MINERALS IN EARTH CRUST

Earth's crust and mantle consist almost entirely of minerals, yet the number of known minerals is less than 3,000. Two factors limit the number of possible and actual minerals. First, a crystal's atoms must be arranged in some periodically repeating, three-dimensional pattern, but only a finite number of such patterns exist. Second, there are only a few score naturally occurring elements, many of which are rare and eight of which—oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium, in order of decreasing commonness comprise 98.5% of Earth's crust by weight. Oxygen alone makes up approximately 47% of the crust by weight and silicon makes up approximately another 27%. The number of minerals that can form is therefore finite, and many of those that could theoretically form do so rarely.
The atoms of the two most common elements on earth, silicon and oxygen, readily arrange themselves into tetrahedra having a silicon atom at the center and an oxygen atom at each point. This unit is the silicate radical, (SiO4)4−. Silicate radicals can link into sheets, chains, or three-dimensional frameworks by sharing oxygen atoms. If every oxygen atom participates in two tetrahedra, then the overall ratio of silicon to oxygen is 1:2, and the resulting chemical formula is that of silica, SiO2. Minerals built mostly of silica are termed silicate minerals. The mineral quartz is pure crystalline silica; other silicate minerals result when atoms of elements other than silicon are introduced at regular intervals. For example, some of the tetrahedra in the silicate framework may be centered on aluminum atoms rather than silicon atoms. In this case, atoms of other elements (usually calcium, potassium, or barium) must be present to balance the ionic charges in the framework. The silicate minerals having this particular structure are the feldspars, which make up approximately 60% of the earth's crust by volume.