Feldspar and other mineral groups
More than half of the crust is, in fact, made up of the various forms of feldspar. The next most abundant mineral groups are the pyroxene and amphibole families, whose most important members are respectively augite and hornblende. These are followed by quartz and then by mica, which includes the dark biotite and the pale muscovite. These relatively few minerals between them
form well over 90 per cent of the earth’s crust. The remainder is made up of olivine and several
18 minerals found in minor quantities in igneous rocks, such as magnetite, apatite, zircon and rutile.
To these can be added the more important minerals of metamorphic rocks, such as chlorite, serpentine and garnet, and the more widespread minerals of sedimentary rocks—the clay minerals and several carbonates, particularly calcite and dolomite—and, in still smaller quantities, haematite, pyrite, limonite, the feldspathoid family, sphene, chromite, corundum, tourmaline, spinel, copper pyrites and pyrrhotite. All these together make up well over 99 per cent of the solid crust. The remaining minerals, which include the beautiful ores that are the pride of many a collection and nearly all the gemstones, form only an exceedingly small, almost negligible, proportion of the crust. The predominant rock types in the crust are the igneous and the related metamorphic rocks, which accounts for the predominance of the minerals of which they are composed.