Eight percent of the Earth's crust is aluminum, and
there are hundreds of aluminum-bearing minerals and
vast quantities of the rocks that contain them. The best
aluminum ore is bauxite, defined as aggregates of alumi-
nous minerals, more or less impure, in which aluminum
is present as hydrated oxides. Bauxite is the richest of all
those aluminous rocks that occur in large quantities, and
it yields alumina, the intermediate product required for
the production of aluminum. Alumina also occurs natu-
rally as the mineral corundum, but corundum is not
found in large deposits of high purity, and therefore it is
an impractical source for making aluminum. Most of
the many abundant nonbauxite aluminous minerals are
silicates, and, like all silicate minerals, they are
refractory, resistant to analysis, and extremely difficult
to process. The aluminum silicates are therefore
generally unsuitable alternatives to bauxite because
considerably more energy is required to extract alumina
from them.