Labradorite 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 (while the third direction is poor). It occurs as clear, white to gray, blocky to lath shaped grains in common maficigneous 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 Poland, Norway, Finland and various other locations worldwide, with notable distribution in Madagascar, China, Australia, Slovakia and the United States.[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 in labradoriteVideo of labradorescence in labradorite, visible as the angle of view changes
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.
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 and 252 nm (5.0×10−6 and 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 (50-70% anorthite) and bytownite (formula: (Ca0.7-0.9,Na0.3-0.1)[Al(Al,Si)Si2O8], i.e., with an anorthite content of ~70 to 90%) particularly exemplify this.[8][11] Another requirement for the lamellar separation is a very slow cooling of the rock containing 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 have 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.
^Hurlbut, Cornelius S.; Klein, Cornelis; Manual of Mineralogy, Wiley, 1985, 20th ed., p. 456, ISBN0-471-80580-7
^ abBøggild, Ove Balthasar (1924), "On the Labradorization of the Feldspars"(PDF), Kongelige Danske Videnskabernes Selskab, Mathematisk-fysiske Meddelelelser, 6 (3): 1–79, archived from the original(PDF) on April 2, 2015
^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, vol. 2, pp. 478–490
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