How to identify Rhyolite Porphyry

To correctly identify Rhyolite Porphyry (Rhyolite Porphyry), 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: pink, reddish-brown, light gray, cream, or sometimes greenish. The color is influenced by the groundmass and the presence of iron oxides.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to vitreous (glassy) in the groundmass, with phenocrysts exhibiting vitreous (quartz, feldspar) or pearly (feldspar) luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porphyritic, characterized by visible phenocrysts (typically quartz and/or feldspar) set within a fine-grained (aphanitic) or glassy groundmass. Flow banding may be present.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Phenocrysts often show euhedral to subhedral crystal forms (e.g., bipyramidal quartz, tabular feldspar). Groundmass crystals are anhedral and microscopic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Phenocrysts of feldspar will exhibit cleavage (typically two directions at or near 90 degrees). Quartz phenocrysts lack cleavage. The groundmass is too fine-grained to observe cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Associated with explosive volcanic eruptions, lava flows, and shallow intrusive bodies (dikes, sills) in continental volcanic arcs, rift zones, and hot spots where felsic magmatism occurs.

Key Facts

Hardness

6-7 on Mohs scale (due to quartz and feldspar phenocrysts and silica-rich groundmass)

Specific Gravity

2.4-2.6 g/cm3

Crystal System

Phenocrysts (e.g., quartz: trigonal; feldspar: monoclinic or triclinic); groundmass is typically microcrystalline or amorphous.

Color

Light-colored: pink, reddish-brown, light gray, cream, sometimes greenish.

Luster

Dull to vitreous (groundmass), vitreous (quartz), pearly (feldspar).

Transparency

Opaque to translucent (phenocrysts can be transparent).

Fracture

Conchoidal to irregular (groundmass), conchoidal (quartz phenocrysts).

Cleavage

Absent in groundmass; present in feldspar phenocrysts (two directions at or near 90 degrees).

Composition

Felsic (high silica content, >69% SiO2). Predominantly quartz, alkali feldspar (orthoclase), plagioclase, with minor biotite, hornblende, and accessory minerals like zircon, apatite, magnetite.

Physical characteristics

• Crystal Habit: Phenocrysts are typically euhedral to subhedral (e.g., bipyramidal quartz, tabular feldspar). Groundmass is aphanitic or glassy.
• Cleavage Type: Feldspar phenocrysts exhibit good cleavage (two directions). Quartz phenocrysts and groundmass lack cleavage.
• Fracture Type: Conchoidal to irregular.
• Tenacity: Brittle.
• Luster Type: Dull to vitreous.

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