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Labradorite ((Calcium, Sodium)(Aluminum, Silicon)4Oxygen8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescence effect (schiller).
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 Crystal 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 .
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.Hurlbut, Cornelius S.; Klein, Cornelis; Manual of Mineralogy, Wiley, 1985, 20th ed., p. 456, It also is found in metamorphic and as a detrital component of some sediments. Common mineral associates in igneous rocks include olivine, , and magnetite.
Contributions to the understanding of the origin and cause of the effect were made by Robert Strutt, 4th Baron Rayleigh (1923), and by Bøggild (1924).
The cause of this optical phenomenon is phase exsolution lamellar 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 (50–70% anorthite) and bytownite (formula: , i.e., with an anorthite content of ~70 to 90%) particularly exemplify this. Another requirement for the lamellar separation is a very slow cooling of the rock containing 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 (they might not have the correct composition, cooled too quickly, or both), and not all plagioclases that exhibit labradorescence are labradorites (they may be bytownite).
Some gemstone varieties of labradorite exhibiting a high degree of labradorescence are called spectrolite.
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