About Labradorite

About Labradorite

 

Formula: (Ca,Na)[Al(Al,Si)Si2O8]
Colour: Pale green, blue, colourless, grey-white
Luster: Sub-Vitreous
Hardness: 6 – 6½
Member of: Feldspar Group

Name: From the occurrence at Ford Harbour, Paul Island, Labrador, Canada (Nain anorthosite).
First Recorded Locality: Ford Harbour, Paul Island, Nain Complex, Labrador, Newfoundland and Labrador, Canada
A variety of Anorthite A plagioclase feldspar with an albite: anorthite molar ratio ranging from 30:70 to 50:50. An intermediate member of the Albite-Anorthite Series. Labradorite can display an iridescent optical effect (or schiller) known as labradorescence.

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 to 252 nm. The lamellae are not necessarily parallel, and the lamellar structure is found to lack long range order. The lamellar separation only occurs in plagioclases of a certain composition, in particular, those of calcic labradorite and bytownite (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.

Originally found at Ford Harbour, Paul Island, near Nain, off the east coast of Labrador, Newfoundland and Labrador [56° 30′ N, 61° 30′ W].

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