Sodalite

Sodalite
General
Category	Tectosilicates without zeolitic H2O
Formula (repeating unit)	Na 8(Al 6Si 6O 24)Cl 2
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
References	[1][2][3][4]
Major varieties
Hackmanite	Tenebrescent; violet-red or green fading to white

Sodalite (/ˈsoʊ.dəˌlaɪt/ SOH-də-lyte) is a royal blue tectosilicate mineral with the formula Na
₈(Al
₆Si
₆O
₂₄)Cl
₂, 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[]

Sodalite is a cubic mineral of space group P43n (space group 215)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 cage structure, similar to zeolites. Each unit cell has two cavities, which have almost the same structure as the borate ion (B
₁₂O
₂₄)¹²⁻
, the ion (Be
₆Si
₆O
₂₄)¹²⁻
, and the aluminate ion (Al
₁₂O
₂₄)¹²⁻
,, and as in the similar mineral tugtupite (Na
₄AlBdSi
₄O
₁₂Cl) (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 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 SiO
₄ tetrahedra and twelve AlO
₄ tetrahedra. Each oxygen atom links between an SiO
₄ tetrahedron and an AlO
₄ 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.^[5]

The structure is a crumpled form of a more symmetric structure with the body-centred space group Im3m (space group 229). As the temperature is raised the structure uncrumples and becomes more like the Im3m structure. In that structure the two cavities are identical and not chiral. The unit cell has full octahedral (or cubic) 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.^[5]

The characteristic blue color arises mainly from caged S−
3 and S
4 clusters.^[6]

Properties[]

A sample of sodalite-carbonate pegmatite from Bolivia, with a polished rock surface.

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.^[7]

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Small specimen of Sodalite from Brazil.

Hackmanite[]

Hackmanite dodecahedron from the Koksha Valley, Afghanistan

Hackmanite is a variety of sodalite exhibiting tenebrescence.^[8] 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.^[1]

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.^[3]

Hippo in sodalite, length 9 cm (3.5 in)

Significant deposits of fine material are restricted to but a few locales: Bancroft, Ontario, 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.^[9] 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.

History[]

The people of the Caral culture traded for sodalite from the Collao altiplano.^[10] Sodalite was also traded for at Lukurmata.^[11]

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,^[12] iodide,^[13] iodate,^[14] permanganate,^[15] perchlorate,^[16] and perrhenate.

See also[]

• Labradorite
• Opal

References[]

• Mineral galleries

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