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Sunstone is a plagioclase feldspar, specifically oligoclase, characterized by a distinctive optical phenomenon known as aventurescence or schiller. This effect manifests as a glittering or spangled appearance caused by the reflection of light from minute, platy inclusions, most commonly hematite or goethite, but sometimes copper. The color of sunstone can range from colorless or yellow to orange, pink, red, and even green, often with a metallic sheen from the inclusions. It is a member of the feldspar group, which is the most abundant mineral group in the Earth's crust.
How to Identify
- Color
- Typically yellow, orange, pink, red, or green, often with a metallic glitter from inclusions.
- Luster
- Vitreous to sub-vitreous on cleavage surfaces; aventurescent (spangled) due to inclusions.
- Texture
- Crystalline, often granular or massive. Individual crystals can be tabular or prismatic.
- Crystal Form
- Typically anhedral to subhedral grains in igneous and metamorphic rocks. Well-formed crystals are monoclinic (for orthoclase component) or triclinic (for albite-anorthite series, including oligoclase), often tabular or prismatic.
- Cleavage
- Two distinct cleavages at approximately 90 degrees (86-94 degrees), characteristic of feldspars.
- Geological Environment
- Felsic to intermediate igneous rocks (volcanic and plutonic), some metamorphic rocks, and placer deposits.
Key Facts
- Hardness: 6 to 6.5 on the Mohs scale
- Specific Gravity: 2.62 to 2.68
- 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 on {001} and good on {010}, intersecting at approximately 86-94 degrees.
- Composition: Sodium calcium aluminum silicate, (Na,Ca)(Al,Si)AlSi2O8, with NaAlSi3O8 (albite) content between 70% and 90% and CaAl2Si2O8 (anorthite) content between 10% and 30%.
Quick Check
- Color: Yellow, orange, pink, red, green, often with metallic glitter.
- Luster: Vitreous to sub-vitreous, with aventurescence.
- Streak: White
Physical Characteristics
- Crystal Habit: Typically anhedral to subhedral grains, massive, or in tabular to prismatic crystals.
- Cleavage Type: Perfect on {001} and good on {010}, forming two planes at nearly 90 degrees.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to sub-vitreous, with aventurescence.
Formation
Sunstone, specifically the aventurescent variety of oligoclase, forms primarily in igneous rocks such as felsic to intermediate volcanic rocks (e.g., rhyolite, dacite, andesite) and plutonic rocks (e.g., granite, granodiorite). It can also occur in metamorphic rocks, particularly those derived from igneous protoliths, and in placers where it has been weathered and transported. The characteristic aventurescence (schiller effect) is due to inclusions of hematite or goethite platelets, which are typically oriented along specific crystallographic planes during crystal growth or exsolution.
Usage
Primarily used as a gemstone, particularly in cabochons, beads, and faceted stones, due to its attractive aventurescence. It is also collected by mineral enthusiasts. Historically, some cultures attributed mystical properties to sunstone.
Age Distribution
Found in rocks of various ages, from Precambrian to Cenozoic, depending on the geological setting.
Where to Find
Oregon, USA
Famous for its copper-bearing sunstone, often exhibiting a schiller effect and a range of colors including green and red. Found in basalt flows.
Norway
Known for oligoclase sunstone with hematite inclusions, often found in pegmatites and gneisses.
Tanzania
Produces sunstone with strong aventurescence, often in orange and red hues.
India
Source of sunstone, typically with hematite inclusions, found in various geological settings.
Canada (Quebec)
Some occurrences of aventurescent feldspar have been noted.
Russia
Ural Mountains region has produced sunstone.
Finding Tips
Look for Igneous Rocks
Sunstone is commonly found in felsic to intermediate volcanic rocks (e.g., rhyolite, dacite) and plutonic rocks (e.g., granite, granodiorite). Examine outcrops of these rock types.
Check Placer Deposits
Due to its relative hardness and resistance to weathering, sunstone can be found in alluvial deposits (riverbeds, gravels) downstream from its primary source.
Identify Aventurescence
The key identifying feature is the spangled or glittery effect caused by internal reflections from tiny mineral inclusions. Rotate the sample under light to observe this phenomenon.
Examine Cleavage
Like all feldspars, sunstone exhibits two distinct cleavage planes at nearly right angles. This can help differentiate it from quartz or glass.
Consider Associated Minerals
Sunstone often occurs with other common igneous minerals such as quartz, biotite, hornblende, and other feldspars.
Similar Rocks
Moonstone
Orthoclase, Albite, or a mixture of both
Also known as: Adularia, Orthoclase, Albite
Labradorite
Labradorite
Also known as: Spectrolite
Goldstone
N/A (man-made material)
Also known as: Monk's Gold
Scientific Classification
- Mineral Class
- Silicates
- 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 ranging from Ab70An30 to Ab90An10 (70-90% albite, 10-30% anorthite).
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