Bauxite is a sedimentary rock composed primarily of aluminum hydroxide minerals, making it the world’s main source of aluminum metal. The principal aluminum-bearing components are gibbsite, boehmite, and diaspore, mixed with impurities like iron oxides, titanium dioxide, and silica. It is these accessory minerals, particularly the iron compounds, that give bauxite its characteristic appearance.
Visual Appearance and Texture
The most recognizable feature of bauxite is its color, which varies widely but often includes hues of reddish-brown, cream, yellow, or tan. The presence of iron oxides such as hematite and goethite in the rock matrix is responsible for the deep reddish-brown and orange-brown staining seen in many deposits. Conversely, bauxite with very low iron content can appear white, gray, or off-white.
Bauxite typically has a dull or earthy luster, meaning it does not reflect light brightly like a metallic or glassy mineral. The rock’s texture is often nodular and can appear massive, but its most distinguishing characteristic is a feature called pisolitic structure. Pisolites are small, rounded, pea-like concretions that give the rock a granular or bumpy feel. They are visible on a broken surface and are formed of concentric bands of different mineral compositions.
The appearance can range dramatically depending on the deposit’s formation; some bauxites are soft, easily crushed, and lack internal structure, while others are hard and dense. Many lateritic bauxites are mottled in appearance, with the iron-rich portions forming a rough, sometimes worm-like structure on exposed surfaces.
Key Physical Properties for Identification
Bauxite is a relatively soft material, which is used for its field identification. Its hardness falls between 1 and 3 on the Mohs scale, meaning it can often be scratched with a knife or even a fingernail, depending on the specific mineral composition. Unlike many common rocks and minerals, bauxite lacks a distinct cleavage, meaning it does not break along smooth, predictable planes.
Despite its softness, bauxite can feel heavier than expected for a light-colored rock due to its specific gravity, which typically ranges from 2.0 to 2.5. The overall texture is often porous, reflecting its formation process. Bauxite is not a crystalline material and is sometimes described as having an amorphous appearance.
Geological Setting and Occurrence
Bauxite forms in the upper layers of the Earth’s crust, usually as a result of intense chemical weathering of aluminum-rich source rocks, such as granite, basalt, or shale. This process, known as lateritization, requires high rainfall and warm, tropical or subtropical climates with good drainage. The weathering leaches out soluble minerals like silica, leaving behind a residue rich in hydrated aluminum oxides.
The rock is found in blanket-like deposits near the surface, often forming a layer within a larger laterite soil profile. Major global deposits are concentrated in tropical regions, including Guinea, Australia, Jamaica, and Brazil. Because of its near-surface occurrence, bauxite is typically extracted using strip mining methods.