Earth Sciences / 29which are separated from the rock industrially by washing and precipitation, leaving a residue of
quartz grains (a white sand) and mica. About 15 per cent of the material is recovered as kaolin
(www.kaolin.com/ccpmin.htm), 10 per cent is mica waste, and 75 per cent is sand, which is also
waste although it has found some use for building and landscaping. In some places the kaolinization
process has been completed from above, possibly by humic or other acids from overlying organic
material, but most of the kaolinite formed at depth is overlain by unaffected granite, probably because
the upward movement of acidic fluids was halted by the absence of veins or joints it could attack.
The resulting deposits are funnel-shaped, extending in places to depths of more than 300 m.
Bauxite, the most important ore of aluminium, is also produced by the chemical weathering of
feldspars, in this case by hydration. Bauxite is a mixture of hydrous aluminium oxides and hydroxides
with various metals as impurities; to be suitable for mining it should contain 25–30 per cent of
aluminium oxide.
Bauxite is a variety of laterite, one product of the kind of extreme weathering of soil called
‘laterization’. The word ‘laterite’ is from the Latin later, meaning ‘brick’, and laterite is brick-hard.
Laterization occurs only in some parts of the seasonal tropics, where soils are derived from granite
parent material, but it is possible that removing the forest or other natural vegetation in such areas
may trigger the formation of laterites. These can be broken by ploughing.
Except on steep slopes, tropical soils overlying granite can be up to 30 m deep. Naturally acidic water
from the surface percolates through them, steadily eating away at the parent rock beneath, and plants
draw the water up again through their roots. Water is also drawn upward by capillary attraction through
tiny spaces between soil particles and evaporates from the surface. If the rainfall is fairly constant
through the year, the movement of water is also constant, but if it is strongly seasonal, evaporation
exceeds precipitation during the dry season and mineral compounds dissolved in the soil water are
precipitated, the least soluble being precipitated first. Provided vegetation cover is adequate, with roots
penetrating deep into the soil, the minerals will not accumulate in particular places and when the rains
return they will be washed away. If there is little plant cover, however, they may accumulate near the
surface. The most insoluble minerals are hydroxides of iron and aluminium (kaolinite) and they are
what give many tropical soils their typically red or yellow colour (HOLMES, 1965, pp. 400–401). Soil
developed over granite will contain sand, or quartz grains, and clays derived from feldspars in varying
amounts. Figure 2.5 shows how these can grade almost imperceptibly from one to the other and from
both into laterite. Laterite layers or nodules are hard, but not usually thick, because, being impermeable,
they prevent further percolation of water downwards into the soil and thus bring the laterization process
to an end. Erosion of the surface layer may then expose the laterite.
Figure 2.5 Gradation of clay and sand to laterite
Source: Holmes, Arthur. 1965. Principles of Physical Geology. Nelson, Walton-
on-Thames