Labradorite (( Ca, Na)( Al, Si)₄ O₈) is a calcium-enriched

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

first identified in

Labrador , nickname = "The Big Land" , etymology = , subdivision_type = Country , subdivision_name = Canada , subdivision_type1 = Province , subdivision_name1 ...

, Canada, which can display an iridescent effect ( schiller). Labradorite is an intermediate to calcic member of the

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

series. It has an anorthite percentage (%An) of between 50 and 70. The

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...

ranges from 2.68 to 2.72. The streak is white, like most

silicates In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name i ...

. The

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

ranges from 1.559 to 1.573 and twinning is common. As with all plagioclase members, the crystal system is

triclinic 180px, Triclinic (a ≠ b ≠ c and α ≠ β ≠ γ ) In crystallography, the triclinic (or anorthic) crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal i ...

, 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 A lath or slat is a thin, narrow strip of straight-wood grain, grained wood used under roof shingles or tiles, on lath and plaster walls and ceilings to hold plaster, and in Latticework, lattice and Trellis (architecture), trellis work. ''Lath ...

shaped grains in common

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...

igneous rocks such as

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

and

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

, as well as in

anorthosite Anorthosite () is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most ...

Occurrence

The geological type area for labradorite is

Paul's Island Paul's Island or Paul Island is an island off the coast of Labrador, near the town of Nain in Canada. The island is the geological type area of the mineral labradorite, which is a plagioclase feldspar. Labradorite is the principal component ...

near the town of Nain in Labrador, Canada. It has also been reported in Poland, Norway, Finland and various other locations worldwide, with notable distribution in Madagascar, China, Australia, Slovakia and the United States. Labradorite occurs in

igneous rocks and is the feldspar variety most common in

and

. The uncommon

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 amphibolites and as a detrital component of some sediments. Common mineral associates in igneous rocks include

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

, pyroxenes, amphiboles and

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...

Labradorescence

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: Contributions to the understanding of the origin and cause of the effect were made by

Robert Strutt, 4th Baron Rayleigh Robert John Strutt, 4th Baron Rayleigh (28 August 1875 – 13 December 1947) was a British peer and physicist. He discovered "active nitrogen" and was the first to distinguish the glow of the night sky. Early life and education Strutt was bo ...

(1923), and by Bøggild (1924). The cause of this optical phenomenon is phase

exsolution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wor ...

lamellar structure, occurring in the Bøggild

miscibility gap A miscibility gap is a region in a phase diagram for a mixture of components where the mixture exists as two or more phases – any region of composition of mixtures where the constituents are not completely miscible. The IUPAC Gold Book defines ...

. 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 and/or they cooled too quickly), 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 Spectrolite is an uncommon variety of labradorite feldspar. Colors Spectrolite exhibits a richer range of colors than other labradorites as for instance in Canada or Madagascar (which show mostly tones of blue-grey-green) and high labradoresce ...

Gallery

File:Labradoryt, Madagaskar.JPG, Polished block from Madagascar File:Labradorite detail.jpg, Detail of labradorite File:LabradoriteAMD.jpg, Polished labradorite 18x20 cm File:Labradorite (UCL Geology Collections).jpg, Polished labradorite from UCL Geology collections File:Labradorite with rare colours.JPG, Labradorite with rare colors File:Ladrador iridescence.jpg, Polished labradorite Grey labradorite

References

External links

* {{Authority control Calcium minerals Feldspar Minerals in space group 2 Provincial symbols of Newfoundland and Labrador Sodium minerals Tectosilicates Triclinic minerals

Occurrence

Labradorescence

Gallery

See also

References

External links