Sodalite In Nepheline Syenite (Precambrian; Bancroft, Ontario, Canada)

Sodalite ( ) is a tectosilicate mineral with the formula , 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.

First discovered by Europeans in 1811 in the Ilimaussaq intrusive complex in Greenland, sodalite did not become important as an ornamental stone until 1891 when vast deposits of fine material were discovered in Ontario, Canada.

Structure

The structure of sodalite was first studied by Linus Pauling in 1930. It is a cubic mineral of space group P3n ( 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 found in the zinc borate , the beryllosilicate cage , and the aluminate cage in ), and as in the similar mineral tugtupite () (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 tetrahedra and twelve 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 tetrahedron and an 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 locations
0.5 0.25 0
0.5 0.75 0
0.25 0 0.5
0.75 0 0.5
0 0.5 0.25
0 0.5 0.75
0.5 0.25 1
0.5 0.75 1
0.25 1 0.5
0.75 1 0.5
1 0.5 0.25
1 0.5 0.75
and the aluminum ions at the locations
0.5 0.25 0
0,5 0.75 0
0.25 0 0.5
0.75 0 0.5
0 0.5 0.25
0 0.5 0.75
0.5 0.25 1
0,5 0.75 1
0.25 1 0.5
0.75 1 0.5
1 0.5 0.25
1 0.5 0.75
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) (et cetera). This adds symmetry (such as mirror planes in the faces of the unit cell) so that the space group becomes Pmn ( 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 and clusters.

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.

Hackmanite

Hackmanite is a variety of sodalite exhibiting tenebrescence. 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 (Burma) 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. Much sodalite will also fluoresce a patchy orange under UV light.

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 Coxygen">O3)2">aluminum">Al6silicon.html" ;"title="luminum.html" ;"title="oxygen.html" ;"tit ...