Labradorite ((Ca,Na)(Al,Si)4O8), a
feldspar mineral, is an intermediate to calcic member of the plagioclaseseries. It is usually defined as having "%An" ( anorthite) between 50 and 70. The specific gravityranges from 2.71 to 2.74. The streak is white, like most silicates. The refractive indexranges from 1.555 to 1.575. Twinning is common. As with all plagioclase members the crystal system is triclinicand 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 rockssuch as basaltand gabbro, as well as in anorthosites.
The geological type area for labradorite is
Paul's Islandnear the town of Nain in Labrador, Canada. It occurs in large crystal masses in anorthosite and shows an iridescenceor "play of colors". The iridescence is the result of light refracting within lamellar intergrowths resulting from phase exsolution on cooling in the Boggild miscibilitygap, An48-An58. Gemstonevarieties of labradorite exhibiting a high degree of iridescence are called spectrolite; moonstone and sunstoneare also commonly used terms, and high-quality samples with good iridescent qualities are desired for jewelry.
* Hurlbut, Cornelius S.; Klein, Cornelis, 1985, "Manual of Mineralogy", 20th ed., Wiley, ISBN 0-471-80580-7
* [http://webmineral.com/data/Labradorite.shtml Webmineral data]
* [http://www.galleries.com/minerals/silicate/labrador/labrador.htm Mineral galleries]
* [http://www.mindat.org/min-2308.html Mindat]
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