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Sodalite is a tectosilicate mineral known for its vibrant blue color, though it can also be gray, green, or pink. It is a member of the sodalite group. Lapis Lazuli is a metamorphic rock prized for its intense blue color, which comes from the mineral lazurite. It is typically composed of lazurite (25-40%), calcite (white streaks), and pyrite (golden flecks), and may also contain minor amounts of diopside, amphibole, feldspar, and mica. The presence of pyrite is a key characteristic distinguishing natural Lapis Lazuli from imitations.
How to Identify
- Color
- Sodalite: Typically intense royal blue, often with white streaks or patches of calcite. Can also be gray, green, or pink. Lapis Lazuli: Deep azure blue to violet-blue, often with golden specks of pyrite and white streaks or patches of calcite.
- Luster
- Sodalite: Vitreous to greasy. Lapis Lazuli: Dull to greasy, sometimes waxy.
- Texture
- Sodalite: Granular, massive, or rarely as distinct dodecahedral crystals. Lapis Lazuli: Fine-grained to medium-grained, massive, often with visible inclusions of pyrite and calcite.
- Crystal Form
- Sodalite: Isometric system, typically massive, granular, or in dodecahedral crystals. Lapis Lazuli: A rock, so it does not have a single crystal form; lazurite within the rock is isometric, typically anhedral to subhedral.
- Cleavage
- Sodalite: Distinct dodecahedral cleavage (three directions at 90 degrees). Lapis Lazuli: No distinct cleavage as a rock; lazurite has imperfect dodecahedral cleavage.
- Geological Environment
- Sodalite: Alkaline igneous rocks (nepheline syenites, phonolites), pegmatites, hydrothermal veins, and some metamorphic rocks. Lapis Lazuli: Contact metamorphosed limestones or dolomites adjacent to silica-poor igneous intrusions.
Key Facts
- Hardness: Sodalite: 5.5-6 on Mohs scale. Lapis Lazuli: 5-6 on Mohs scale (due to varying mineral composition).
- Specific Gravity: Sodalite: 2.13-2.29. Lapis Lazuli: 2.7-2.9.
- Crystal System: Sodalite: Isometric. Lapis Lazuli: Not applicable as a rock; lazurite is isometric.
- Color: Sodalite: Blue, gray, green, pink. Lapis Lazuli: Deep blue, violet-blue.
- Luster: Sodalite: Vitreous to greasy. Lapis Lazuli: Dull to greasy.
- Transparency: Sodalite: Transparent to translucent. Lapis Lazuli: Opaque.
- Fracture: Sodalite: Uneven to conchoidal. Lapis Lazuli: Uneven to splintery.
- Cleavage: Sodalite: Distinct dodecahedral (3 directions at 90°). Lapis Lazuli: None as a rock; lazurite has imperfect dodecahedral.
- Composition: Sodalite: Sodium aluminum silicate with chloride. Lapis Lazuli: A rock composed primarily of lazurite, calcite, and pyrite.
Quick Check
- Color: Sodalite: Royal blue with white streaks. Lapis Lazuli: Deep azure to violet-blue with golden pyrite flecks and white calcite.
- Luster: Sodalite: Vitreous to greasy. Lapis Lazuli: Dull to greasy.
- Streak: Sodalite: White. Lapis Lazuli: Light blue (due to lazurite).
Physical Characteristics
- Crystal Habit: Sodalite: Massive, granular, dodecahedral crystals. Lapis Lazuli: Massive, granular.
- Cleavage Type: Sodalite: Distinct dodecahedral. Lapis Lazuli: None (as a rock).
- Fracture Type: Sodalite: Uneven to conchoidal. Lapis Lazuli: Uneven to splintery.
- Tenacity: Sodalite: Brittle. Lapis Lazuli: Brittle.
- Luster Type: Sodalite: Vitreous to greasy. Lapis Lazuli: Dull to greasy.
