Aluminum (aluminium) is a staple of civilization, but you'd never know it even exists without the help of chemistry. In nature, aluminum invariably exists in two kinds of mineral compounds: silicates and oxides. Aluminum metal combines with oxygen so quickly that a molecule-thin skin of aluminum oxide forms almost instantly on its surface and protects it from further corrosion. Powdered aluminum oxidizes so quickly and explosively that it's used as rocket fuel and firework propellant. Only the rarest conditions, notably some highly reduced volcanic materials, allow the metal to exist in nature.
Aluminum is the most abundant metallic element in the Earth's crust and the crust's third-most abundant element, after oxygen and silicon (a semi-metal). These three elements are therefore intimately mixed in many minerals, aluminum and silicon fighting for oxygen's attention. When the tidy ranks of linked silica tetrahedra are forced to admit some of these abundant aluminum atoms, the result is one of a large number of aluminosilicate minerals.
The major aluminosilicates in sedimentary rocks, by volume, are feldspar, mica and clay. In metamorphic rocks, feldspar and mica are joined by the three sister minerals kyanite, andalusite and sillimanite (Al2SiO5). Aluminosilicates in igneous rocks include feldspar, mica and some of the garnets. Down in the mantle, the main aluminosilicate is garnet.
The mineral form of straight aluminum oxide (Al2O3) is the metamorphic mineral corundum, second only to diamond in hardness. Spinel, which also includes magnesium and other metals, is the other major oxide compound of aluminum and is found in metamorphic and some mafic igneous rocks. In the mantle, spinel is the main aluminum oxide.
The aluminum minerals, silicates and oxides alike, have tended over geologic time to collect in the rocks near the Earth's surface. The continental crust is much higher in aluminum than the mantle (about 8.5 percent versus about 2 percent) and correspondingly lower in iron (Fe) and especially in magnesium (Mg). Geology students are taught, therefore, that the continental crust is sialic (from "Si-Al") while the oceanic crust is simatic (from "Si-Mg") and the mantle is mafic (for "Mg-Fe"). Because the aluminum minerals are less dense than mafic minerals, the continents are permanently floating on top of the denser mantle where they resist mixing in the plate-tectonic rock cycle. In a word, aluminum is what holds us all above sea level.
The minerals I've named are important to geologists and interesting to geochemists, but none of them are useful aluminum ores. Today we produce the metal exclusively from surface deposits of highly weathered material making up bauxite. Bauxite is a very soft rock or sediment made of powdery, microcrystalline aluminum hydroxides. It forms as a laterite, that is, a soil or residue left behind after rainwater has dissolved and removed everything except the most insoluble minerals.
A good bauxite results from deep, intensive weathering of a granitic rock, which is high in aluminosilicates and low in iron minerals. (A laterite with too much iron makes a decent iron ore instead.)
Bauxite is refined into aluminum metal first by converting it to a single hydroxide (the Bayer process), then roasting that into a pure oxide (alumina). After that the alumina is dissolved in a molten formula (cryolite) so that the metal can be harvested by electrolysis at a much lower temperature than it would take to actually melt the oxide (the Hall process). About.com Metals Expert Terrence Bell gives more details.