Theoretical composition of bauxite (crystal chemistry) (wB%): Al2O365.4, H2O34.6. Common isomorphic substitutions are Fe and Ga, Fe2O3 can reach 2%, and Ga2O3 can reach 0.006%. In addition, it often contains impurities CaO, MgO, SiO2, etc.
Bauxite
Monoclinic: 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 voids in the two layers of OH- which are formed by OH- in the hexagonal closest packing layer (∥(001)), because Al3 has a higher charge than Mg2, so it is less The number of Al3 can balance the charge of OH-.
Orthorhombic crystals: C2h-2/m (L2PC). The crystals are in the form of pseudo-hexagonal plates, which are very rare. Main simplex: parallel double-sided a, c, rhomboid column m. Often according to (100) and (110) into twins. Common polycrystalline twins. The aggregates are radially fibrous, scaly, cortical, stalactite or oolitic, bean-like, spherical nodules or fine-grained earthy blocks. Mainly colloidal amorphous or fine-grained crystalline.
Physical properties: white or light gray, light green and light red due to impurities. Vitreous luster, cleavage surface pearl luster. Transparent to translucent. Cleavage is very complete. Hardness 2.5~3.5. The relative density is 2.30~2.43. Mud odor. Under a polarizer, it is colorless. Biaxial crystals. Ng=1.587, Nm=Np=1.566.
Occurrence and combination: mainly from the decomposition and hydrolysis of aluminosilicates. Tropical and subtropical climates favor the formation of gibbsite. In regional metamorphism, it can be transformed into boehmite and diaspore (140~200℃) after dehydration; as the degree of metamorphism increases, it can be transformed into corundum.
