Sodalite

Sodalite is a tectosilicate mineral with the formula Na8(Al6Si6O24)Cl2, with royal blue varieties widely used as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent. Sodalite is a member of the sodalite group with hauyne, nosean, lazurite and tugtupite.

The people of the Caral culture traded for sodalite from the Collao altiplano. First discovered by Europeans in 1811 in the Ilimaussaq intrusive complex in Greenland, sodalite did not become widely important as an ornamental stone until 1891 when vast deposits of fine material were discovered in Ontario, Canada.

Sodalite

General

Category Tectosilicate minerals, feldspathoid group, sodalite group

Formula Na8(Al6Si6O24)Cl2

IMA symbol Sdl

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

Ultraviolet fluorescence Bright red-orange cathodoluminescence and fluorescence under LW and SW UV, with yellowish phosphorescence; may be photochromic in magentas

Fusibility Easily to a colourless glass; sodium yellow flame

Solubility Soluble in hydrochloric acid and nitric acid

Major varieties

Hackmanite Tenebrescent; violet-red or green fading to white

Properties

A light, relatively hard yet fragile mineral, sodalite is named after its sodium content; in mineralogy it may be classed as a feldspathoid. 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 in jewellery, where it is fashioned into cabochons and beads. Lesser material is more often seen as facing or inlay in various applications.

Although somewhat similar to lazurite and lapis lazuli, sodalite rarely contains pyrite (a common inclusion in lapis) and its blue color is more like traditional royal blue rather than ultramarine. 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 will fluoresce orange under ultraviolet light, and hackmanite exhibits tenebrescence.

Structure

The structure of sodalite was first studied by Linus Pauling in 1930. It is a cubic mineral of space 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 a zeolite cage structure. Each unit cell has two cavities, which have almost the same structure as the borate cage (B24O48)24− found in the zinc borate Zn4O(BO2)6, the beryllosilicate cage (Be12Si12O48)24−, and the aluminate cage (Al24O48)24− in Ca8(Al12O24)(WO4)2, and as in the similar mineral tugtupite (Na4AlBeSi4O12Cl) (see Haüyne#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 has chiral tetrahedral symmetry, and the cavities around these two chloride locations are mirror images one of the other (a glide plane or a four-fold improper 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 twelve SiO4 tetrahedra and twelve AlO4 tetrahedra. The silicon and aluminum atoms are located at the corners of a truncated octahedron with the chloride and four sodium atoms inside. (A similar structure called "carbon sodalite" may occur as a very high pressure form of carbon — see illustration in reference.) Each oxygen atom links between an SiO4 tetrahedron and an AlO4 tetrahedron. All the oxygen atoms are equivalent, but one half are in environments that are enantiomorphic to the environments of the other half. The silicon atoms are at the location (0,1/2,1/4) and symmetry-equivalent positions, and the aluminum ions at the location (1/2,0,1/4) and 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.

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 no indirect 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 the thermal 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 (1/4,1/4,1/4). 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 on pyritohedral symmetry. Natural sodalite holds primarily chloride anions in the cages, but they can be substituted by other anions such as sulfate, sulfide, hydroxide, trisulfur with other minerals in the sodalite group representing end member compositions. The sodium can be replaced by other alkali group elements, and the chloride by other halides. Many of these have been synthesized.

The characteristic blue color arises mainly from caged S−3 and S4 clusters.

Crystal chemistry

Sodalite serves as the leader of a group of tectosilicates. The sodalite group is composed of minerals with a similar isometric structure and chemically close; all derived from feldspathoids, minerals of igneous rocks, poor in silica. Like zeolites, feldspathoids and the sodalite group have widely open crystal structures, which is perfectly reflected in their low density.

Coloration and appearance

What is most striking about sodalite is its sometimes beautiful coloration. These are generally shades of blue or light lilac, and these specimens are the most prized by collectors. It is also possible to find sodalite in white, gray, or green, which increases its collector's value as varied samples of the same mineral are available. Beyond its color, the most spectacular specimens of sodalite are those that appear in the form of dodecahedral crystals, very rare, whose transparency varies. However, this mineral is most commonly found in the form of amorphous, opaque masses that retain their beautiful coloration but not their interesting crystalline structure.

Modifications and varieties

Depending on the location, sodalite exhibits strong orange-red fluorescence and yellow phosphorescence under long-wave and short-wave UV light.

Sodalite is readily soluble in weak to moderately strong acids, such as hydrochloric acid. It initially loses color and then dissolves after some time, precipitating silica gel. Under the influence of heat, the reactions, especially the color loss, occur more rapidly. Even boiling water can remove sodium and chlorine from sodalite.

Hackmanite

Hackmanite is a variety of sodalite exhibiting tenebrescence. A special property of hackmanite is its photochromism (tenebrescence), probably caused by color centers. Unlike "normal" hackmanite, its color does not fade under sunlight, but rather becomes more intense. The effect is even more pronounced when exposed to a UV lamp or an X-ray source, under whose influence the color can be intensified to a strong violet within tenths of a second. Additionally, a pink to orange fluorescence occurs. Hackmanite from other localities, in contrast, recharges its color in the dark. 

