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 ).
Chemical Composition:
Aluminum silicate of potassium, sodium and calcium. KAlSi
3 O
8 .
Isomorphous series made up of Albite (Na), Oligoclase, Andesine,
Labradorite, Bytownite and Anorthite (Ca) where the two end member are
NaAlSi
3 O
8 and CaAl
2 Si
2 O
8 .
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, starPlagioclase :
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.
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
Geological Occurrence:
Igneous rocks, pegmatite.
