore and boehmite) and iron, gibbsite is found in bauxite ores. Gibbsite bauxites are among the best aluminum ores.