Laterite

Description

Laterite is a residual product of rock weathering, characterized by its high content of iron and aluminum oxides and hydroxides, and low content of silica. It typically forms a hard, crust-like layer near the surface. Its color is predominantly reddish-brown to dark red due to the presence of hematite and goethite. The texture can vary from earthy and friable to hard and indurated, often exhibiting pisolitic or oolitic structures. The term 'laterite' was coined by Francis Buchanan-Hamilton in 1807 from the Latin word 'later', meaning brick, referring to its use as a building material.

How to Identify

Color

Typically reddish-brown to dark red, often mottled with yellow, brown, or black. The color is due to the presence of iron oxides (hematite, goethite).

Luster

Dull to earthy, sometimes sub-vitreous in more indurated varieties.

Texture

Highly variable, ranging from earthy, porous, and friable to hard, dense, and indurated. Often exhibits pisolitic (pea-sized concretions) or oolitic (egg-shaped concretions) structures. Can also be massive or vesicular.

Crystal Form

Amorphous to cryptocrystalline aggregates of various iron and aluminum oxyhydroxides. Individual crystals are rarely macroscopic.

Cleavage

None, as it is a rock composed of fine-grained or amorphous minerals.

Geological Environment

Forms in tropical and subtropical regions with high rainfall, high temperatures, and distinct wet and dry seasons. Requires well-drained upland surfaces or gently sloping terrains where chemical weathering is intense and prolonged, leading to the leaching of soluble components from parent rocks (igneous, metamorphic, or sedimentary).

Key Facts

• Hardness: Variable, from 1 (earthy) to 7 (indurated, due to quartz or highly cemented iron oxides) on the Mohs scale.
• Specific Gravity: 2.5 to 3.5, depending on porosity and mineralogical composition.
• Crystal System: Amorphous to cryptocrystalline aggregates; individual mineral components (e.g., goethite, hematite, gibbsite) have their own crystal systems (e.g., orthorhombic for goethite, trigonal for hematite, monoclinic for gibbsite).
• Color: Reddish-brown, dark red, yellow, brown, black, often mottled.
• Luster: Dull, earthy, sometimes sub-vitreous.
• Transparency: Opaque.
• Fracture: Earthy, conchoidal (in indurated varieties), irregular.
• Cleavage: None.
• Composition: Primarily hydrated oxides of iron (goethite, hematite, ferrihydrite) and aluminum (gibbsite, boehmite, diaspore), with minor amounts of kaolinite, quartz, and residual primary minerals.

Quick Check

• Color: Reddish-brown to dark red
• Luster: Dull to earthy
• Streak: Reddish-brown

Physical Characteristics

• Crystal Habit: Massive, earthy, pisolitic, oolitic, vesicular, concretionary.
• Cleavage Type: Not applicable (no cleavage).
• Fracture Type: Earthy, irregular, sub-conchoidal.
• Tenacity: Friable to brittle, depending on induration.
• Luster Type: Dull to earthy.

Formation

Laterite is a soil and rock type rich in iron and aluminum, formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of iron oxides. They are formed by intense and prolonged weathering of the underlying parent rock. Tropical weathering (laterization) is a prolonged process of chemical weathering which produces a wide variety of rocks and soils. The primary mechanism is the leaching of soluble elements (such as silicon, sodium, potassium, calcium, and magnesium) by percolating rainwater, leaving behind insoluble elements, predominantly iron and aluminum oxides and hydroxides. This process is most effective in areas with high rainfall, high temperatures, and good drainage, allowing for continuous removal of leached components.

Usage

Laterite is a significant source of iron ore (ferricrete), aluminum ore (bauxite), nickel, and cobalt. It is also used as a building material, particularly in tropical regions, where it can be cut into blocks and hardens upon exposure to air. Its use in road construction and as a low-grade aggregate is also common.

Age Distribution

Predominantly Cenozoic to Recent, but can form on older geological units under suitable conditions.

Where to Find

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Scientific Classification

Mineral Class

Not a single mineral, but a rock type composed of various oxide and hydroxide minerals.

Group

Residual weathering product.

Crystal System

Not applicable for the rock as a whole; constituent minerals have their own crystal systems.

Chemical Formula

Variable, but generally represented as FeO(OH)·nH2O (goethite), Fe2O3 (hematite), Al(OH)3 (gibbsite), AlO(OH) (boehmite, diaspore).

Composition

Hydrated iron oxides (e.g., goethite, hematite), hydrated aluminum oxides (e.g., gibbsite), kaolinite, quartz, and other residual minerals. The exact proportions vary widely.

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