Obsidian with phenocrysts

Description

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Obsidian with phenocrysts differs from pure obsidian by containing visible, larger crystals (phenocrysts) embedded within the glassy matrix. These phenocrysts represent minerals that began to crystallize from the magma at slower cooling rates within the magma chamber before the final rapid extrusion and cooling of the lava. Common phenocrysts include quartz, feldspar (plagioclase or sanidine), and biotite. The presence and type of phenocrysts can provide clues about the magma's history and crystallization conditions.

How to Identify

Color

Typically dark, ranging from black to dark brown, dark green, or rarely red. The phenocrysts are usually white (feldspar, quartz) or black/dark brown (biotite).

Luster

Vitreous (glassy) to sub-vitreous.

Texture

Aphanitic (glassy) groundmass with scattered, larger, well-formed crystals (phenocrysts) that are typically euhedral to subhedral. The texture is porphyritic-glassy.

Crystal Form

The glassy matrix has no crystal form. Phenocrysts exhibit their characteristic crystal habits (e.g., prismatic for feldspar, bipyramidal for quartz, tabular for biotite).

Cleavage

None in the glassy matrix. Phenocrysts may exhibit their characteristic cleavage (e.g., two directions at ~90 degrees for feldspar, one perfect basal for biotite).

Geological Environment

Volcanic flows, domes, and margins of lava flows where felsic magma cooled rapidly. Often found in areas of recent or geologically young volcanic activity.

Key Facts

• Hardness: 5 to 5.5 on the Mohs scale (for the glass matrix); phenocrysts like quartz (7) or feldspar (6-6.5) will be harder.
• Specific Gravity: 2.35 to 2.60 g/cm3
• Crystal System: Amorphous (glassy matrix); phenocrysts will have their respective crystal systems (e.g., trigonal for quartz, triclinic/monoclinic for feldspar).
• Color: Black, dark brown, dark green, rarely red; phenocrysts are typically white, gray, or black.
• Luster: Vitreous (glassy)
• Transparency: Opaque to translucent in thin sections or at very thin edges.
• Fracture: Conchoidal (very sharp edges)
• Cleavage: None (in the glass); phenocrysts may exhibit cleavage.
• Composition: Primarily amorphous SiO (70-75% by weight), with minor amounts of AlO, FeO, MgO, CaO, NaO, KO, and HO. Phenocrysts are typically quartz (SiO), plagioclase feldspar (NaAlSiO-CaAlSiO), sanidine (KAlSiO), or biotite (K(Mg,Fe)AlSiO(OH)).

Quick Check

• Color: Dark (black, brown, green) with lighter or darker phenocrysts
• Luster: Vitreous (glassy)
• Streak: White (due to its high silica content and lack of opaque minerals in the glass)

Physical Characteristics

• Crystal Habit: Amorphous (glassy matrix); phenocrysts are typically euhedral to subhedral, exhibiting their characteristic crystal forms.
• Cleavage Type: None (in the glass); phenocrysts may have distinct cleavage planes.
• Fracture Type: Conchoidal, producing extremely sharp edges.
• Tenacity: Brittle
• Luster Type: Vitreous (glassy)

Formation

Obsidian forms from the rapid cooling of felsic (rhyolitic) lava, preventing the growth of large crystals. When some crystals (phenocrysts) begin to grow within the magma chamber before eruption, and then the remaining melt cools rapidly, the resulting rock is obsidian with phenocrysts. These phenocrysts are typically quartz, feldspar (plagioclase or sanidine), or biotite, which crystallized at depth.

Usage

Historically, obsidian was highly valued for tools, weapons (arrowheads, spear points, knives), and cutting implements due to its conchoidal fracture producing extremely sharp edges. Modern uses include surgical scalpels (due to its superior sharpness compared to steel), decorative objects, jewelry, and as a geological specimen for studying volcanic processes and magma crystallization. Snowflake obsidian is particularly popular for ornamental purposes.

Age Distribution

Typically Cenozoic to recent, as it is an unstable glass that devitrifies over geological time. Most obsidian deposits are less than 20 million years old.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Not a mineral (it's a rock, specifically a volcanic glass)

Group

Volcanic Glass

Crystal System

Amorphous

Chemical Formula

Predominantly SiO (amorphous), with various oxides of Al, Fe, Mg, Ca, Na, K. Phenocrysts have their own specific formulas.

Composition

Felsic (rhyolitic) volcanic glass, typically 70-75% SiO, with embedded crystals of quartz, feldspar, or biotite.

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