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Sunstone is a plagioclase feldspar mineral, specifically a variety of oligoclase, known for its distinctive optical effect called aventurescence or schiller. This phenomenon is a glittering or spangled effect caused by light reflecting off minute, platy inclusions within the mineral. These inclusions are typically hematite or goethite, and in the case of Oregon Sunstone, native copper. The color of sunstone can range from colorless, yellow, orange, pink, red, to green, often with a metallic sheen from the inclusions. It is a solid solution mineral between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), with oligoclase having a composition of 10-30% anorthite and 70-90% albite.
How to Identify
- Color
- Typically yellow, orange, pink, red, or green, often with a metallic glitter. The color is often unevenly distributed.
- Luster
- Vitreous to sub-vitreous, with a distinct metallic glitter (aventurescence) from inclusions.
- Texture
- Smooth to slightly granular, depending on crystal size and presence of inclusions. Often exhibits a shimmering internal play of light.
- Crystal Form
- Usually found as anhedral to subhedral grains within igneous rocks. When well-formed, it can occur as tabular or blocky crystals, though gem-quality material is often massive or in irregular fragments.
- Cleavage
- Perfect in two directions, nearly at 90 degrees (86° and 94°), characteristic of plagioclase feldspars.
- Geological Environment
- Primarily in mafic igneous rocks (e.g., basalt flows, gabbro), but also in some felsic igneous rocks (e.g., granite, syenite) and occasionally in metamorphic rocks.
Key Facts
- Hardness: 6 to 6.5 on the Mohs scale
- Specific Gravity: 2.62 to 2.67
- Crystal System: Triclinic
- Color: Colorless, yellow, orange, pink, red, green; often with 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 (86° and 94°).
- Composition: (Na,Ca)(Al,Si)AlSi2O8 - a solid solution of albite and anorthite, specifically oligoclase (An10-An30).
Quick Check
- Color: Yellow, orange, pink, red, green, often with metallic glitter.
- Luster: Vitreous with aventurescence (schiller).
- Streak: White
Physical Characteristics
- Crystal Habit: Typically anhedral to subhedral grains; rarely well-formed tabular or blocky crystals. Often massive or granular.
- Cleavage Type: Perfect on {001} and {010}, intersecting at approximately 86°.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to sub-vitreous, with a characteristic metallic glitter (aventurescence) due to inclusions.
Formation
Sunstone, specifically the aventurescent variety of oligoclase, forms in igneous rocks, particularly in mafic lavas (like basalt) and some felsic intrusive rocks (like granite or syenite). The characteristic aventurescence (schiller effect) is caused by inclusions of hematite, goethite, or sometimes copper platelets, which are exsolved during cooling or incorporated during crystallization. These inclusions are typically oriented along specific crystallographic planes.
Usage
Primarily used as a gemstone in jewelry due to its unique optical phenomenon (aventurescence). High-quality specimens are cut into cabochons, beads, and faceted stones. It is also collected by mineral enthusiasts.
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 'Oregon Sunstone,' which contains native copper inclusions, giving it a unique schiller and often bi-color or tri-color zones (red, green, yellow). Found in basalt flows in Harney and Lake Counties.
Norway
Historically significant source, particularly for material with hematite inclusions, found in pegmatites and metamorphic rocks.
India
Produces sunstone with hematite inclusions, often from pegmatitic and metamorphic environments.
Tanzania
Known for sunstone with strong aventurescence, often with red and orange hues.
Canada
Found in various locations, often associated with anorthosite complexes.
Russia
Occurrences reported in several regions.
Madagascar
Produces sunstone with good clarity and aventurescence.
Finding Tips
Look for Igneous Rocks
Sunstone is most commonly found in igneous rocks, particularly basalt flows or certain granitic/syenitic intrusions. Search in areas known for these rock types.
Identify Aventurescence
The key identifying feature is the internal glitter or schiller. Rotate the specimen under a light source to observe this effect. This distinguishes it from other feldspars.
Check for Cleavage
Like all feldspars, sunstone exhibits two distinct cleavage planes at nearly 90 degrees. This can be observed on broken surfaces.
Examine Inclusions
Under magnification, the tiny metallic inclusions (hematite, goethite, or copper) responsible for the aventurescence may be visible.
Consider Associated Minerals
In basaltic environments, sunstone may be found with other minerals typical of such rocks, like pyroxenes or olivine. In granitic settings, it might be with quartz and other feldspars.
Similar Rocks
Aventurine
Aventurine Quartz
Also known as: Green Aventurine, Red Aventurine
Goldstone
Synthetic Glass
Also known as: Monk's Gold
Labradorite
Labradorite
Also known as: Spectrolite
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Feldspar Group, Plagioclase Series
- Crystal System
- Triclinic
- Chemical Formula
- (Na,Ca)Al(Si,Al)Si2O8 (specifically Oligoclase: Na0.9-0.7Ca0.1-0.3Al1.1-1.3Si2.9-2.7O8)
- Composition
- Sodium calcium aluminum silicate, with a composition between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). Oligoclase is defined as having 10-30% anorthite component.
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