| Sodalite | |
|---|---|
 | |
| General | |
| Category | Tectosilicateminerals,feldspathoidgroup, sodalite group |
| Formula | Na 8(Al 6Si 6O 24)Cl 2 |
| IMA symbol | Sdl[1] |
| Strunz classification | 9.FB.10 |
| Crystal system | Cubic |
| Crystal class | Hextetrahedral (43m) H–M symbol: (4 3m) |
| Space group | P43n |
| Unit cell | a = 8.876(6) Å; Z = 1 |
| Identification | |
| Color | Rich royal blue, green, yellow, violet, white veining common |
| Crystal habit | Massive; rarely as dodecahedra |
| Twinning | Common on {111} forming pseudohexagonal prisms |
| Cleavage | Poor on {110} |
| Fracture | Conchoidal to uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 5.5–6 |
| Luster | Dull vitreous to greasy |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.27–2.33 |
| Optical properties | Isotropic |
| Refractive index | n = 1.483 – 1.487 |
| Ultravioletfluorescence | Bright red-orangecathodoluminescence and fluorescence under LW and SW UV, with yellowishphosphorescence; may bephotochromic in magentas |
| Fusibility | Easily to a colourless glass; sodium yellow flame |
| Solubility | Soluble inhydrochloric acid andnitric acid |
| References | [2][3][4][5] |
| Major varieties | |
| Hackmanite | Tenebrescent; violet-red or green fading to white |
Sodalite (/ˈsoʊ.dəˌlaɪt/SOH-də-lyte) is atectosilicate mineral with the formulaNa
8(Al
6Si
6O
24)Cl
2, withroyal blue varieties widely used as anornamentalgemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent. Sodalite is a member of the sodalite group withhauyne,nosean,lazurite andtugtupite.
The people of theCaral culture traded for sodalite from theCollao altiplano.[6] First discovered by Europeans in 1811 in theIlimaussaq intrusive complex inGreenland, sodalite did not become widely important as an ornamental stone until 1891 when vast deposits of fine material were discovered inOntario, Canada.
The structure of sodalite was first studied byLinus Pauling in 1930.[7] It is a cubic mineral ofspace group P43n (space group 218) which consists of an aluminosilicate cage network with Na+ cations and chloride anions in the interframework. (There may be small amounts of other cations and anions instead.) This framework forms azeolite cage structure. Each unit cell has two cavities, which have almost the same structure as theborate cage(B
24O
48)24−
found in thezinc borateZn
4O(BO
2)
6,[8] theberyllosilicate cage(Be
12Si
12O
48)24−
,[7] and thealuminate cage(Al
24O
48)24−
inCa
8(Al
12O
24)(WO
4)
2,[9] and as in the similar mineraltugtupite (Na
4AlBeSi
4O
12Cl) (seeHaüyne#Sodalite group). There is one cavity around each chloride ion. One chloride is located at the corners of the unit cell, and the other at the centre. Each cavity haschiral tetrahedral symmetry, and the cavities around these two chloride locations are mirror images one of the other (aglide plane or a four-foldimproper rotation takes one into the other). There are four sodium ions around each chloride ion (at one distance, and four more at a greater distance), surrounded by twelveSiO
4 tetrahedra and twelveAlO
4 tetrahedra. The silicon and aluminum atoms are located at the corners of atruncated octahedron with the chloride and four sodium atoms inside.[8] (A similar structure called "carbon sodalite" may occur as a very high pressure form of carbon — see illustration in reference.[10]) Each oxygen atom links between anSiO
4 tetrahedron and anAlO
4 tetrahedron. All the oxygen atoms are equivalent, but one half are in environments that areenantiomorphic to the environments of the other half. The silicon atoms are at the location and symmetry-equivalent positions, and the aluminum ions at the locationand symmetry-equivalent positions. The three silicon atoms and the three aluminum atoms listed above closest to a given corner of the unit cell form a six-membered ring of tetrahedra, and the four in any face of the unit cell form a four-membered ring of tetrahedra. The six-membered rings can serve as channels in which ions can diffuse through the crystal.[11]
