How to identify Sodalite and Lapis Lazuli

To correctly identify Sodalite and Lapis Lazuli (Sodalite (Na8(Al6Si6O24)Cl2) and Lapis Lazuli (a rock composed primarily of lazurite, calcite, and pyrite)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Sodalite: Vitreous to greasy. Lapis Lazuli: Dull to greasy, sometimes waxy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Sodalite: Distinct dodecahedral cleavage (three directions at 90 degrees). Lapis Lazuli: No distinct cleavage as a rock; lazurite has imperfect dodecahedral cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

Explore Sodalite and Lapis Lazuli