| Labradorite | |
|---|---|
.jpg) Labradorite in a polished rock slab | |
| General | |
| Category | Tectosilicateminerals,feldspargroup,plagioclase series |
| Formula | (Ca,Na)(Al,Si)4O8, where Na is 30-50% & Ca is 50–70% |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (sameH-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 | |
| Color | Gray, gray-white, brown, greenish, pale green, blue, orange, pink, yellow, colorless |
| Crystal habit | Crystals typically thin and tabular, rhombic in cross section, striated; massive |
| Twinning | Common byalbite,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 hardness | 6–6.5 |
| Luster | Vitreous to pearly on cleavages |
| Streak | White |
| Diaphaneity | Translucent to transparent |
| Specific gravity | 2.68 to 2.72 |
| Optical properties | Biaxial (+) |
| 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 characteristics | Labradorescence (iridescence,schiller optical effect) |
| References | [1][2][3] |
Labradorite ((Ca,Na)(Al,Si)4O8) is a calcium-enrichedfeldsparmineral first identified inLabrador, Canada, which can display aniridescent effect (schiller).
Labradorite is an intermediate to calcic member of theplagioclase series. It has ananorthite percentage (%An) of between 50 and 70. Thespecific gravity ranges from 2.68 to 2.72. The streak is white, like mostsilicates. Therefractive index ranges from 1.559 to 1.573 andtwinning is common. As with all plagioclase members, the crystal system istriclinic, and three directions ofcleavage 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 tolath shaped grains in commonmaficigneous rocks such asbasalt andgabbro, as well as inanorthosites.
Thegeological type area for labradorite isPaul's Island near the town ofNain 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.[2]
Labradorite occurs inmafic igneous rocks and is the feldspar variety most common inbasalt andgabbro. The uncommonanorthosite bodies are composed almost entirely of labradorite.[4] It also is found in metamorphicamphibolites and as a detrital component of some sediments. Common mineral associates in igneous rocks includeolivine,pyroxenes,amphiboles andmagnetite.[1]
Labradorite can display aniridescent optical effect (orschiller) known as labradorescence. The termlabradorescence was coined byOve 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 byRobert Strutt, 4th Baron Rayleigh (1923), and by Bøggild (1924).[5][6][7]
The cause of this optical phenomenon is phaseexsolution lamellar structure,[8] occurring in the Bøggildmiscibility gap.[9] The effect is visible when the lamellar separation is between 128 and 252 nm (5.0×10−6 and 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 (50–70% anorthite) andbytownite (formula:(Ca0.7-0.9,Na0.3-0.1)[Al(Al,Si)Si2O8],i.e., with ananorthite content of ~70 to 90%) particularly exemplify this.[8][11] 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).
Somegemstone varieties of labradorite exhibiting a high degree of labradorescence are calledspectrolite.
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