Labradorite

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Labradorite
Labradorite polie 3(Madagascar).jpg
Labradorite in a polished rock slab
General
Category Adularescence, tectosilicate
Formula
(repeating unit)
(Ca,Na)(Al,Si)4O8, where Ca/(Ca + Na) (% anorthite) is between 50%–70%
Crystal system Triclinic
Crystal class Pinacoidal (1)
(same H-M symbol)
Unit cell a = 8.155  Å, b = 12.84 Å
c = 10.16 Å; α = 93.5°
β = 116.25°, γ = 89.133°; Z = 6
Identification
ColorGray, gray-white, brown, greenish, pale green, blue, yellow, colorless
Crystal habit Crystals typically thin and tabular, rhombic in cross section, striated; massive
Twinning Common by Albite, Pericline, Carlsbad, Baveno, or Manebach twin laws
Cleavage Perfect on {001}, less perfect on {010}, intersecting at near 90°; distinct on {110}
Fracture Uneven to conchoidal
Mohs scale hardness6 – 6.5
Luster Vitreous to pearly on cleavages
Streak white
Diaphaneity Translucent to transparent
Specific gravity 2.68 to 2.72
Optical propertiesBiaxial (+)
Refractive index nα = 1.554 - 1.563 nβ = 1.559 - 1.568 nγ = 1.562 - 1.573
Birefringence δ = 0.008 - 0.010
2V angle Measured: 85°
Dispersion None
Other characteristicsLabradorescence (iridescent)
References [1] [2] [3]

Labradorite ((Ca, Na)(Al, Si)4 O 8), a feldspar mineral, 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. (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.

Contents

Occurrence

The geological type area for labradorite is Paul 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. [2]

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. [4] 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. [1]

Labradorescence

Labradorescence in labradorite Labradorescence.jpg
Labradorescence in labradorite
Video of labradorescence in labdradorite, visible as the angle of view changes

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: [5]

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.

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). [5] [6] [7]

The cause of this optical phenomenon is phase exsolution lamellar structure, [8] occurring in the Bøggild miscibility gap. [9] The effect is visible when the lamellar separation is between 128 to 252 nm (5.0×10−6 to 9.9×10−6 in); the lamellae are not necessarily parallel; [9] and the lamellar structure is found to lack long range order. [10]

The lamellar separation only occurs in plagioclases of a certain composition; those of calcic labradorite and bytownite (anorthite content of ~60 to 90%) particularly exemplify this. [8] [11] 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 (they might not be 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.

See also

Related Research Articles

Gabbro A coarse-grained mafic intrusive rock

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 chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term "gabbro" may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic".

Feldspar A group of rock-forming tectosilicate minerals

Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight.

Plagioclase Type of feldspar

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 properly known as the plagioclase feldspar series. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or 'record-groove' effect.

Baddeleyite

Baddeleyite is a rare zirconium oxide mineral (ZrO2 or zirconia), occurring in a variety of monoclinic prismatic crystal forms. It is transparent to translucent, has high indices of refraction, and ranges from colorless to yellow, green, and dark brown. See etymology below.

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)[Al(Al,Si)Si2O8]). Like others of the series, bytownite forms grey to white triclinic crystals commonly exhibiting the typical plagioclase twinning and associated fine striations.

Anorthite

Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is CaAl2Si2O8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth but abundant on the Moon.

Amphibolite A metamorphic rock containing mainly amphibole and plagioclase

Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase.

Anorthosite A mafic intrusive igneous rock composed predominantly of plagioclase

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 commonly present.

Scapolite

The scapolites (Gr. σκάπος, rod, and λίθος, stone) are a group of rock-forming silicate minerals composed of aluminium, calcium, and sodium silicate with chlorine, carbonate and sulfate. The two endmembers are meionite (Ca4Al6Si6O24CO3) and marialite (Na4Al3Si9O24Cl). Silvialite (Ca,Na)4Al6Si6O24(SO4,CO3) is also a recognized member of the group.

Oligoclase

Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). The albite:anorthite molar ratio of oligoclase ranges from 90:10 to 70:30.

Theralite is, in petrology, the name given to calcic foidal gabbro, a plutonic hylocrystalline rock consisting of augite, olivine, calcic plagioclase (labradorite), and nepheline, along with accessories including biotite, magnetite, ilmenite and analcime.

Moon rock Rock from the Moon

Moon rock or lunar rock refers to rock that is found on the Earth's Moon. This includes lunar material collected during the course of human exploration of the Moon, and rock that has been ejected naturally from the Moon's surface and landed on the Earth as meteorites.

