How to identify Obsidian with phenocrysts

To correctly identify Obsidian with phenocrysts (Volcanic glass (felsic igneous rock)), 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

   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).

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

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

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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)).

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)

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