Labradorite

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 and 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.

Labradorite
Labradorite in a polished rock slab
General
Category	Adularescence, 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, gray-white, brown, greenish, pale green, 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	[1][2][3]

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, Finland and various other locations worldwide, with notable distribution in Madagascar, China, Australia, Slovakia and the USA.[2]

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.[4] 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.[1]

Labradorescence

See also: adularescence

Labradorescence in labradorite

Labradorite can display an iridescent optical effect (or schiller) known as labradorescence. The term labradorescence was coined by Ove Balthasar Bøggild, who defined it (labradorization) as follows:[5]

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 Robert Strutt, 4th Baron Rayleigh (1923), and by Bøggild (1924).[5][6][7]

The cause of this optical phenomenon is phase exsolution lamellar structure,[8] occurring in the Bøggild miscibility gap.[9] The effect is visible when the lamellar separation is between 128 to 252 nm (5.0×10⁻⁶ to 9.9×10⁻⁶ in); the lamellae are not necessarily parallel;[9] and the lamellar structure is found to lack long range order.[10]

The lamellar separation only occurs in plagioclases of a certain composition; those of calcic labradorite and bytownite (anorthite content of ~60 to 90%) particularly exemplify this.[8][11] Another requirement for the lamellar separation is very slow cooling of the rock that contains the plagioclase. Slow cooling is required to allow the Ca, Na, Si, and Al ions to diffuse through the plagioclase and produce the lamellar separation. Therefore, not all labradorites exhibit labradorescence (they might not be the correct composition and/or they cooled too quickly), and not all plagioclases that exhibit labradorescence are labradorites (they may be bytownite).

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

Gallery

• 
  Polished block from Madagascar
• 
  Detail of labradorite
• 
  Polished labradorite 18x20 cm
• 
  Polished labradorite from UCL Geology collections
• 
  Labradorite with rare colors
• 
  Polished labradorite

See also

• Aventurescence
• Larvikite
• Opal
• Lapis lazuli

References

1. Handbook of Mineralogy
2. Mindat.org
3. Webmineral data
4. Hurlbut, Cornelius S.; Klein, Cornelis; Manual of Mineralogy, Wiley, 1985, 20th ed., p. 456 ISBN 0-471-80580-7
5. Bøggild, Ove Balthasar (1924), "On the Labradorization of the Feldspars" (PDF), Kongelige Danske Videnskabernes Selskab, Mathematisk-fysiske Meddelelelser, 6 (3): 1–79
6. Raman, Chandrasekhara Venkata; Jayaraman, Aiyasami (July 1950). "The structure of labradorite and the origin of its iridescence". Proceedings of the Indian Academy of Sciences, Section A. 32 (1): 1–16. doi:10.1007/BF03172469.
7. Lord Rayleigh (3 April 1923), "Studies of Iridescent Colour and the Structure Producing it. III. The Colours of Labrador Felspar", Proceedings of the Royal Society of London. Series A, The Royal Society, 103 (720): 34–45, doi:10.1098/rspa.1923.0037, JSTOR 94093
8. Yan-ju, Peng; Xue-mei, He; Qin-fang, Fang (May 2008), "Exsolution lamellar structure causes of iridescence in labradorite: evidence from TEM", Acta Petrologica et Mineralogica
9. Hao, Xie; Jing-cheng, Pei; Li-ping, Li (February 2006), "Relation Between Labradorescence and Internal Structure of Labradorite", Geological Science and Technology Information
10. Bolton, Herbert Cairns; Bursill, Leslie Arthur; McLaren, Alexander Clark; Turner, Robin G. (1966). "On the origin of the colour of labradorite". Physica Status Solidi B. 18: 221. doi:10.1002/pssb.19660180123.
11. MacKenzie, William Scott; Zussman, Jack, eds. (1974), "23. Electron-optical study of a schiller labradorite", The Feldspars: Proceedings of a NATO Advanced Study Institute, Manchester, 11–21 July 1972, Manchester University Press, 2, pp. 478–490

• Chisholm, Hugh, ed. (1911). "Labradorite" . Encyclopædia Britannica (11th ed.). Cambridge University Press.