What is a laterite in geology ?

Laterite : definition

A laterite is a residual soil rich in iron and aluminum hydroxides which develops in a humid tropical climate, mainly at the expense of eruptive or grainy metamorphic rocks.

Laterite results from the dissolution of minerals and then the evacuation of most of the elements contained in the rock (sodium, calcium, magnesium, potassium, silica). Iron and aluminum, insoluble under these conditions, then concentrate on the spot in the form of hydroxides to form a thick armor sometimes several meters : the lateritic layer. This breastplate is the upper part of the lateritic profile : it surmounts a zone of mottled clays, strongly transformed, then the saprolite zone, before arriving at the healthy bedrock. The lateritic profile can exceed 100 meters in thickness above the bedrock.

When aluminum is sufficiently abundant, laterite takes the name of bauxite. But other metals can also be concentrated during this process : the nickel deposits of New Caledonia and many gold deposits of Australia are of lateritic origin.