BÖHMITE
Böhmite (or “bohemite”) – AlO(OH) – is a very common hydroxide, typical of residual soils. It is one of the main constituents of bauxite and thus an important Al ore.
It is dimorphic with diaspore and isostructural with lepidocrocite. May contain Fe, Mn and Cr.
Due to the generally submicroscopic size of its crystals, the reliable identification of böhmite requires analytical methods other than microscopy. For example, X-ray diffraction is used. The tiny crystals are only visible with the Scanning Electron Microscope (SEM).
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: White, pale grayish-brown. When impure, yellowish or reddish.
Habit: Rare crystals, tabular to short-prismatic, up to 2 mm. Generally extremely fine-grained, disseminated or in pisolitic aggregates.
Cleavage: {010} very good, {100} good, {001} poor.
Tenacity: No information available.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 3.5
Parting: No.
Streak: White.
Lustre: Vitreous to pearly
Diaphaneity: Transparent.
Density (g/cm³): 3.02 – 3.05
2. Geology and Deposits
Boehmite forms from the alteration of aluminosilicates and, following the leaching of the altered rock, is found in residual soils, laterites, latosols, bauxites, and related deposits, typically in tropical climates.
In bauxite, it is one of the three aluminum ore minerals, alongside diaspore and gibbsite.
It occurs as a product of low-temperature hydrothermal decomposition of corundum and nepheline in nepheline pegmatites, syenites, and mid-ocean ridge basalts.
It may occur in association with coal.
3. Mineral Associations
In laterites and bauxites, it occurs associated with other Al oxides and hydroxides such as diaspore, gibbsite, and nordstrandite, as well as quartz, clay minerals (kaolinite), and Fe and Mn oxides and hydroxides.
In nepheline pegmatites, it occurs with various zeolites such as natrolite (including the “Spreustein” variety), thomsonite-(Ca), and phillipsite-(K), as well as with vesuvianite, garnet (grossular, hibschite variety), and rare minerals like kochsandorite and sudoite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.644 – 1.648 nβ: 1.654 – 1.657 nγ: 1.661 – 1.668
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Moderate.
Cleavage: {010} very good, {100} good, and {001} poor. Cleavages are rarely visible.
Habits: Generally extremely fine-grained, submicroscopic, below the resolution of an optical microscope. Rare crystals are tabular or short-prismatic, up to 2 mm.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.017 to 0.020, corresponding to interference colors up to the end of the first order: gray, yellow, orange, and red.
Extinction: Parallel in the prismatic crystals.
Elongation sign: ES(+) or ES(-), it is not diagnostic.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(+) or B(-). Generally, it is not possible to obtain the interference figure due to the minute size of the grains.
2V angle: 74 – 88º
Alterations: Dehydration can lead to the formation of cubic alumina. The addition of silica can transform boehmite into clay minerals.
May be confused with: other minerals, including others from the same paragenesis.
Gibbsite has lower refractive indices and oblique extinction.
Diaspore has higher refractive index and birefringence.
5. Reflected Light Microscopy
Reflected-light microscopy is evidently not the recommended analytical method for the identification of boehmite. It is not possible to identify boehmite under reflected light.