The most abundant mineral group found in the Earth's crust is the silicate group. The silicate minerals contain both oxygen and silicon, which are also the two most abundant elements contained in the Earth's mantle. Silicates are also the most common minerals in the rock-formation process, and it has been estimated that they comprise 75 to 90 percent of the Earth's crust.
The chemical formula for the most commonly occurring silicate is (SiO4)4-. The superscript "4-" indicates that it is a negatively charged ion, or anion, with a net negative charge of 4. Referred to as the silicon-oxygen tetrahedron, this anion is the basic building block of the minerals in the silicate group.
The additional elements that combine with the silicon-oxygen tetrahedron correspond to the other elements commonly found in the Earth's crust and mantle. These are aluminum, calcium, iron, magnesium, potassium and sodium. Most sedimentary and igneous rocks contain some combination of these silicate minerals.
The most abundant individual silicate mineral compound is quartz, which represents about 12 percent of the Earth's crust. Feldspar represent the most abundant group of silicate minerals and are estimated to account for about 50 percent of the material found in the Earth's crust. There are 22 different mineral species, or, types, of feldspars.