Labradorite

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 symmetry	Triclinic pinacoidal 1
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
Crystal system	Triclinic
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 form nearly right angle prisms. 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.

Occurrence

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.

Labradorescence

See also: adularescence

Labradorescence in labradorite

Labradorite can display an iridescent optical effect (or schiller) known as labradorescence. The term labradoresence was coined by Ove Balthasar Bøggild, who defined it (labradorization) as follows:

Labradorization is the peculiar reflection of the light from submicroscopical planes orientated in one direction (rarely in two directions); these planes have never such a position that they can be expressed by simple indices, and they are not directly visible under the microscope.

Contributions to the understanding of the origin and cause of the effect were made by Rayleigh (1923), and by Bøggild (1924).

The cause of this optical phenomenon is phase exsolution lamellar structure, occurring in the Bøggild miscibility gap. The effect is visible when the lamellar separation is between 128 to 252 nanometres (5.0×10⁻⁶ to 9.9×10⁻⁶ in); the lamellae are not necessarily parallel; and the lamellar structure is found to lack long range order.

Relationships between lamellar separation and chemical composition have been found for the An (Anorthite) content (calcium rich plagioclase) of the mineral.

Some gemstone varieties of labradorite exhibiting a high degree of labradorescence are called spectrolite.

Gallery

Labradorite, Ylämaa, Finland  Polished labradorite 18x20cm  Detail of Labradorite  Polished labradorite from UCL Geology collections  Labradorite slabs used as floor tiles  Labradorite on a feldspar phase diagram

See also

• Aventurescence