Labradorite may be the plagioclase feldspar most often faceted like a gem. It happens in orange, yellow, colourless and red however the material that shows a play of colour, or “schiller”, is easily the most popular to be used in jewellery.
Happens in metamorphic and igneous rocks in Labrador (Canada), Finland, Norway and the first kind USSR.
Properties 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:
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:
Triclinic System
Specific Gravity:
2.63 - 2.72
Cleavage / Fracture:
Easy 2 directional / Uneven.
Optic Character:
Anisotropic,
D.R. ; Biaxial positive
Refractive Index / Birefringence:
1.560 - 1.572 / 0.009
Pleochroism:
In dark colored labradorite.
Magnification:
Oriented needles or platelets of magnetite, twin planes, fingerprints and crystal inclusions.
U.V. Fluorescence:
Variable.
Spectrum:
Not characteristic.
Cause of Color:
Play of color due to interference of light from twin planes.
Treatment (Enhancement):
None.
Simulants (with key separation tests):
Transparent Labradorite from quartz (optic figure), scapolite (optic figure), beryl (optic figure), Iolite (pleochroism, inclusions, U.V. fluorescence) Spectrolite from black opal (R.I. , S.G. , structure) Geological Occurrence:
Igneous rocks, pegmatite.
Sources:
U.S.A. , India, Canada, Australia, Madagascar, Russia, Mexico.
Cuts & Uses:
Facetted, cabochon, beads and carving.
