Feldspar

The name feldspar has come from the German “feld” meaning field and “spar” meaning
stone. Feldspar is a group of minerals which are primarily high-silica aluminosilicates with
varying amounts of sodium, potassium and calcium and a large number of combinations. The
common feldspars are orthoclase (potassium feldspar, K2O.Al2O3. SiO2 ), albite (sodium
feldspar, Na2O.Al2O3.6SiO2) and anorthite (calcium feldspar, CaO.Al2O3.2SiO2). Albite and
anorthite are completely miscible and one may progressively merge into the other forming an
isomorphous series known as plagioclase series with each of them occupying the two end
positions. Members of the plagioclase series are:

  1. Albite (100-90% albite and rest anorthite)
  2. Oligoclase (90-70% albite and rest anorthite)
  3. Andesine (70-50% albite and rest anorthite)
  4. Labradorite (50-30% albite and rest anorthite)
  5. Bytownite (30-10% albite and rest anorthite)
  6. Anorthite (10-0% albite and rest anorthite)
    Out of the feldspar minerals, only orthoclase is monoclinic while the rest are all triclinic.
    But, there are two triclinic variations of orthoclase, namely, microcline and anorthoclase
    (contains sodium in addition to potassium). Hyalophane and celsian are two relatively rare
    barium-containing feldspars.
    Feldspars especially orthoclase are the most abundant rock forming minerals in the
    earth’s crust making up about 60% of igneous, 30% of metamorphic and 10% of sedimentary
    rocks. The sodium- and potassium-bearing acidic feldspars are common in acidic rocks like
    granite, pegmatite, syenite, etc., while the basic calcium-bearing ones are found in basic rocks
    like basalt, gabbro, etc.
    From the economic point of view, the most significant are the two potassium feldspars
    (orthoclase and microcline) that occur as segregations in large masses. Generally, microcline
    is included in orthoclase as they cannot be differentiated without microscopic examination.
    Some of the other varieties have only some minor uses. In the commercial circle, orthoclase
    (including microcline) is generally understood by the name feldspar. Massive quantities of
    feldspar of commercial importance are mostly found in pegmatite, which is its main source.