Labradorescence

Labradorite
Labradorite in a polished rock slab
General
Category	Feldspar, tectosilicate
Formula (repeating unit)	(Ca,Na)(Al,Si)4O8, where Ca/(Ca + Na) (% anorthite) is between 50%–70%
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Unit cell	a = 8.155 Å, b = 12.84 Å c = 10.16 Å; α = 93.5° β = 116.25°, γ = 89.133°; Z = 6
Identification
Color	Gray, brown, greenish, blue, yellow, colorless
Crystal habit	Crystals typically thin and tabular, rhombic in cross section, striated; massive
Twinning	Common by Albite, Pericline, Carlsbad, Baveno, or Manebach twin laws
Cleavage	Perfect on {001}, less perfect on {010}, intersecting at near 90°; distinct on {110}
Fracture	Uneven to conchoidal
Mohs scale hardness	6 – 6.5
Luster	Vitreous to pearly on cleavages
Streak	white
Diaphaneity	Translucent to transparent
Specific gravity	2.68 to 2.72
Optical properties	Biaxial (+)
Refractive index	nα = 1.554 - 1.563 nβ = 1.559 - 1.568 nγ = 1.562 - 1.573
Birefringence	δ = 0.008 - 0.010
2V angle	Measured: 85°
Dispersion	None
Other characteristics	Labradorescence (iridescent)
References

Labradorite ((Ca, Na)(Al, Si)₄O₈), a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573. Twinning is common. As with all plagioclase members, the crystal system is triclinic, and three directions of cleavage are present, two of which are nearly at right angles and are more obvious, being of good to perfect quality. (The third direction is poor.) It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites.

The geological type area for labradorite is Paul's Island near the town of Nain in Labrador, Canada. It has also been reported in Norway and various other locations worldwide.

Labradorite occurs in mafic igneous rocks and is the feldspar variety most common in basalt and gabbro. The uncommon anorthosite bodies are composed almost entirely of labradorite. It also is found in metamorphic amphibolites and as a detrital component of some sediments. Common mineral associates in igneous rocks include olivine, pyroxenes, amphiboles and magnetite.