QAPF diagram Classification system for igneous rocks

A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition. The acronym QAPF stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". These are the mineral groups used for classification in QAPF diagram. Q, A, P and F percentages are normalized.

Cumulate rock

Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks. Cumulates can be deposited on top of other older cumulates of different composition and colour, typically giving the cumulate rock a layered or banded appearance.

Myrmekite

Myrmekite describes a vermicular, or wormy, intergrowth of quartz in plagioclase. The intergrowths are microscopic in scale, typically with maximum dimensions less than 1 millimeter. The plagioclase is sodium-rich, usually albite or oligoclase. These quartz-plagioclase intergrowths are associated with and commonly in contact with potassium feldspar. Myrmekite is formed under metasomatic conditions, usually in conjunction with tectonic deformations. It has to be clearly separated from micrographic and granophyric intergrowths, which are magmatic.

Pauls Island

Paul's Island or Paul Island is an island off the coast of Labrador, near the town of Nain in Canada.

Thin section Laboratory preparation of a rock, mineral, soil, pottery, bones, or metal for use with a petrographic microscope

In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a diamond saw and ground optically flat. It is then mounted on a glass slide and then ground smooth using progressively finer abrasive grit until the sample is only 30 μm thick. The method involved using the Michel-Lévy interference colour chart. Typically quartz is used as the gauge to determine thickness as it is one of the most abundant minerals.

Litchfieldite

Litchfieldite is a rare igneous rock. It is a coarse-grained, foliated variety of nepheline syenite, sometimes called nepheline syenite gneiss or gneissic nepeheline syenite. Litchfieldite is composed of two varieties of feldspar, with nepheline, sodalite, cancrinite and calcite. The mafic minerals, when present, are magnetite and an iron-rich variety of biotite (lepidomelane).

Southern Oklahoma Aulacogen A failed rift in the western and southern US of the triple junction that became the Iapetus Ocean

The Southern Oklahoma Aulacogen(ah-lah-coh-jin)  is a failed rift, or failed rift arm (aulacogen), of the triple junction that became the Iapetus Ocean spreading ridges. It is a significant geological feature in the Western and Southern United States. It formed sometime in the early to mid Cambrian Period and spans the Wichita Mountains, Taovayan Valley, Anadarko Basin, and Hardeman Basin in Southwestern Oklahoma. The Southern Oklahoma Aulacogen is primarily composed of basaltic dikes, gabbros, and units of granitic rock.

References

  1. 1 2 Handbook of Mineralogy
  2. 1 2 Mindat.org
  3. Webmineral data
  4. Hurlbut, Cornelius S.; Klein, Cornelis; Manual of Mineralogy, Wiley, 1985, 20th ed., p. 456 ISBN   0-471-80580-7
  5. 1 2 Bøggild, Ove Balthasar (1924), "On the Labradorization of the Feldspars" (PDF), Kongelige Danske Videnskabernes Selskab, Mathematisk-fysiske Meddelelelser, 6 (3): 1–79[ permanent dead link ]
  6. Raman, Chandrasekhara Venkata; Jayaraman, Aiyasami (July 1950). "The structure of labradorite and the origin of its iridescence". Proceedings of the Indian Academy of Sciences, Section A. 32 (1): 1–16. doi:10.1007/BF03172469. S2CID   128235557.
  7. Lord Rayleigh (3 April 1923), "Studies of Iridescent Colour and the Structure Producing it. III. The Colours of Labrador Felspar", Proceedings of the Royal Society of London. Series A, The Royal Society, 103 (720): 34–45, doi: 10.1098/rspa.1923.0037 , JSTOR   94093
  8. 1 2 Yan-ju, Peng; Xue-mei, He; Qin-fang, Fang (May 2008), "Exsolution lamellar structure causes of iridescence in labradorite: evidence from TEM", Acta Petrologica et Mineralogica
  9. 1 2 Hao, Xie; Jing-cheng, Pei; Li-ping, Li (February 2006), "Relation Between Labradorescence and Internal Structure of Labradorite", Geological Science and Technology Information
  10. Bolton, Herbert Cairns; Bursill, Leslie Arthur; McLaren, Alexander Clark; Turner, Robin G. (1966). "On the origin of the colour of labradorite". Physica Status Solidi B. 18: 221–230. doi:10.1002/pssb.19660180123.
  11. MacKenzie, William Scott; Zussman, Jack, eds. (1974), "23. Electron-optical study of a schiller labradorite", The Feldspars: Proceedings of a NATO Advanced Study Institute, Manchester, 11–21 July 1972, Manchester University Press, 2, pp. 478–490