Interesting Things: What is Labradorite?
Labradorite is a feldspar mineral of the plagioclase series that is most often found in mafic igneous rocks such as basalt, gabbro, and norite. It is also found in anorthosite, an igneous rock in which labradorite can be the most abundant mineral.
Some specimens of labradorite exhibit a schiller effect, which is a strong play of iridescent blue, green, red, orange, and yellow colors as shown in the photographs. Labradorite is so well known for these spectacular displays of color that the phenomenon is known as “labradorescence.” Specimens with the highest quality labradorescence are often selected for use as gemstones.
What Causes Labradorescence?
Labradorescence is not a display of colors reflected from the surface of a specimen. Instead, light enters the stone, strikes a twinning surface within the stone, and reflects from it. The color seen by the observer is the color of light reflected from that twinning surface. Different twinning surfaces within the stone reflect different colors of light. Light reflecting from different twinning surfaces in various parts of the stone can give the stone a multi-colored appearance.
Physical Properties of Labradorite
Color Usually clear, white, or gray in reflected light. Labradorescent colors can include blue, green, yellow, orange, and red.
Streak White Luster Vitreous, pearly on cleavage faces
Diaphaneity Transparent to translucent
Cleavage Two directions of perfect cleavage intersecting at about 86 degrees
Mohs Hardness 6 to 6.5
Specific Gravity 2.68 to 2.72
Diagnostic Properties Hardness, cleavage (twinning and labradorescence are only shown by some specimens)
Chemical Composition (Na,Ca)(Al,Si)4O8 with Na (30-50%) and Ca (70-50%) Crystal System Triclinic
Properties of Labradorite
Labradorite is a mineral in the plagioclase series, and it shares many of the properties of plagioclase minerals. It has a Mohs hardness of about 6 to 6 1/2 and two distinct directions of cleavage that intersect at an angle of about 86 degrees or 94 degrees. Plagioclase minerals frequently exhibit twinning and striations on cleavage faces.
Labradorite is the only mineral in the plagioclase series that exhibits strong labradorescence; however, many specimens of labradorite do not exhibit the phenomenon. Without seeing labradorescence, distinguishing labradorite from other members of the plagioclase series can be difficult. The methods used for distinguishing them are x-ray diffraction, chemical analysis, optical tests, and specific gravity determinations on pure specimens.