Aluminous ore produced by intense humid tropical
weathering of limestone, basalt, shale, marl, or virtually
any igneous, metamorphic, or sedimentary rock. Intense tropical
weathering of the parent rocks removes soluble material,
leaving the insoluble alumina, iron, and some silica behind in
laterite and bauxite deposits. It is typically a reddish brown,
yellow, or whitish rock consisting of amorphous or crystalline
hydrous aluminum oxides and hydroxides (gibbsite,
boehmite, and diaspore) along with free silica, silt, iron
hydroxides, and clay minerals. The bauxite may form concretionary,
pisolitic, or oolitic forms, and may form dense or
earthy horizons in the laterites. It may be either a residual or
transported component of lateritic clay deposits in tropical
and subtropical environments, and it is the principal economic
ore of aluminum. Several different types of bauxite may be
distinguished. Orthobauxites consist of massive red gibbsite
and are typically overlain by an iron-rich crust called a hardcap.
Metabauxites are white, boehmitic deposits that have
pisolitic structures, are poor in iron, and are typically underlain
by kaolinitic nodular iron-rich crusts known as ferricrete.
They are thought to represent orthobauxites that have
been transformed into metabauxites under arid weathering
conditions. Cryptobauxites are orthobauxites that form
under extremely humid environmental conditions and are
overlain by an organic rich soft kaolinite horizon. Bauxites
are common in Jamaica and the Caribbean and occur with
laterites in India, Africa, and Asia. The term bauxite comes
from Les Baux de Provence, near Arles in southern France.
See also LATERITE.
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