Boehmite is a mineral from the class of hydroxides with the chemical formula γ-AlO (OH). It was named after the German mineralogist I. Böhm ( Johannes Böhm ) (1857–1938), who, by means of X-ray diffraction studies, established the presence of this mineral in bauxite , similar in its crystalline structure to lepidocrocite . Opened in 1927
| Bohemite | |
|---|---|
 | |
| Formula | γ-AlO (OH) |
| Impurity | Fe 2 O 3 , SiO 2 , TiO 2 , sometimes CaO, MgO, Na 2 O + K 2 O, Ga 2 O 3 , Sc |
| Physical properties | |
| Colour | often colorless or white with a yellowish, reddish, pinkish, greenish tint |
| Trait color | white, gray |
| Shine | glass, on the cleavage planes - pearlescent; in thin plate aggregates sometimes silky |
| Transparency | transparent to translucent |
| Hardness | 3,5 |
| Cleavage | very perfect in {010} , perfect in {100} , and very imperfect in {001} |
| Kink | uneven, in aggregates - up to conchoidal, sometimes fine-grained |
| Density | 3.01-3.06 g / cm³ |
| Syngonia | rhombic (orthorhombic) |
| Refractive index | 1.640-1.645 |
MMA status is valid, described for the first time until 1959 (before MMA)
Content
Classification
| Strunz Classification (8th Edition) | 4 / F.06 - 20 |
| Nickel-Strunz Classification (10th Edition) | 4.FE.15 |
| Dana Classification (8th Edition) | 6.1.2.1 |
| Hey's chemical index of minerals | 7.6.2 |
Physical Properties
| Colour | often colorless or white with a yellowish, reddish, pinkish, greenish tint |
| Trait color | white, gray |
| Shine | glass, on the cleavage planes - pearlescent; in thin plate aggregates sometimes silky |
| Transparency | transparent to translucent |
| Hardness (Mohs scale) | 3,5 |
| Strength | fragile |
| Cleavage | very perfect in {010} , perfect in {100} , and very imperfect in {001} |
| Kink | uneven, in aggregates - up to conchoidal, sometimes fine-grained |
| Density (measured) | 3.02-3.05 g / cm 3 |
| Density (estimated) | 3.08 g / cm 3 |
Optical Properties
| Type of | biaxial (+) |
| Refractive indices | n α = 1.644-1.648; n β = 1.654-1.657; n γ = 1,661—1,668 |
| 2V angle | measured: 74 ° to 88 °, calculated: 80 ° |
| Maximum birefringence | δ = 0.017-0.020 |
| Optical relief | moderate |
| Optical axis dispersion | weak |
Crystallographic Properties
| Syngonia | rhombic (orthorhombic) |
| Symmetry class | rhombo, bipyramidal |
| Cell options | a = 3.693 Å; b = 12.221 Å; c = 2.865 Å |
| Axis ratio | a: b: c = 0.302: 1: 0.234 |
| Number of formula units (Z) | four |
| Unit cell volume | V 129.30 Å 3 (calculated according to unit cell parameters) |
Chemical Composition
Rarely does an analysis show up to 98% AlO (OH). The usual impurities are Fe 2 O 3 (up to 3.2%), SiO 2 (up to 5.3%), TiO 2 (up to 4.1%), sometimes CaO (up to 1.8%), MgO (up to 1%) , Na 2 O + K 2 O (up to 0.6%), associated mainly with mechanical impurities and adsorption , although some of them are isomorphic (Fe). Of particular note isomorphic impurities Ga 2 O 3 (up to 0.07%) and Sc.
Crystal Form
In cracks and pores among bauxite , as well as decomposition products, nepheline is established in the form of very small lamellar or lenticular crystals . It is usually distributed in the form of cryptocrystalline mass or in collomorphic formations (in bauxite)
Diagnostic signs
Macroscopically, it is difficult to distinguish from gibbsite , with which it often forms very close mixtures ( bauxites ). Differences can be detected only in explicit crystalline precipitates or radiographically and thermographically. Before the soldering tube does not melt, whitens and splits according to cleavage . It releases water in a glass tube. It is insoluble in acids.
Mineral Association
Hydrargillite (gibbsite) , diasporas , kaolinite . Kaolinite, gibbsite, diaspora ( bauxite ). Nepheline , gibbsite, diaspora, natrolite , analcime ( nepheline pegmatitis ).
Origin
It is usually formed during the decomposition and hydrolysis of aluminum silicates (especially when weathering feldspars) in tropical climates. Boehmite is an integral part of aluminum ore - bauxite .
Deposits
For some time, boehmite was known only in exogenous bauxite deposits . It was found in the ancient weathering crust in the Yakovlevsky deposit ( KMA ) in association with kaolinite on metamorphic schists of an iron-magnesian-alumina composition. Later it was established as a low - temperature hydrothermal mineral in the form of small crystals in voids among pegmatites of alkaline rocks in the Cherry Mountains ( Middle Urals ) in association with water-transparent gibbsite on needle crystals of zeolite (natrolite).
Practical Importance
Ore to aluminum .
Literature
- Betekhtin A. G. Mineralogy course: study guide / A. G. Betekhtin; under the scientific. ed. B.I. Pirogov and B. B. Shkursky. - M.: KDU, 2008 .-- 374 p.: Ill., Tab. - ISBN 978-5-98227-122-8
- R. Ubelt Key to minerals = Mineral - bestimmungsbuch / R. Ubelt; per. with him. T. B. Zdorik and V. P. Kolchanova. - M.: Mir, 1978.- 102 p.
- Godovikov A. A. Mineralogy / A. A. Godovikov. - M.: Nedra, 1975 .-- 192 p.
- S. A. Kirillova, A. V. Smirnov, B. A. Fedorov, A. A. Krasilin, A. N. Bugrov, K. G. Gareev, I. E. Gracheva, V. I. Almyashev / Morphology and dimensional parameters of boehmite nanocrystals obtained under hydrothermal conditions. - The journal "Nanosystems" NRU ITMO. - SPb.- UDC 544.08