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Sunstone is a plagioclase feldspar mineral, specifically a variety of oligoclase, known for its distinctive optical phenomenon called aventurescence or schiller. This effect is caused by minute, platy inclusions of hematite, goethite, or sometimes copper, which reflect light at various angles, producing a glittering or spangled appearance. The color of sunstone can range from colorless to yellow, orange, red, and brown, often with a metallic sheen. The intensity and color of the aventurescence depend on the size, density, and composition of the inclusions.
How to Identify
- Color
- Typically yellow, orange, red, or brown, often with a metallic or glittery sheen due to aventurescence. Can also be colorless.
- Luster
- Vitreous to sub-vitreous, with a distinct metallic glitter (aventurescence) on cleavage surfaces or polished faces.
- Texture
- Crystalline, often granular or massive. Individual crystals can be tabular or prismatic.
- Crystal Form
- Typically occurs as anhedral to subhedral grains within igneous or metamorphic rocks. Euhedral crystals are less common but can be tabular or prismatic.
- Cleavage
- Perfect in two directions, nearly at 90 degrees (specifically, 94 degrees for oligoclase), characteristic of feldspar minerals.
- Geological Environment
- 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).
Quick Check
- Color: Yellow, orange, red, brown, or colorless with a metallic glitter.
- Luster: Vitreous to sub-vitreous with aventurescence.
- Streak: White
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).
Formation
Sunstone, specifically the oligoclase variety, forms primarily in igneous rocks such as granites, syenites, and anorthosites, and also in some metamorphic rocks. Its characteristic aventurescence (schiller effect) is due to inclusions of hematite or goethite, which are typically exsolved during cooling or later hydrothermal alteration. These inclusions are often oriented along specific crystallographic planes, leading to the directional sparkle.
Usage
Primarily used as a gemstone for jewelry due to its attractive aventurescence. It is cut into cabochons, beads, and faceted stones. Collector's specimens are also sought after. Historically, some cultures attributed mystical properties to it.
Age Distribution
Found in rocks of various ages, from Precambrian to Cenozoic, depending on the geological setting of its formation.
Where to Find
Oregon, USA
Famous for its copper-bearing sunstone, which exhibits a unique schiller effect and can be found in various colors, including green and red. Occurs in basaltic lava flows.
Norway
Known for sunstone with hematite inclusions, often found in pegmatites and metamorphic rocks.
India
Produces sunstone with hematite or goethite inclusions, often from metamorphic terrains.
Tanzania
Source of sunstone with good aventurescence.
Canada
Found in various localities, often associated with anorthosite complexes.
Finding Tips
Look for Igneous and Metamorphic Rocks
Sunstone is typically found in specific geological environments. Focus your search in areas known for granites, syenites, anorthosites, basalts, or certain metamorphic rocks.
Identify the Schiller Effect
The most distinctive feature of sunstone is its aventurescence. Look for a glittery or spangled effect when rotating the specimen under a light source. This is crucial for distinguishing it from other feldspars.
Check for Cleavage
Like all feldspars, sunstone exhibits two perfect cleavage planes at nearly 90 degrees. This can help confirm it's a feldspar, though not specifically sunstone.
Consider Associated Minerals
Sunstone often occurs with other minerals typical of its host rock, such as quartz, hornblende, biotite, and other feldspars. Familiarize yourself with these associations.
Similar Rocks
Aventurine Quartz
Quartz (variety)
Also known as: Green Aventurine, Red Aventurine
Goldstone
Synthetic glass with copper inclusions
Also known as: Monk's Gold
Labradorite
Labradorite (variety of Plagioclase Feldspar)
Also known as: Spectrolite
Scientific Classification
- Mineral Class
- Silicate
- Group
- Feldspar Group (Plagioclase Series)
- Crystal System
- Triclinic
- Chemical Formula
- (Na,Ca)(Al,Si)AlSi2O8
- Composition
- Oligoclase is an intermediate member of the plagioclase solid solution series, with a composition between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). Specifically, it contains 70-90% albite component (Ab70-90) and 10-30% anorthite component (An10-30). The aventurescence is due to inclusions of hematite, goethite, or copper.
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