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).

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.

Classification / Type: 

-

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.

Specific Tests & Remarks: 

-

Synthesis: 

None.

Geological Occurrence: 

Igneous rocks, pegmatite.

Cuts & Uses: 

Facetted, cabochon, beads and carving.