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Sodalite is a beautiful blue mineral, often found in massive form, but can also occur as distinct dodecahedral or cubic crystals. It is characterized by its rich blue color, which can range from light to dark blue, often with white streaks or veins of calcite or other minerals. It is a member of the sodalite group of tectosilicates, which are framework silicates. Sodalite is relatively soft and has a vitreous to greasy luster. It is distinguished from lapis lazuli by the absence of pyrite inclusions and its white streak.
How to Identify
- Color
- Typically a rich, royal blue to violet-blue, but can also be gray, green, yellow, or pink (hackmanite). Often exhibits white streaks or veins.
- Luster
- Vitreous to greasy.
- Texture
- Massive, granular, or as disseminated grains. Crystals are rare but can be dodecahedral or cubic.
- Crystal Form
- Typically massive or granular. When euhedral, it forms dodecahedral or cubic crystals, though these are uncommon.
- Cleavage
- Distinct dodecahedral cleavage in three directions (110), but often difficult to observe in massive material.
- Geological Environment
- Found in silica-undersaturated, alkali-rich igneous rocks such as nepheline syenites, phonolites, and related pegmatites. Also occurs in metasomatic deposits.
Key Facts
- Hardness: 5.5-6 on the Mohs scale
- Specific Gravity: 2.13-2.33 g/cm³
- Crystal System: Cubic (isometric)
- Color: Blue, gray, green, yellow, pink (hackmanite)
- Luster: Vitreous to greasy
- Transparency: Transparent to translucent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: Distinct dodecahedral (110) in three directions
- Composition: Sodium aluminum silicate with chloride: Na8(Al6Si6O24)Cl2
Quick Check
- Color: Blue (royal to violet-blue), often with white streaks/veins
- Luster: Vitreous to greasy
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, granular, disseminated grains; rarely euhedral dodecahedral or cubic crystals.
- Cleavage Type: Distinct dodecahedral (110) in three directions, often imperfect or difficult to observe in massive material.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous to greasy.
Formation
Sodalite is a tectosilicate mineral that crystallizes from silica-undersaturated, alkali-rich magmas. It is a primary mineral in nepheline syenites, phonolites, and other alkali igneous rocks. It can also form in metasomatic environments, where existing rocks are altered by hot, chemically active fluids, often associated with contact metamorphism of limestones or other carbonate rocks adjacent to alkali intrusions. Sodalite is part of the feldspathoid group, which are minerals that form in magmas that lack sufficient silica to form feldspars.
Usage
Sodalite is primarily used as an ornamental stone, for carvings, cabochons, beads, and inlays. Its attractive blue color makes it popular in jewelry. It is also used as a decorative building material, particularly in slabs and tiles. The hackmanite variety, which exhibits tenebrescence (reversible photochromism), is of scientific interest and sometimes collected for its unique property.
Age Distribution
Sodalite forms in igneous rocks, particularly those that are silica-undersaturated and alkali-rich. These types of rocks can form throughout geological time, from Precambrian to Cenozoic, wherever the specific magmatic conditions are met.
Where to Find
Bancroft, Ontario, Canada
Known for large deposits of high-quality blue sodalite, often associated with nepheline syenite intrusions.
Ilimaussaq intrusive complex, Greenland
A classic locality for sodalite and other rare alkali minerals.
Litchfield, Maine, USA
Occurrences of sodalite in alkali igneous rocks.
Kola Peninsula, Russia
Significant deposits of sodalite in various alkali complexes.
Bahia, Brazil
Source of ornamental sodalite, often with white calcite veins.
Namibia
Known for good quality sodalite specimens.
Finding Tips
Look for Alkali Igneous Rocks
Sodalite is almost exclusively found in silica-undersaturated, alkali-rich igneous rocks. Familiarize yourself with the appearance of nepheline syenites and phonolites.
Check for Blue Coloration
While not all sodalite is blue, the most sought-after and easily identifiable forms are. Look for distinct blue patches or veins within the host rock.
Perform a Streak Test
Sodalite has a white streak, which helps differentiate it from other blue minerals like azurite (blue streak) or lapis lazuli (light blue streak).
Test for Fluorescence (Hackmanite)
The hackmanite variety of sodalite exhibits tenebrescence (reversible photochromism) and often strong fluorescence under UV light, changing color from white/pink to violet/red.
Acid Test (Caution Advised)
Sodalite will slowly gelatinize in hydrochloric acid, releasing hydrogen sulfide (rotten egg smell) if sulfur is present in its structure. This test should be performed with caution and proper ventilation.
Similar Rocks
Lapis Lazuli
Lapis Lazuli (a rock composed primarily of lazurite, calcite, and pyrite)
Also known as: Lapis
Azurite
Azurite (Cu3(CO3)2(OH)2)
Also known as: Chessylite
Dumortierite
Dumortierite (Al7(BO3)(SiO4)3O3)
Also known as: Blue Quartz (when included in quartz)
Scientific Classification
- Mineral Class
- Tectosilicate (Framework Silicate)
- Group
- Sodalite Group
- Crystal System
- Cubic (Isometric)
- Chemical Formula
- Na8(Al6Si6O24)Cl2
- Composition
- Sodium aluminum silicate with chloride. Minor substitutions can occur, leading to varieties like hackmanite (containing sulfur).
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