- Labradorite
Labradorite ((Ca,Na)(Al,Si)4O8), a
feldspar mineral , is an intermediate to calcic member of theplagioclase series. It is usually defined as having "%An" (anorthite ) between 50 and 70. Thespecific gravity ranges from 2.71 to 2.74. The streak is white, like most silicates. Therefractive index ranges from 1.555 to 1.575. Twinning is common. As with all plagioclase members the crystal system istriclinic and 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 commonmafic igneous rocks such asbasalt andgabbro , as well as inanorthosite s.The geological type area for labradorite is
Paul's Island near the town of Nain inLabrador ,Canada . It occurs in large crystal masses in anorthosite and shows aniridescence or "play of colors". The iridescence is the result of light refracting within lamellar intergrowths resulting from phase exsolution on cooling in the Boggildmiscibility gap, An48-An58.Gemstone varieties of labradorite exhibiting a high degree of iridescence are calledspectrolite ; moonstone andsunstone are also commonly used terms, and high-quality samples with good iridescent qualities are desired for jewelry.References
* 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]
Wikimedia Foundation. 2010.