How to identify Sunstone

To correctly identify Sunstone (Plagioclase 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 on cleavage surfaces, with a distinctive metallic or spangled aventurescent effect.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Crystalline, often granular in massive forms. The aventurescence is the key textural feature.

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. Cleavage fragments are common.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, intersecting at approximately 90 degrees (specifically, 86° and 94°), characteristic of plagioclase feldspar.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in mafic igneous rocks (basalts, andesites, gabbros), some felsic igneous rocks (granites, granodiorites), and occasionally in metamorphic rocks. Also found in placer deposits where it has weathered out of its host rock.

Key Facts

Hardness

6 to 6.5 on the Mohs scale

Specific Gravity

2.62 to 2.67 g/cm³

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 (pinacoidal), intersecting at approximately 86° and 94°.

Composition

Sodium calcium aluminum silicate (Na,Ca)(Al,Si)AlSi2O8, specifically the oligoclase variety with 10-30% anorthite (CaAl2Si2O8) and 70-90% albite (NaAlSi3O8) end-members, plus inclusions of hematite or goethite.

Physical characteristics

• Crystal Habit: Typically anhedral to subhedral grains; less commonly tabular or prismatic crystals. Massive forms are common.
• Cleavage Type: Perfect in two directions, {001} and {010}, intersecting at nearly 90 degrees (86° and 94°).
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous, with aventurescence (schiller effect).

Explore Sunstone