Gibbsite Al(OH)3 Gibbsite is a hydroxide mineral of aluminum, which is the main component in the bauxite deposit. The crystals of gibbsite are very fine, and the crystals cluster together into a nodule, bean, or earth, usually white, with a shiny glass, and red if they contain impurities. They are mainly secondary minerals produced after the weathering of feldspar and other aluminum-containing minerals.
The chemical composition of the Al(OH)3? crystals is a monoclinic P21/n hydroxide mineral in the space group. It is homogeneous with bayerite and nordstrandite. The old name is trihydrate or sillimanite. It was named 1822 after the name of the mineral collector CG Gibbs. The crystal structure is similar to that of brucite. It consists of sandwich-shaped (OH)-Al-(OH)-coordinated octahedral layers that are stacked in parallel. Only Al3+ does not occupy all the octahedral voids in the sandwich. It only occupies them. 2/3. The crystals of gibbsite are generally extremely small, pseudo-hexagonal, and often twin-crystal, usually nodular, bean-like and earthy aggregates. White or pale red to red due to staining of impurities. Glass sheen? Cleavage with pearl sheen. Underlying cleavage is extremely complete. Morse hardness 2.5 to 3.5? Specific gravity 2.40. Is gibbsite mainly a secondary product of chemical weathering of feldspar and other aluminum-containing minerals? It is the main mineral component of lateritic bauxite. However, it can also be caused by hypothermia. The hydrothermal veins in Zlatustovsk, Southern Ural, Russia, produce crystals up to 5 cm in size. See bauxite for use.
Gibbsite [Crystal Chemistry] Theoretical composition (wB%): Al2O3 65.4, H2O 34.6. Common isomorphisms are replaced by Fe and Ga, with Fe2O3 up to 2% and Ga2O3 up to 0.006%. In addition, often contain impurities such as CaO, MgO, SiO2.
[Structure and Morphology] Monoclinic system, a0=0.864 nm, b0=0.507 nm, c0=0.972 nm; Z=8. The crystal structure is similar to that of brucite, which is a typical layered structure. The difference is that Al3 fills only the octahedral space between two layers of OH-, which is a close packed layer of OH-hexagonal (001 (001)). This is because Al3 has a higher charge than Mg2, so it has less The Al3 number balances the charge of OH-.
Orthorhombic columnar crystals, C2h-2/m (L2PC). The crystal is pseudo-six-sided and rarely seen. The main simplex: parallel two-sided a, c, oblique square column m. Frequently (100) and (110) twinning. Common polycrystalline double crystals. The aggregates were radial fibrous, scaly, hull-like, stalactite-like or pod-like, bean-shaped, spherulitic tuberculosis or a fine-grained earth-like block. Mainly colloidal amorphous or fine grained.
[Physical properties] White or light gray, light green, light red tone due to impurities. Glass sheen, cleavage surface pearl luster. Transparent to translucent. Cleavage is extremely complete. Hardness 2.5~3.5. Relative density 2.30~2.43. With mud odor.
Polarizer: Colorless. Biaxial crystals. Ng=1.587, Nm=Np=1.566.
[Presentation and combination] Mainly consists of aluminosilicate decomposition and hydrolysis. The tropical and subtropical climate is conducive to the formation of gibbsite. In the regional metamorphism, dehydration can be converted into boehmite and diaspore (140~200°C); with the increase of metamorphism, it can be transformed into corundum.
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