Laterite

Hardness: Varies greatly, often soft when wet and hard when dry (approx. 2.5-5.0 on Mohs scale when indurated). Color: Primarily reddish-brown to dark red, often mottled with yellow, brown, or black. Luster: Dull to earthy. Crystal structure: Non-crystalline; composed of a mixture of minerals. Often pisolitic (pea-sized concretions) or vermiform (worm-like structures).

Laterite forms in hot, humid tropical and subtropical regions with high rainfall. It results from intense and prolonged weathering of various underlying rocks (igneous, metamorphic, or sedimentary). This weathering process, known as laterization, leaches away soluble base cations and silica, leaving behind a residual concentration of insoluble iron oxides (like goethite and hematite), aluminum hydroxides (like gibbsite), and clay minerals (like kaolinite). The geological age varies depending on the region and the duration of the weathering process, often forming over millions of years.

Laterite has several uses. It is a significant source of iron ore (iron laterite) and aluminum ore (bauxite, which is aluminum-rich laterite). Due to its hardness when dry, it has historically been used as a building material (cut into blocks) in tropical regions, particularly for ancient monuments and temples (e.g., Angkor Wat). It can also be used as a road base material.

The term 'laterite' was first coined by Francis Buchanan-Hamilton in 1807 to describe an iron-rich soil found in parts of South India that could be cut into brick-like shapes when soft and then hardened on exposure to air. This hardening property makes it suitable for construction. Bauxite, the primary ore of aluminum, is a type of laterite that is particularly rich in aluminum hydroxides. Laterites can also host significant deposits of nickel, cobalt, and manganese.

In the field, laterite is identified by its characteristic reddish-brown to dark red color, blocky or nodular appearance, and its tendency to be relatively soft when freshly exposed and wet, but hardening considerably upon drying and exposure to air. The presence of pisolitic structures (small, rounded concretions) is also a strong indicator. It's commonly found in tropical and subtropical regions such worldwide, including India, Southeast Asia, Africa, South America, and parts of Australia. Collectors might be interested in specimens showing distinct nodular or concretional forms, or those with interesting color variations. To confirm, one might test for the presence of iron (e.g., using a magnet if iron content is high, or observing reddish streak).