Labradorite

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Labradorite, a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It is usually defined as having "%An" (anorthite) between 50 and 70. The specific gravity ranges from 2.71 to 2.74. The refractive index ranges from 1.555 to 1.575. Twinning is common. As with all plagioclase members the crystal system is triclinic 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 common mafic igneous rocks.

The geological type area for labradorite is Paul's Island near the town of Nain in Labrador, Canada. It occurs in large crystal masses in anorthosite and shows an iridescence or play of colors. The iridescence is the result of lamellar intergrowths resulting from a phase change on cooling. Gemstone varieties of labradorite exhibiting a high degree of iridescence are called spectrolite. Labradorite was named for the Labrador occurrence. Labradorite is the common feldspar in basalt and gabbro as well as the anorthosites.

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