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Feldspar, any of a group of aluminosilicate minerals that contain calcium, sodium, orpotassium. Feldspars make up more than half of Earth’s crust, and professional literature about them constitutes a large percentage of the literature of mineralogy.
Of the more than 3,000 known mineral species, less than 0.1 percent make up the bulk of Earth’s crust and mantle. These and an additional score of minerals serve as the basis for naming most of the rocks exposed on Earth’s surface.
Description and Identifying Characteristics ‘Feldspar’ is derived from the German word feld,which means ‘field’, and spar, which means ‘crystal’. This name reflects the minerals’ abundance, as most crystalline material found in the field is feldspar. Potassium feldspars are the feldspar minerals in which the silicate tetrahedral and aluminum tetrahedra are bound with potassium ions, rather than sodium or calcium ions as in the plagioclase feldspar subgroup. The potassium feldspar group is composed of three mineral polymorphs, each having the same chemical composition, but slightly different crystal structures. Technically these are distinct minerals, but their physical properties are so similar that they are usually only identified as ‘potassium feldspar’ in the field. Using other minerals in the rock to determine the host rock’s identity is often the most useful guide to their probable identity. felsic plutonic igneous rocks and metamorphic rocks Microcline
