How to identify Sunstone

To correctly identify Sunstone (Feldspar (Oligoclase variety)), 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

   Typically yellow, orange, red, or brown, often with a metallic or glittery sheen due to aventurescence. Can also be colorless.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-vitreous, with a distinct metallic glitter (aventurescence) on cleavage surfaces or polished faces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Crystalline, often granular or massive. Individual crystals can be tabular or prismatic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically occurs as anhedral to subhedral grains within igneous or metamorphic rocks. Euhedral crystals are less common but can be tabular or prismatic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, nearly at 90 degrees (specifically, 94 degrees for oligoclase), characteristic of feldspar minerals.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in mafic igneous rocks (e.g., basalt, gabbro), felsic igneous rocks (e.g., granite, syenite), and some metamorphic rocks. Often associated with pegmatites and anorthosites. The Oregon Sunstone, a notable variety, is found in lava flows.

Key Facts

Hardness

6 - 6.5 on the Mohs scale

Specific Gravity

2.62 - 2.67

Crystal System

Triclinic

Color

Colorless, yellow, orange, red, brown, often with a metallic glitter (aventurescence).

Luster

Vitreous to sub-vitreous, with aventurescence.

Transparency

Transparent to translucent.

Fracture

Uneven to conchoidal.

Cleavage

Perfect in two directions, nearly at 90 degrees (specifically {001} and {010} at 94 degrees).

Composition

Sodium calcium aluminum silicate, (Na,Ca)(Al,Si)AlSi2O8, with a sodium content (albite component) ranging from 70% to 90% (An10-An30).

Physical characteristics

• Crystal Habit: Typically anhedral to subhedral grains, massive, or as tabular to prismatic crystals.
• Cleavage Type: Perfect in two directions, nearly perpendicular (94 degrees).
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous, with aventurescence (schiller effect).

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