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Black Obsidian with Phenocrysts is a natural volcanic glass characterized by its typically dark, often black, color and the presence of visible, larger mineral crystals (phenocrysts) embedded within its glassy matrix. The rapid cooling of high-silica lava prevents the formation of a crystalline structure, resulting in an amorphous glass. The phenocrysts represent minerals that crystallized slowly within the magma chamber before the rapid eruption and cooling of the remaining melt.
How to Identify
- Color
- Predominantly black, but can also be dark brown or dark gray. The phenocrysts will have their characteristic mineral colors (e.g., white/gray for feldspar, dark for pyroxene/amphibole).
- Luster
- Vitreous (glassy) to sub-vitreous.
- Texture
- Aphanitic (glassy) groundmass with macroscopic, well-formed crystals (phenocrysts) embedded within it, giving it a porphyritic texture.
- Crystal Form
- The glassy matrix is amorphous. Phenocrysts exhibit their characteristic euhedral to subhedral crystal forms (e.g., tabular for feldspar, prismatic for pyroxene/amphibole).
- Cleavage
- None in the glassy matrix; exhibits conchoidal fracture. Phenocrysts will display their characteristic cleavage planes if present (e.g., two at ~90 degrees for feldspar, two at ~90 degrees for pyroxene, two at ~60/120 degrees for amphibole).
- Geological Environment
- Associated with explosive volcanic eruptions of felsic (rhyolitic) magma, typically found in lava flows, domes, and margins of dikes and sills where rapid cooling occurred.
Key Facts
- Hardness: 5 to 5.5 on the Mohs scale (for the glass matrix). Phenocrysts will have their own hardness (e.g., quartz 7, feldspar 6-6.5).
- Specific Gravity: 2.35 to 2.60 g/cm3.
- Crystal System: Amorphous (glassy) for the matrix. Phenocrysts will have their specific crystal systems (e.g., monoclinic for feldspar, quartz is trigonal).
- Color: Black, dark brown, dark gray; phenocrysts vary.
- Luster: Vitreous (glassy).
- Transparency: Opaque to translucent on thin edges.
- Fracture: Conchoidal (for the glass matrix).
- Cleavage: None (for the glass matrix).
- Composition: Primarily SiO2 (70-75% by weight) with minor amounts of Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, and H2O. Phenocrysts are typically quartz, feldspar (plagioclase or orthoclase), biotite, hornblende, or pyroxene.
Quick Check
- Color: Black to dark gray/brown with visible lighter or darker mineral inclusions.
- Luster: Vitreous (glassy).
- Streak: White (due to its amorphous nature and high silica content).
Physical Characteristics
- Crystal Habit: Amorphous (glassy) matrix; phenocrysts are typically euhedral to subhedral crystals.
- Cleavage Type: Absent in the glass matrix. Phenocrysts may exhibit cleavage (e.g., perfect in two directions for feldspar).
- Fracture Type: Conchoidal (for the glass matrix).
- Tenacity: Brittle.
- Luster Type: Vitreous.
Formation
Formed from the rapid cooling of felsic lava (rhyolitic composition) with pre-existing crystals (phenocrysts) that had already formed within the magma chamber prior to eruption.
Usage
Historically used for cutting tools, weapons, and ceremonial objects due to its sharp conchoidal fracture. Modern uses include surgical scalpels, jewelry, and decorative objects. The phenocrysts add aesthetic appeal.
Age Distribution
Typically Cenozoic to recent, as its glassy nature is prone to devitrification over geological time.
Where to Find
Newberry Volcano, Oregon, USA
Known for extensive obsidian flows, including varieties with phenocrysts.
Glass Mountain, California, USA
Part of the Long Valley Caldera, famous for its obsidian deposits, some containing visible crystals.
Lipari Island, Italy
A classic locality for obsidian, often with phenocrysts of feldspar.
Mexico
Various volcanic regions, particularly in central Mexico, yield obsidian with phenocrysts.
Finding Tips
Look for Volcanic Regions
Obsidian is exclusively found in areas of recent (geologically speaking) volcanic activity, particularly where rhyolitic lavas have erupted.
Examine Lava Flows and Domes
Obsidian forms on the outer margins of lava flows and domes where cooling is most rapid. Look for glassy, dark rocks.
Check for Conchoidal Fracture
A key identifying feature of obsidian is its sharp, curved, shell-like fracture surfaces. The presence of phenocrysts will not alter this characteristic of the glassy matrix.
Safety Precautions
Obsidian edges can be extremely sharp, comparable to surgical scalpels. Handle with care, and consider wearing gloves and eye protection when collecting or breaking samples. Avoid inhaling obsidian dust, as it contains crystalline silica (from the phenocrysts) and amorphous silica, which can be a respiratory irritant.
Similar Rocks
Obsidian
Volcanic Glass (SiO2)
Also known as: Volcanic Glass
Pumice
Highly vesicular volcanic glass
Also known as: Volcanic Foam
Scoria
Vesicular basaltic rock
Also known as: Basaltic Scoria
Pitchstone
Hydrated volcanic glass
Also known as: Waxy Obsidian
Scientific Classification
- Mineral Class
- Not a true mineral due to its amorphous structure, but classified as a volcanic glass (rock).
- Group
- Volcanic Glass
- Crystal System
- Amorphous
- Chemical Formula
- Primarily SiO2 (amorphous), with various mineral formulas for the phenocrysts.
- Composition
- Felsic (rhyolitic) volcanic glass with embedded mineral crystals.
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