Bauxite
The formation of bauxite, the principal one of aluminum, is one important example of an ore created as a result of enrichment by weathering processes(Figure 422) Although aluminum is the third most abundant element in Earth's crust, economically valuable concentrations of this important metal are not common because most aluminum is tied up in silicate minerals from which it is extremely difficult to extract.
Bauxite forms in rainy tropical climates When aluminum rich source rocks are subjected to the intense and prolonged chemical weathering of the tropics most of the common elements including calcium, sodium, and potassium, are removed by leaching Because aluminum is extremely insoluble, it becomes concentrated in the soil(as bauxite, a hydrated aluminum oxide) Thus, the formation of bauxite depends on climatic conditions in which chemical weathering and leaching are pronounced plus of course, the presence of aluminum-rich source rock In a similar manner, important deposits of nickel and cobalt develop from igneous rock rich in silicate minerals such as olivine.
There is significant concern regarding the mining of bauxite and other residual deposits because they tend to occur in environmentally sensitive areas of the tropics. Mining is preceded by the removal tropical vegetation, thus destroying rain forest ecosystems. Moreover , the thin moisture- retaining layer of organic matter is also disturbed. When the soil dries out in the hot sun, it becomes bricklike and loses its moisture- retaining qualities. Such soil cannot be productively armed nor can it support significant forest growth. The long- term consequences of bauxite mining are clearly of concern for developing countries in the tropics where this important is mined.