saprolite

Saprolite is a weathered, chemically altered rock that has remained in place, forming a thick, soft, clay-rich layer derived from its parent rock. It results from in situ chemical weathering near the surface, during which primary minerals are partly or wholly depleted while the rock’s structure can be preserved.

The mineralogy of saprolite is dominated by secondary products such as kaolinite, halloysite, gibbsite, and iron oxides like goethite and hematite. Quartz and more resistant minerals may persist, giving a texture that can be finely layered or massive and friable. The exact composition depends on the parent rock and the climatic regime.

Formation and context: Saprolite develops through prolonged chemical weathering with leaching of mobile elements (alkalis, calcium, magnesium, silica) and simultaneous formation of secondary minerals. It is common beneath tropical and subtropical soils where high rainfall drives deep weathering, though it can occur in other climates. Groundwater movement can promote redistribution of iron and other elements within the layer.

Distribution and relation to other units: Saprolite grades into saprock at depth and is overlain by regolith or soil above. Thickness varies from a few centimeters to tens of meters, reflecting landscape and climate history. In many regions it forms the weathered basement for lateritic soils and associated ore deposits.

Economic and practical significance: Saprolite hosts important ore deposits in tropical regions, notably nickel-cobalt laterites derived from saprolite. It is also a major source of kaolin clay and is a focus in mining and geotechnical studies due to its variable strength and water sensitivity.

Etymology: The term saprolite derives from Greek sapros, meaning rotten, and lithos, meaning stone.