Feldspar the name of a group of minerals that are the most abundant constituents of the igneous rocks and are the most important constituents of pegmatites and veins. They are major constituents of most gneisses and schists and are second in abundance to quartz in the arenaceous sediments.
The majority of feldspars may be classified chemically as members of the ternary system NaAlSi3O8-KAlSi3O8-CaAl2Si2O8. These compositions are referred to as sodium, potassium and calcium feldspar respectively. Members of the series between NaAlSi3O8 and KAlSi3O8 are called alkali feldspars, and those between NaAlSi3O8 and CaAl2Si2O8 are plagioclase feldspars.
The pure calcium end-member, anorthite, is triclinic, and no essentially different names are used to describe structural states at different temperatures. The potassium end-member can exist in a number of different structural states. Those with structures corresponding to the highest temperatures of crystallisation are monoclinic and are called sanidines, while a lower temperature monoclinic potassium feldspar is called orthoclase. The lowest temperature potassium feldspars are microclines. A variety of potassium feldspar, adularia, is one which is structurally inhomogeneous. The pure sodium end-member is called low-albite or high-albite (or intermediate-albite) according to the structural state. The albites are normally triclinic. Pericline is the sodium feldspar counterpart of adularia.
In Bangladesh feldspars are found in the sandstone, siltstone and shale of most of the geological group-formations that constitute the stratigraphy of the country. They are mostly altered, though, a small portion of them can be authigenic. Fresh feldspars are found in the boulders and gravels that are being collected from the border area of sylhet in the river beds of the Sari, Bolla, dauki etc. Feldspars are also reported to be present in the Hardrock that is being excavated/mined in North Bengal. [Munir Ahmed]
See also rocks and minerals.