Feldspar

Feldspar is a group of minerals which comprises a number of species with varying degrees of isomorphism. Feldspars are subdivided into two sub group, potassium feldspars (orthoclase, microcline) and plagioclase feldspar.

Of gem interest are orthoclase, microcline and plagioclase (albite, labradorite and oligoclase).





Feldspar Properties

Chemical Composition: 
Aluminum silicate of potassium, sodium and calcium. KAlSi3O8. Isomorphous series made up of Albite (Na), Oligoclase, Andesine, Labradorite, Bytownite and Anorthite (Ca) where the two end member are NaAlSi3O8 and CaAl2Si2O8.
Colors / Varieties: 
  • Orthoclase: Moonstone (all colors); Transparent: yellow, pink and white; Phenomena: sheen, chatoyancy, asterism, adularescence (blue sheen)
  • Microcline: Amazonite (mottled green blue, grayish white, brownish red); Phenomena: sheen, chatoyancy, star
  • Plagioclase:
    • Albite: Moonstone (orthoclase-albite)
    • Labradorite: Transparent (white, grey, yellow); Phenomena: chatoyancy, asterism, play of color i.e., labradorescence); Spectrolite: dark background with play of color.
    • Oligoclase: Sunstone: Phenomena: aventurescent, chatoyancy
Crystal System / Forms: 
Well formed crystals as well as massive or tabular habits. Commonly exhibit lamellar twinning.
Specific Gravity: 
2.55 - 2.72
Cleavage / Fracture: 
Easy 2 directional / Uneven.
Optic Character: 
Anisotropic, D.R.
Lustre: 
Vitreous
Dispersion: 
0.012
U.V. Fluorescence: 
Variable
Spectrum: 
Not characteristic
Cause of Color: 
In all species chatoyancy and asterism is due to the presence of parallel, oriented inclusions.
Treatment (Enhancement): 
None
Synthesis: 
None
Geological Occurrence: 
Igneous rocks, pegmatite.