How to identify Black Obsidian with Phenocrysts

To correctly identify Black Obsidian with Phenocrysts (Volcanic Glass (SiO2) with mineral inclusions), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to sub-vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Aphanitic (glassy) groundmass with macroscopic, well-formed crystals (phenocrysts) embedded within it, giving it a porphyritic texture.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   The glassy matrix is amorphous. Phenocrysts exhibit their characteristic euhedral to subhedral crystal forms (e.g., tabular for feldspar, prismatic for pyroxene/amphibole).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

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