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Snowflake Obsidian is a naturally occurring volcanic glass characterized by its dark, typically black or dark gray, glassy matrix interspersed with white to grayish-white, radially structured mineral inclusions known as spherulites. These spherulites, composed of cristobalite, resemble snowflakes, giving the rock its common name. The contrast between the dark glass and the light spherulites creates a striking visual effect. It is an extrusive igneous rock, meaning it formed from lava that cooled quickly on the Earth's surface.
How to Identify
- Color
- Dark (black, dark gray, sometimes dark brown) with white to grayish-white 'snowflake' patterns.
- Luster
- Vitreous (glassy) to sub-vitreous.
- Texture
- Aphanitic (glassy) matrix with visible spherulitic inclusions. Smooth to the touch, but can feel slightly granular where spherulites are prominent.
- Crystal Form
- Amorphous (glassy) matrix; spherulites are microcrystalline radial aggregates of cristobalite.
- Cleavage
- None (amorphous glass).
- Geological Environment
- Associated with silicic volcanic eruptions, typically found in lava flows, domes, and volcanic necks where rapid cooling of high-silica magma occurred.
Key Facts
- Hardness: 5-5.5 on Mohs scale (for the glass matrix); cristobalite is slightly harder at 6-7.
- Specific Gravity: 2.35-2.60 g/cm³
- Crystal System: Amorphous (glass) for the matrix; spherulites are microcrystalline aggregates of cristobalite (tetragonal or orthorhombic at room temperature, but formed as high-temperature cubic polymorph).
- Color: Black, dark gray, or dark brown with white to grayish-white spherulites.
- Luster: Vitreous (glassy).
- Transparency: Opaque to translucent on thin edges.
- Fracture: Conchoidal (for the glass matrix).
- Cleavage: None.
- Composition: Primarily SiO2 (70-75%), with minor amounts of Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, and H2O. The spherulites are composed of cristobalite (SiO2).
Quick Check
- Color: Black with white 'snowflakes'
- Luster: Vitreous (glassy)
- Streak: White (due to the amorphous nature and lack of significant chromophores)
Physical Characteristics
- Crystal Habit: Amorphous (glassy) matrix; spherulites are radial aggregates of microcrystalline cristobalite.
- Cleavage Type: None.
- Fracture Type: Conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy).
Formation
Snowflake Obsidian forms from the rapid cooling of viscous, silica-rich (rhyolitic) lava. The rapid quenching prevents the formation of a crystalline structure, resulting in volcanic glass (obsidian). The distinctive 'snowflake' patterns are spherulites, which are radial aggregates of microcrystalline cristobalite (a high-temperature polymorph of SiO2) that crystallize within the obsidian matrix. These spherulites form during devitrification, a process where the unstable glass begins to crystallize, often initiated by the presence of water or other volatiles, or by slow cooling within the obsidian flow.
Usage
Primarily used as an ornamental stone, for carving, cabochons, beads, and decorative objects. Historically, obsidian was used for tools, weapons, and surgical blades due to its conchoidal fracture producing extremely sharp edges. Snowflake obsidian is less commonly used for such utilitarian purposes due to the presence of the cristobalite spherulites which can make it more brittle and less uniform than pure obsidian.
Age Distribution
Typically Cenozoic to recent, associated with silicic volcanic activity.
Where to Find
United States
Significant deposits are found in the western United States, particularly in Oregon (e.g., Glass Buttes), Utah, Arizona, and California.
Mexico
Known occurrences in various volcanic regions.
Iceland
Found in areas of recent volcanic activity.
Japan
Occurs in volcanic terrains.
Argentina
Deposits are present in volcanic regions.
Canada
Some occurrences in British Columbia and other volcanic areas.
Finding Tips
Look in Volcanic Areas
Search in regions with past or present silicic volcanic activity, especially near rhyolitic lava flows and domes.
Identify Glassy Texture
Look for dark, glassy rocks with a conchoidal fracture. The 'snowflakes' will be visible upon closer inspection.
Check for Spherulites
The defining characteristic is the presence of white, radial, snowflake-like patterns within the dark glass.
Safety Precautions
Obsidian can have extremely sharp edges due to its conchoidal fracture. Handle with care to avoid cuts. Wear appropriate gloves and eye protection when breaking or shaping it. The cristobalite spherulites are generally stable within the glass matrix and do not pose an inhalation hazard unless the material is crushed into fine dust. Avoid inhaling dust if cutting or grinding.
Similar Rocks
Obsidian
Obsidian
Also known as: Volcanic Glass
Apache Tears
Obsidian nodules
Also known as: Marekanite
Pumice
Pumice
Also known as: Volcanic Foam
Scientific Classification
- Mineral Class
- Not a true mineral, but a volcanic glass. The spherulites are composed of cristobalite, a tectosilicate mineral.
- Group
- Volcanic Glass (Obsidian); Cristobalite belongs to the Silica Group.
- Crystal System
- Amorphous (glass); Cristobalite is typically tetragonal or orthorhombic at ambient conditions, but forms as a high-temperature cubic polymorph.
- Chemical Formula
- SiO2 (amorphous) + SiO2 (cristobalite)
- Composition
- Silica-rich volcanic glass with microcrystalline cristobalite inclusions.
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