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Sodalite is a tectosilicate mineral belonging to the sodalite group. It is well-known for its vibrant blue color, though it can also be white, grey, green, or pink. It often occurs in massive form, but can also be found as disseminated grains or euhedral crystals. Its distinctive blue color, often with white calcite veins, makes it a popular decorative stone. Sodalite exhibits tenebrescence (reversible photochromism) in some varieties, meaning its color can change upon exposure to ultraviolet light and revert in the dark.
How to Identify
- Color
- Typically deep blue, often with white streaks or patches of calcite. Can also be grey, green, pink, or white.
- Luster
- Vitreous to greasy.
- Texture
- Massive, granular, or as disseminated grains. Crystals are rare but can be dodecahedral.
- Crystal Form
- Isometric system, typically dodecahedral crystals, but more commonly found in massive or granular aggregates.
- Cleavage
- Poor to distinct on {110}.
- Geological Environment
- Alkaline igneous rocks (nepheline syenites, phonolites), metasomatic rocks, and contact metamorphic limestones.
Key Facts
- Hardness: 5.5-6 on the Mohs scale.
- Specific Gravity: 2.13-2.30 g/cm³.
- Crystal System: Isometric (Cubic).
- Color: Typically deep blue, but can be grey, green, pink, or white. Often mottled with white calcite.
- Luster: Vitreous to greasy.
- Transparency: Transparent to opaque.
- Fracture: Conchoidal to uneven.
- Cleavage: Poor to distinct on {110} (dodecahedral).
- Composition: Sodium aluminum silicate with chloride.
Quick Check
- Color: Deep blue, often with white calcite.
- Luster: Vitreous to greasy.
- Streak: White.
Physical Characteristics
- Crystal Habit: Massive, granular, disseminated grains; rarely as dodecahedral crystals.
- Cleavage Type: Poor to distinct dodecahedral.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous to greasy.
Formation
Sodalite forms in silica-undersaturated igneous rocks, particularly nephehe syenites, phonolites, and other alkaline intrusive and extrusive rocks. It can also occur in metasomatic rocks and contact metamorphic limestones.
Usage
Primarily used as an ornamental stone, for carvings, cabochons, beads, and inlays. It is also used as a decorative building material and occasionally as a gemstone. Historically, it was used as a pigment (ultramarine, though this is more commonly associated with Lazurite, a related mineral).
Age Distribution
Found in various geological ages, typically associated with alkaline igneous rocks which can range from Precambrian to Cenozoic.
Where to Find
Bancroft, Ontario, Canada
Known for large deposits of blue sodalite, often associated with nepheline syenite intrusions.
Ilimaussaq intrusive complex, Greenland
A classic locality for sodalite, found in alkaline igneous rocks.
Litchfield, Maine, USA
Occurrences in nepheline syenite.
Kola Peninsula, Russia
Significant deposits in alkaline massifs.
Bahia, Brazil
Source of high-quality blue sodalite.
Namibia
Known for fine-grained, deep blue sodalite.
Mount Vesuvius, Italy
Found in volcanic ejecta.
Finding Tips
Look for Alkaline Igneous Rocks
Sodalite is almost exclusively found in silica-undersaturated alkaline igneous rocks. Familiarize yourself with the appearance of nepheline syenites and related rock types.
Identify Blue Coloration
The most striking characteristic of sodalite is its deep blue color. Look for blue minerals within a matrix of lighter-colored feldspars or nepheline.
Check for White Veins
Sodalite often occurs with white calcite veins or patches, which can help distinguish it from other blue minerals like lazurite (which often has pyrite).
Test for Tenebrescence (Hackmanite variety)
Some varieties of sodalite, particularly Hackmanite, exhibit tenebrescence. If you have a UV light, expose the mineral to it and observe if the color changes (e.g., from pale to pink/violet) and then fades in the dark.
Perform a Hardness Test
Sodalite has a Mohs hardness of 5.5-6. This can help differentiate it from softer minerals like azurite (3.5-4) or harder ones like quartz (7).
Observe Cleavage
Sodalite has poor to distinct dodecahedral cleavage. While not always obvious in massive forms, it can be a useful identifier in crystalline samples.
Similar Rocks
Lapis Lazuli
Lapis Lazuli (a rock composed primarily of Lazurite, Calcite, Pyrite, and Sodalite)
Also known as: None
Lazurite
Lazurite
Also known as: None
Dumortierite
Dumortierite
Also known as: None
Azurite
Azurite
Also known as: None
Scientific Classification
- Mineral Class
- Tectosilicate (Framework Silicate)
- Group
- Sodalite Group
- Crystal System
- Isometric (Cubic)
- Chemical Formula
- Na4Al3Si3O12Cl
- Composition
- Sodium aluminum silicate with chloride. Idealized formula: Na8(Al6Si6O24)Cl2. The formula Na4Al3Si3O12Cl is a simplified representation, often seen as Na8[Al6Si6O24]Cl2, where the bracketed part represents the framework structure.
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