Bauxite
Gibbsite (Gibbsite) Al(OH)3gibbsite is a crystalline hydrate of aluminum hydroxide, which is the main component in bauxite. The crystals of gibbsite are very small, and the crystals gather together to form nodules, beans or soils, generally white, with glass luster, and red if it contains impurities. They are mainly secondary minerals produced by the weathering of aluminum-containing minerals such as feldspar. The chemical composition is Al(OH)3, and the crystal belongs to the hydroxide mineral of the monoclinic system P21/n space group. It is homogeneous with bayerite and nordstrandite. Formerly known as gibbsite or basophile. Named in 1822 after mineral collector C.G. Gibbs (Gibbs). The crystal structure is similar to that of brucite, which is composed of sandwich biscuit-like (OH)-Al-(OH) coordination octahedral layers stacked in parallel, except that Al3+ does not occupy all the octahedral voids in the interlayer, but only occupies one of them. 2/3. The crystals of gibbsite are generally very small, in the shape of pseudo-hexagonal sheets, and often form twin crystals, usually in the form of nodules, beans, and soil-like aggregates. White, or light red to red due to impurity staining. Vitreous luster, cleavage surface shows pearly luster. The underside is very well cleaved. The Mohs hardness is 2.5-3.5, and the specific gravity is 2.40. Gibbsite is mainly a secondary product of chemical weathering of aluminum-containing minerals such as feldspar, and is the main mineral component of laterite-type bauxite. But it can also be caused by low-temperature hydrothermal fluids. Crystals up to 5 cm in size have been produced from hydrothermal veins at Zlatoustovsk in the southern Urals of Russia.
Characteristic description
Bauxite (crystal chemistry) theoretical composition (wB%): Al2O365.4, H2O34.6. The common isomorphic substitutes are Fe and Ga, Fe2O3 can reach 2%, and Ga2O3 can reach 0.006%. In addition, it often contains impurities such as CaO, MgO, SiO2, etc.
Bauxite
Monoclinic system: a0=0.864nm, b0=0.507nm, c0=0.972nm; Z=8. The crystal structure is similar to brucite, which is a typical layered structure. The difference is that Al3 only fills 2/3 of the octahedral gaps in the two layers of OH- between the two layers of OH- in the hexagonal closest packing layer (∥(001)), because Al3 has a higher charge than Mg2, so it uses less The number of Al3 can balance the charge of OH-.
Orthorhombic columnar crystals: C2h-2/m (L2PC). The crystals are pseudo-hexagonal plates, which are extremely rare. Main simplex: parallel double-sided a, c, orthorhombic column m. Often according to (100) and (110) into twin crystals. Polycrystalline twin crystals are common. The aggregates are radial fibrous, scale-like, shell-like, stalactite-like or oolitic-like, bean-like, spherical nodules or fine-grained soil-like masses. It is mainly colloidal amorphous or fine-grained crystalline.
Physical properties: white or light gray, light green, light red due to impurities. Vitreous luster, cleavage surface pearl luster. Transparent to translucent. Very complete cleavage. Hardness 2.5~3.5. The relative density is 2.30~2.43. With earthy smell. Under a polarizer, it is colorless. biaxial crystal. Ng=1.587, Nm=Np=1.566.
Occurrence and combination: It is mainly formed by decomposition and hydrolysis of aluminosilicate. Tropical and subtropical climates favor the formation of gibbsite. In regional metamorphism, it can be transformed into boehmite and diaspore (140~200°C) after dehydration; as the degree of metamorphism increases, it can be transformed into corundum.
