How to identify Rhyolite / Dacite

To correctly identify Rhyolite / Dacite (Rhyolite or Dacite), 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, gray, white, light green. Can be darker if rich in mafic phenocrysts or if glassy (obsidian is black).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to vitreous (glassy) in the groundmass; phenocrysts can be vitreous (quartz, feldspar) or pearly (mica).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Aphanitic (fine-grained), porphyritic (phenocrysts in a fine-grained groundmass), or glassy (obsidian). Can be vesicular (pumice) or flow-banded.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual crystals (phenocrysts) are typically euhedral to subhedral, often quartz (hexagonal bipyramids), feldspar (tabular), or biotite (hexagonal flakes). Groundmass crystals are too small to discern without magnification.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No distinct cleavage planes visible in the fine-grained groundmass. Phenocrysts may exhibit their characteristic cleavage (e.g., feldspar).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Associated with explosive volcanic eruptions, forming lava flows, domes, and pyroclastic deposits (tuffs, ignimbrites). Common in continental volcanic arcs (e.g., Andes, Cascades), continental rifts, and hot spots.

Key Facts

Hardness

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

Specific Gravity

2.3-2.7 g/cm³ (lower for vesicular varieties like pumice, which can be <1 g/cm³).

Crystal System

Not applicable for the rock as a whole; constituent minerals have their own crystal systems (e.g., quartz is trigonal, feldspars are triclinic or monoclinic).

Color

Light-colored (pink, gray, white, light green); can be dark (black) if glassy (obsidian).

Luster

Dull to vitreous.

Transparency

Opaque to translucent (thin edges). Obsidian is opaque.

Fracture

Conchoidal (especially in glassy varieties like obsidian), irregular, or splintery.

Cleavage

Absent in the fine-grained groundmass; present in individual phenocrysts (e.g., feldspar).

Composition

High silica content (Rhyolite >69% SiO2, Dacite 63-69% SiO2). Predominantly quartz, plagioclase feldspar, potassium feldspar, with minor biotite, hornblende, or pyroxene.

Physical characteristics

• Crystal Habit: Aphanitic (microcrystalline), porphyritic (phenocrysts in a fine-grained groundmass), or glassy. Vesicular or flow-banded textures are common.
• Cleavage Type: Not applicable for the rock as a whole; constituent minerals exhibit their characteristic cleavage (e.g., feldspar has two good cleavages at ~90 degrees).
• Fracture Type: Conchoidal (especially glassy varieties), irregular, or splintery.
• Tenacity: Brittle.
• Luster Type: Dull to vitreous.

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