Description

Labradorite ((Ca, Na)(Al, Si)4O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada.

Labradorite can display an iridescent optical effect (or schiller) known as labradorescence. The term labradorescence was coined by Ove Balthasar Bøggild, who defined it (labradorization) as follows: Labradorization is the peculiar reflection of the light from submicroscopical planes orientated in one direction (rarely in two directions); these planes have never such a position that they can be expressed by simple indices, and they are not directly visible under the microscope.

Labradorite is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573 and twinning is common. As with all plagioclase members, the crystal system is triclinic, and three directions of cleavage are present, two of which are nearly at right angles and are more obvious, being of good to perfect quality (while the third direction is poor). It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites.