polished labradorite
Photo by A. Dougan

LABRADORITE

rough labradorite
Photo by A. Dougan

by Adam Dougan

Introduction

    Labradorite is a very interesting mineral.  Depending on the angle from which the specimen is viewed, it can appear to be a nondescript gray-green rock or it can display a grand range of colors known as labradorescence.  This phenomenon can be seen in specimens which are uncut, but it is fantastically clear in cut and polished specimens.
polished labradorite
Photo by A. Dougan

Table Of Contents

rough labradorite
Photo by A. Dougan

History

    Labradorite is a plagioclase feldspar.  It is formed with hornblende in basalt of volcanic rocks.  Labradorite gets its name from the Labrador Peninsula in Canada, the first location where labradorite was discovered. Labradorite occurs in Labrador, Newfoundland, Finland , and Russia.(3)   The finest specimens display all colors from blue through gold.  The most common colors seen are blue and green.  The finest specimens are found in Finland and these specimens are referred to by their trade name of Spectrolite.

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Physical Properties (4)


Hardness 6-6.5
Density
 2.65-2.75
Crystal System
 Triclinic
Streak
 White
Refractive Index
 1.559-1.570
Cleavage
 Perfect
Fracture
 Uneven, Splintery

    On of the physical properties used when identifying labradorite is the twinning that is common in all the plagioclase feldspar.  This twinning causes the cleaved surfaces to appear striated. (3) The twinning is called polysynthetic and is a repeated twin.

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Chemical Properties


Chemical Formula
 (Ca, Na) (Si, Al)4 O8

Chemical Composition (2)
Na
 3.38%
Ca
 8.85%
Si
 24.80%
Al
 15.88%
O
 47.09%

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Optical Properties


    The most evident feature used to identify labradorite is the labradorescence.  Labradorescence is the color display resulting from lamellar intergrowths inside the crystal. These intergrowths result from compatible chemistries at high temperatures becoming incompatible at lower temperatures and thus a separating and layering of these two phases. The resulting color effect happens because a ray of light enters a layer and is refracted back and forth by deeper layers. This refracted ray is slowed by the extra travel through the layers and mixes with other rays to produce a light ray coming out that has a different wavelength than when it went in. The wavelength could correspond to the wavelength of a particular color, such as blue. The effect depends on the thickness and orientation of the layers. (1)

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labradorite en cabochon
Photo by A. Dougan

Gem Use

 labradorite en cabochon
Photo by A. Dougan

    While the rare transparent crystals are faceted into gem stones, labradorite is usually cut and polished into ornamental objects or often used in building stone.  Translucent to opaque labradorite is polished into beads and fashioned en cabochon to be used in jewelry. (4)

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Sources

1. http://mineral.galleries.com/minerals/silicate/labrador/labrador.htm
2. http://webmineral.com/data/Labradorite.shtml
3. Chesterman, Charles W.  National Audubon Society Field Guide to North American Rocks and Minerals.  New York: Chanticleer Press, Inc., 1998.
4. Schumann, Walter.  Gemstones of the World .  New York:  Sterling Publishing Co., Inc.

This website was created for GO 340 Gemstones and Gemology, a course taught at Emporia State University under the instruction of Susan W. Aber.  If you have any questions, feel free to e-mail the author Adam Dougan.

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