Labradorite, a feldsparmineral, is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage 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. It occurs as clear, white to gray, blocky to lath shaped grains in common maficigneous rocks such as basalt and gabbro, as well as in anorthosites.
Occurrence
The geological type area for labradorite is Paul's Island near the town of Nain in Labrador, Canada. It has also been reported in Norway, Finland and various other locations worldwide, with notable distribution in Madagascar, China, Australia, Slovakia and the USA. Labradorite occurs in mafic igneous rocks and is the feldspar variety most common in basalt and gabbro. The uncommon anorthosite bodies are composed almost entirely of labradorite. It also is found in metamorphic amphibolites and as a detrital component of some sediments. Common mineral associates in igneous rocks include olivine, pyroxenes, amphiboles and magnetite.
Labradorescence
Labradorite can display an iridescentoptical effect known as labradorescence. The term labradorescence was coined by Ove Balthasar Bøggild, who defined it as follows: Contributions to the understanding of the origin and cause of the effect were made by Robert Strutt, 4th Baron Rayleigh, and by Bøggild. The cause of this optical phenomenon is phase exsolutionlamellar structure, occurring in the Bøggild miscibility gap. The effect is visible when the lamellar separation is between ; 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; those of calcic labradorite and bytownite particularly exemplify this. 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. Therefore, not all labradorites exhibit labradorescence, and not all plagioclases that exhibit labradorescence are labradorites. Some gemstonevarieties of labradorite exhibiting a high degree of labradorescence are called spectrolite.
