How to identify Sodalite with Feldspar

To correctly identify Sodalite with Feldspar (Sodalite (Na4Al3Si3O12Cl) with Feldspar (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8)), 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

   Dominantly blue (sodalite) with white, pink, or grey (feldspar). The blue can range from pale to deep violet-blue. Feldspar can be orthoclase (pink/white), albite (white/grey), or anorthite (grey/white).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Sodalite: Vitreous to greasy. Feldspar: Vitreous to pearly.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically granular, medium to coarse-grained, interlocking crystals. Can be massive.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Sodalite: Dodecahedral crystals, often anhedral to subhedral in massive forms. Feldspar: Tabular, prismatic, or anhedral crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Sodalite: Imperfect dodecahedral cleavage (three directions at 60°/120°). Feldspar: Two distinct cleavages at or near 90°.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Alkaline igneous rocks, particularly nepheline syenites, alkali syenites, and phonolites, which are silica-undersaturated. Also found in contact metamorphic rocks adjacent to such intrusions.

Key Facts

Hardness

Sodalite: 5.5-6 (Mohs). Feldspar: 6-6.5 (Mohs).

Specific Gravity

Sodalite: 2.13-2.30. Feldspar: 2.55-2.76.

Crystal System

Sodalite: Isometric. Feldspar: Monoclinic (Orthoclase) or Triclinic (Plagioclase).

Color

Sodalite: Blue, grey, white, green, pink. Feldspar: White, grey, pink, green.

Luster

Sodalite: Vitreous to greasy. Feldspar: Vitreous to pearly.

Transparency

Sodalite: Transparent to translucent. Feldspar: Transparent to translucent.

Fracture

Sodalite: Uneven to conchoidal. Feldspar: Uneven to conchoidal.

Cleavage

Sodalite: Imperfect dodecahedral (three directions). Feldspar: Two distinct cleavages at or near 90°.

Composition

Sodalite: Sodium aluminum silicate with chloride. Feldspar: Potassium, sodium, or calcium aluminum silicate.

Physical characteristics

• Crystal Habit: Sodalite: Massive, granular, dodecahedral crystals. Feldspar: Tabular, prismatic, massive.
• Cleavage Type: Sodalite: Imperfect dodecahedral. Feldspar: Perfect to good in two directions.
• Fracture Type: Uneven to conchoidal for both.
• Tenacity: Brittle for both.
• Luster Type: Vitreous to greasy (sodalite), Vitreous to pearly (feldspar).

Explore Sodalite with Feldspar