The structure is a crumpled form of a structure in which the three-fold axes of each tetrahedron lie in planes parallel to the faces of the unit cell, thus putting half the oxygen atoms in the faces. As the temperature is raised the sodalite structure expands and uncrumples, becoming more like this structure. In this structure the two cavities are still chiral, because noindirect isometry centred on the cavity (i.e. a reflexion, inversion, or improper rotation) can superimpose the silicon atoms onto silicon atoms and the aluminum atoms onto aluminum atoms, while also superimposing the sodium atoms on other sodium atoms. A discontinuity of thethermal expansion coefficient occurs at a certain temperature when chloride is replaced by sulfate or iodide, and this is thought to happen when the framework becomes fully expanded or when the cation (sodium in natural sodalite) reaches the coordinates (et cetera).[11] This adds symmetry (such as mirror planes in the faces of the unit cell) so that the space group becomes Pm3n (space group 223), and the cavities cease to be chiral and take onpyritohedral symmetry.Natural sodalite holds primarilychloride anions in the cages, but they can be substituted by other anions such assulfate,sulfide,hydroxide,trisulfur with other minerals in the sodalite group representing end member compositions. The sodium can be replaced by otheralkali group elements, and the chloride by otherhalides. Many of these have been synthesized.[11]
The characteristic blue color arises mainly from cagedS−3 andS4 clusters.[12]
A light, relatively hard yet fragile mineral, sodalite is named after itssodium content; inmineralogy it may be classed as afeldspathoid. Well known for its blue color, sodalite may also be grey, yellow, green, or pink and is often mottled with white veins or patches. The more uniformly blue material is used injewellery, where it is fashioned intocabochons andbeads. Lesser material is more often seen as facing or inlay in various applications.
Although somewhat similar tolazurite andlapis lazuli, sodalite rarely containspyrite (a common inclusion in lapis) and its blue color is more like traditionalroyal blue rather thanultramarine. It is further distinguished from similar minerals by its white (rather than blue) streak. Sodalite's six directions of poor cleavage may be seen as incipient cracks running through the stone.
Most sodalite willfluoresce orange underultraviolet light, and hackmanite exhibitstenebrescence.[13]
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| Small specimen of sodalite from Brazil. | |||
Hackmanite is a variety of sodalite exhibitingtenebrescence.[14] When hackmanite fromMont Saint-Hilaire (Quebec) or Ilímaussaq (Greenland) is freshly quarried, it is generally pale to deep violet but the color fades quickly to greyish or greenish white. Conversely, hackmanite fromAfghanistan andthe Myanmar Republic starts off creamy white but develops a violet to pink-red color in sunlight. If left in a dark environment for some time, the violet will fade again. Tenebrescence is accelerated by the use of longwave or, particularly, shortwaveultraviolet light.
Sodalite was first described in 1811 for the occurrence in itstype locality in theIlimaussaq complex, Narsaq,West Greenland.[2]
Occurring typically in massive form, sodalite is found as vein fillings in plutonicigneous rocks such asnepheline syenites. It is associated with other minerals typical of silica-undersaturated environments, namelyleucite,cancrinite andnatrolite. Other associated minerals includenepheline, titanianandradite,aegirine,microcline,sanidine,albite,calcite,fluorite,ankerite andbaryte.[4]
Significant deposits of fine material are restricted to but a few locales:Bancroft, Ontario (Princess Sodalite Mine), andMont-Saint-Hilaire, Quebec, in Canada; andLitchfield, Maine, andMagnet Cove, Arkansas, in the US. The Ice River complex, nearGolden, British Columbia, contains sodalite.[15] Smaller deposits are found in South America (Brazil and Bolivia), Portugal, Romania, Burma and Russia. Hackmanite is found principally in Mont-Saint-Hilaire andGreenland.
Euhedral, transparent crystals are found in northernNamibia and in thelavas ofVesuvius, Italy.
Sodalitite is a type ofextrusive igneous rock rich in sodalite.[16] Itsintrusive equivalent issodalitolite.[16]
The people of theCaral culture traded for sodalite from the Collao altiplano.[17]
Themesoporous cage structure of sodalite makes it useful as a container material for many anions. Some of the anions known to have been included in sodalite-structure materials includenitrate,[18]iodide,[19]iodate,[20]permanganate,[21]perchlorate,[22] andperrhenate.
Media related toSodalite at Wikimedia Commons
