Minerals of Felspar group

As in the previous post we have mentioned that silicate minerals are the most abundant minerals on the earth’s crust. They constitute about 90% of the earth’s crust. Apart from the silicate minerals, there are other silicate and non-silicate minerals, which are known as common minerals, among which the felspars are the most abundant of all minerals. Feldspars are a large group of rock-forming silicate minerals, that makes up over 50% of earth’s crust. They fall into two main series; (i) K-Na felspars, called the ”alkali felspars”, and (ii) Ca-Na felspars, called the ”plagioclase felspars”.

Various colours of Feldspars

Minerals belonging to the felspars group show related physical properties.

  • Feldspars have good cleavage in two directions at an angle of 90° approximately.
  • They have a specific gravity of about 2.55-2.76.
  • They have a vitreous luster with a white streak.
  • They are found in white, grey, pink, and green colors.
  • They generally have a tabular habit.
  • Feldspars are used in the manufacturing of porcelain.

There are two sub-groups of feldspars; K-feldspars and plagioclase feldspars.

K-feldspars: 

The important members of this group are ”orthoclase” and ”microcline”. Each of these has the composition of NaAlSi3O8 but orthoclase crystallizes in the orthorhombic system and microcline in the triclinic system. Potash felspars alter readily into clay minerals, especially ”kaolinite”.

Plagioclase Felspars:

The plagioclase felspars form a complete solution series from pure ”albite”, NaAlSi3O to pure ”anorthite” CaAl2Si2O8. 

The important intermediate members are ”oligolclase”, ”andesine”, labradorite”, and ”bytownite”. The plagioclase felspars by the presence of repeated albite twin striations visible on the cleavage surface. The properties of various members of the plagioclase series vary in a uniform manner with the change in the chemical composition. The plagioclase felspars usually alter to clay minerals or ”sericite” (a fine grained micaceous material).

Occurrence:

  1. The alkali feldspars which include orthoclase, microcline, and albite have a similar occurrence. They are commonly found in igneous rocks such as granite, pegmatite, syenites, rhyolites, and trachytes.
  2. Oligoclase is commonly found in granodiorites and monzonites.
  3. Labradorite is a common constitute of gabbros, basalts, and anorthosites.

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