Formation
Sodalite: Forms in silica-undersaturated igneous rocks such as nepheline syenites, phonolites, and related pegmatites and hydrothermal veins. It can also occur in metamorphic rocks. Lapis Lazuli: Forms through contact metamorphism of limestones or dolomites with silica-poor igneous intrusions, leading to the metasomatic replacement of carbonate minerals by lazurite, diopside, and other silicates. The presence of sulfur is crucial for lazurite formation.
Usage
Both Sodalite and Lapis Lazuli are primarily used as ornamental stones, for carvings, cabochons, beads, and inlays. Lapis Lazuli was historically ground to produce ultramarine pigment, a highly valued blue pigment.
Age Distribution
Sodalite: Found in various geological ages, typically associated with alkaline igneous rocks. Lapis Lazuli: Formed during regional or contact metamorphism, often in Precambrian to Paleozoic carbonate rocks.
Where to Find
Sodalite: Bancroft, Ontario, Canada
Known for large deposits of high-quality blue sodalite, often with associated white calcite.
Sodalite: Ilimaussaq Complex, Greenland
A significant source of sodalite, often found in nepheline syenites.
Sodalite: Bahia, Brazil
Produces sodalite, sometimes with a more violet hue.
Lapis Lazuli: Badakhshan Province, Afghanistan
The most historically significant and primary source of the finest quality Lapis Lazuli for over 6,000 years, found in ancient marble deposits.
Lapis Lazuli: Lake Baikal, Siberia, Russia
Another important source, producing high-quality material.
Lapis Lazuli: Ovalle, Coquimbo Region, Chile
Produces Lapis Lazuli, often with a lighter blue color and more prominent calcite.
Finding Tips
Sodalite: UV Fluorescence
Some sodalite exhibits orange fluorescence under longwave UV light, which can aid in identification.
Sodalite: Hydrochloric Acid Test
Sodalite will slowly dissolve in hydrochloric acid, forming a gel and releasing hydrogen sulfide (rotten egg smell) if sulfur is present as an impurity. This test should be performed with caution in a well-ventilated area.
Lapis Lazuli: Pyrite Inclusions
The presence of golden pyrite flecks is a strong indicator of natural Lapis Lazuli. Absence of pyrite or presence of uniform, glittery specks might indicate imitation.
Lapis Lazuli: Calcite Streaks
White calcite streaks or patches are common in Lapis Lazuli and are a natural component of the rock. Excessive white can lower its value.
Differentiation from Dyed Materials
Both sodalite and lapis lazuli are sometimes dyed. Examine for uneven color distribution, dye concentrations in fractures, or a 'plastic' appearance. A cotton swab with acetone can sometimes reveal dye on the surface.
Similar Rocks
Dumortierite
(Al,Fe3+)7(SiO4)3(BO3)O3
Also known as: Blue Quartz
Azurite
Cu3(CO3)2(OH)2
Also known as: Chessylite
Howlite
Ca2B5SiO9(OH)5
Also known as: White Turquoise
Dyed Jasper
SiO2 (with impurities)
Also known as: Blue Jasper
Scientific Classification
- Mineral Class
- Sodalite: Tectosilicate; Lapis Lazuli: Rock (composed of silicates, carbonates, sulfides)
- Group
- Sodalite: Sodalite Group; Lapis Lazuli: Not applicable (rock)
- Crystal System
- Sodalite: Isometric; Lapis Lazuli: Not applicable (rock)
- Chemical Formula
- Sodalite: Na8(Al6Si6O24)Cl2; Lapis Lazuli: A rock composed primarily of lazurite ((Na,Ca)8(AlSiO4)6(S,SO4,Cl)2), calcite (CaCO3), and pyrite (FeS2).
- Composition
- Sodalite: Sodium, aluminum, silicon, oxygen, chlorine. Lapis Lazuli: Sodium, calcium, aluminum, silicon, oxygen, sulfur, iron, carbon.
Explore Sodalite and Lapis Lazuli
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