How to identify Porphyritic Rhyolite

To correctly identify Porphyritic Rhyolite (Rhyolite with phenocrysts of feldspar and possibly quartz), 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 light-colored, ranging from white, light gray, pink, tan, to reddish-brown. Darker colors can occur if it contains more mafic minerals or is weathered.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Groundmass can be dull to earthy if fine-grained, or vitreous (glassy) if obsidian-like. Phenocrysts will exhibit their characteristic lusters (e.g., vitreous for quartz, pearly to vitreous for feldspar).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porphyritic texture is the key identifying feature, with visible phenocrysts (typically 1 mm to several cm) set in an aphanitic (fine-grained) or glassy groundmass. Flow banding may also be present.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Phenocrysts of quartz often appear as bipyramidal or anhedral grains. Feldspar phenocrysts can be euhedral to subhedral, often tabular or blocky. Groundmass crystals are too small to discern without a microscope.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Cleavage is not a characteristic of the rock as a whole, but individual phenocrysts will exhibit their characteristic cleavage (e.g., feldspar has two good cleavages at or near 90 degrees; quartz has no cleavage).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Associated with explosive volcanic eruptions, lava flows, and domes in continental volcanic arcs, rift zones, and hot spots. Often found in caldera complexes and ignimbrite sheets.

Key Facts

Hardness

6-7 on Mohs scale (due to quartz and feldspar content)

Specific Gravity

2.3-2.7 g/cm³

Crystal System

Phenocrysts: Quartz (trigonal), Feldspar (monoclinic or triclinic). Groundmass: Microcrystalline or amorphous.

Color

Light-colored: white, light gray, pink, tan, reddish-brown.

Luster

Vitreous (glassy) for quartz phenocrysts, pearly to vitreous for feldspar phenocrysts. Groundmass can be dull to earthy.

Transparency

Opaque to translucent (phenocrysts can be transparent to translucent).

Fracture

Conchoidal (especially if glassy groundmass or quartz phenocrysts) to irregular.

Cleavage

Absent in the rock as a whole. Feldspar phenocrysts exhibit 2 good cleavages.

Composition

Felsic (high silica content, >69% SiO2). Predominantly quartz, alkali feldspar (orthoclase/sanidine), and plagioclase, with minor biotite, hornblende, or pyroxene.

Physical characteristics

• Crystal Habit: Phenocrysts are typically euhedral to subhedral, often blocky or tabular for feldspar, and bipyramidal or anhedral for quartz. Groundmass is microcrystalline or glassy.
• Cleavage Type: Not applicable to the rock as a whole. Feldspar phenocrysts exhibit good cleavage.
• Fracture Type: Conchoidal to irregular.
• Tenacity: Brittle.
• Luster Type: Vitreous to dull.

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