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) feld ...

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. ...

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 Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...

effect (

schiller Johann Christoph Friedrich von Schiller (, short: ; 10 November 17599 May 1805) was a German playwright, poet, and philosopher. During the last seventeen years of his life (1788–1805), Schiller developed a productive, if complicated, friends ...

). 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 Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is Ca Al2 Si2O8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth but abundant on the Moon. Minera ...

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 (molecule), wa ...

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, o ...

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-grained wood used under roof shingles or tiles, on lath and plaster walls and ceilings to hold plaster, and in lattice and trellis work. ''Lath'' has expanded to mean any type of backing ma ...

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 includ ...

igneous rocks Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or l ...

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% of a ...

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 ...

, 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 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

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flak ...

s 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 qui ...

pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

amphibole Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is ...

s 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 ...

Labradorescence

Labradorite can display an

optical effect (or ''

'') 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 bor ...

(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 word ...

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 Bytownite is a calcium rich member of the plagioclase solid solution series of feldspar minerals with composition between anorthite and labradorite. It is usually defined as having between 70 and 90% An (formula: (Ca0.7-0.9,Na0.3-0.1) l(Al,Si)Si2O ...

(formula: , ''i.e.'', with an

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

). Some gemstone varieties of labradorite exhibiting a high degree of labradorescence are called spectrolite.

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