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Labradorite
A variety ofAnorthite
This page kindly sponsored byLeslie Almberg
Aplagioclase feldspar with an albite : anorthite molar ratio ranging from 30 : 70 to 50 : 50.
An intermediate member of theAlbite-Anorthite Series.
Labradorite can display an iridescent optical effect (or schiller) known aslabradorescence.
The cause of this optical phenomenon is lamellar structure caused by an exsolution process due to a miscibility gap. The effect is visible when the lamellar separation is between 128 and 252 nm. The lamellae are not necessarily parallel, and the lamellar structure is found to lack long range order (Bolton et al.,1966). Results of a modern electron diffraction study were published by Götz et al. (2022).
The lamellar separation only occurs in plagioclases of a certain composition, in particular, those of calcic labradorite andbytownite (anorthite content of ~60 to 90%). 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. Thus, not all labradorites exhibit labradorescence (they might not have the correct composition and/or they cooled too quickly). Also, not all plagioclases that exhibit labradorescence are labradorites (they may be bytownite).
Originally found at Ford Harbour, Paul Island, near Nain, off the east coast of Labrador, Newfoundland and Labrador [56° 30' N, 61° 30' W].
Visit gemdat.org forgemological information about Labradorite.
An intermediate member of theAlbite-Anorthite Series.
Labradorite can display an iridescent optical effect (or schiller) known aslabradorescence.
The cause of this optical phenomenon is lamellar structure caused by an exsolution process due to a miscibility gap. The effect is visible when the lamellar separation is between 128 and 252 nm. The lamellae are not necessarily parallel, and the lamellar structure is found to lack long range order (Bolton et al.,1966). Results of a modern electron diffraction study were published by Götz et al. (2022).
The lamellar separation only occurs in plagioclases of a certain composition, in particular, those of calcic labradorite andbytownite (anorthite content of ~60 to 90%). 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. Thus, not all labradorites exhibit labradorescence (they might not have the correct composition and/or they cooled too quickly). Also, not all plagioclases that exhibit labradorescence are labradorites (they may be bytownite).
Originally found at Ford Harbour, Paul Island, near Nain, off the east coast of Labrador, Newfoundland and Labrador [56° 30' N, 61° 30' W].
Visit gemdat.org forgemological information about Labradorite.Unique Identifiers
Streak:
White
Hardness:
6 - 6½ onMohs scale
Cleavage:
Perfect