How to identify Sunstone

To correctly identify Sunstone (Plagioclase Feldspar (Oligoclase or Labradorite 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, pink, red, or reddish-brown, often with a metallic glitter. Can also be colorless, green, or blue.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-vitreous, with a distinct metallic glitter (aventurescence) from inclusions.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically anhedral to subhedral grains within igneous or metamorphic rocks. When well-formed, it exhibits triclinic crystal habits, often tabular or prismatic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, nearly at 90 degrees (86 degrees and 94 degrees), characteristic of plagioclase feldspars.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in mafic igneous rocks (e.g., basalts, gabbros), felsic igneous rocks (e.g., granites, pegmatites), and some metamorphic rocks. The most famous occurrences are in basaltic lava flows where it forms phenocrysts.

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, pink, red, reddish-brown, green, blue, often with metallic glitter.

Luster

Vitreous to sub-vitreous, with aventurescence (schiller effect).

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions (pinacoidal), nearly at 90 degrees (86° and 94°).

Composition

Sodium calcium aluminum silicate (Na,Ca)(Al,Si)AlSi2O8)

Physical characteristics

• Crystal Habit: Typically anhedral to subhedral grains, massive, granular. Can form tabular or prismatic crystals.
• Cleavage Type: Perfect on {001} and {010}, nearly at right angles.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous, with aventurescence.

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