 When hackmanite from Mont 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 from Afghanistan and the 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, shortwave ultraviolet light.

In addition, researchers led by Sami Vuori were able to demonstrate in 2022 that hackmanite also possesses "radiochromic" or "radiochromatic" properties. Irradiation with radioactive substances that emit alpha, beta, or gamma rays causes the mineral to change its color to pinkish-purple to reddish-violet, which becomes more intense the greater the exposure. The coloration is very similar to that caused by UV and X-ray radiation, but occurs more slowly. However, as with the aforementioned sources, the coloration caused by ionizing radiation is reversible, meaning that hackmanite loses its color once the radiation exposure has ended. However, the mineral "remembers" past radiation exposures because these cause tiny defects in its crystal structure. 

According to Vuori and his colleagues, due to its reversible, dose-dependent coloration, hackmanite would be suitable as an environmentally friendly indicator in dosimeters. In contrast to non-toxic hackmanite, current radiochromic dosimeter materials mostly consist of toxic or non-reusable substances. 

Related minerals

Lazurite (also ultramarine) is a component of the mineral compound lapis lazuli. Sodalite with S 3 − and S 2 − radicals produces an intense blue color due to the arrangement (coordination) in the sodalite cages.

Nosean also has the framework structure of sodalite, but only every second cage is occupied by the divalent sulfate anion. The compound is colorless.

Occurrence

Sodalite was first described in 1811 for the occurrence in its type locality in the Ilimaussaq complex, Narsaq, West Greenland.

Occurring typically in massive form, sodalite is found as vein fillings in plutonic igneous rocks such as nepheline syenites. It is associated with other minerals typical of silica-undersaturated environments, namely leucite, cancrinite and natrolite. Other associated minerals include nepheline, titanian andradite, aegirine, microcline, sanidine, albite, calcite, fluorite, ankerite and baryte.

Significant deposits of fine material are restricted to but a few locales: Bancroft, Ontario (Princess Sodalite Mine), and Mont-Saint-Hilaire, Quebec, in Canada; and Litchfield, Maine, and Magnet Cove, Arkansas, in the US. The Ice River complex, near Golden, British Columbia, contains sodalite. Smaller deposits are found in South America (Brazil and Bolivia), Portugal, Romania, Burma and Russia. Hackmanite is found principally in Mont-Saint-Hilaire and Greenland.

Euhedral, transparent crystals are found in northern Namibia and in the lavas of Vesuvius, Italy.

Sodalitite is a type of extrusive igneous rock rich in sodalite. Its intrusive equivalent is sodalitolite.

History

The people of the Caral culture traded for sodalite from the Collao altiplano.

Use

Due to its often vibrant mottled color, sodalite is often crafted into gemstones in the form of cameos and small sculptures, as well as spheres or cabochons for necklaces and faceted for rings. Due to its beautiful coloration, which varies from specimen to specimen, sodalite is used in the manufacture of necklaces, earrings, and bracelets, as well as for figurines and industrial art. 

The most common cuts are spheres or cabochons, except for the rare transparent or vitreous specimens found, which are faceted to maximize their luminosity.Large, deep blue stones are sometimes called "Royal Blue," blue stones "Blue Sapo," blue stones with a few light inclusions "Blue Tiger," and light blue stones with white inclusions "Nuvolato."

Large deposits in Bolivia, Brazil, Zambia, and Namibia, among others, are processed into floor and wall tiles and facade panels. The Namibian deposit is not currently being mined. Bolivian material is very rarely available on the market because mining takes place under difficult conditions. Blue King from Zambia is also no longer mined because it is not visually appealing as a decorative stone. Only the Brazilian material, trade name Azul Bahia, is available on the market. Sodalite is also used as decoration in aquariums.

The physical properties of sodalite, both its coloration and crystallization, have allowed it to be used as a source of pigments. Sodalite powder mixed with a substance such as casein produces a blue-colored mass with optical properties far superior to those of the synthetic pigments commonly used today. The characteristic shine and texture of these natural pigments give them a very special luminosity, although no distinction can be made based on color theory. Sodalite is of secondary importance as a pigment. Its related mineral lazurite and the mineral mixture lapis lazuli are preferred as pigment sources.

In science, synthetic sodalites, whose composition often differs from that of the mineral, serve as a model system for the group of substances known as zeolites. The sodalite cage is a structural component of the technically important compounds zeolite A, zeolite X, and zeolite Y. The technical synthesis of sodalites is usually carried out hydrothermally.

Synthesis

The mesoporous 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 include nitrate, iodide, iodate, permanganate, perchlorate, and perrhenate.

Erroneous relationship between sodalite and lapis lazuli

Due to its appearance, sodalite can be confused with lapis lazuli, especially when the former occurs in massive specimens. In some cases, it has been claimed that sodalite is a component of lapis lazuli, an error likely due to the presence of lazurite among the latter's components.

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