Phonolite Identification Guide
Recognizing phonolite, a fine-grained feldspathoid volcanic rock that rings when struck, and separating it from trachyte and basalt.
Read the full Phonolite encyclopedia entry →
What Phonolite Looks Like
Phonolite is a fine-grained, silica-undersaturated volcanic rock — the extrusive equivalent of nepheline syenite. It is composed mainly of alkali feldspar (sanidine) plus feldspathoids (nepheline or leucite). Color is typically greenish-gray to bluish-gray, sometimes brown or pale, often with a slightly greasy or platy look. It is frequently porphyritic, with glassy sanidine or nepheline phenocrysts set in a fine groundmass. Its famous trait is that thin slabs ring with a clear metallic tone when struck — the source of the name ("sounding stone").
Step-by-Step Field ID Checklist
- Tap a thin slab. A clear ringing or clinking tone is a strong phonolite clue.
- Note the color. Greenish to bluish-gray, often with a platy, foliated tendency.
- Look for phenocrysts. Glassy sanidine tablets or gray nepheline crystals in a fine matrix.
- Check grain size. Aphanitic (fine) groundmass — individual grains are hard to see.
- Identify the volcanic setting. Found in alkaline volcanic provinces, plugs, and domes.
Key Diagnostic Tests
- Hardness: Feldspar/feldspathoid minerals ~5.5–6; scratches glass.
- Density: ~2.5–2.6, moderate; lighter than basalt.
- Texture: Fine-grained, often porphyritic and platy.
- Feldspathoid test: Nepheline gelatinizes in acid (lab), distinguishing it from feldspar-only rocks.
- Acid: No fizz unless carbonate-altered.
- Magnetism: Generally weak.
Common Look-Alikes and How to Tell Them Apart
- Trachyte: Closely related and also feldspar-rich, but trachyte is silica-saturated (no feldspathoids) and usually does not ring as distinctly; phonolite contains nepheline or leucite.
- Basalt: Darker, denser, heavier, and rich in pyroxene and plagioclase with no feldspathoids; basalt does not ring like phonolite.
- Andesite/dacite: Lighter intermediate lavas but silica-saturated; lack the feldspathoid minerals.
- Rhyolite: Lighter colored, very silica-rich, often flow-banded; opposite chemistry to undersaturated phonolite.
- Slate (for the ringing): Slate also rings but is a foliated metamorphic rock, splits into flat sheets, and lacks volcanic phenocrysts.
The ring test plus a feldspathoid-bearing, greenish-gray fine groundmass identifies phonolite.
Where Phonolite Is Found
Phonolite occurs in alkaline, silica-poor volcanic provinces: the East African Rift, the Eifel region of Germany, the Massif Central in France, the Canary Islands, Mount Kilimanjaro's flanks, and Devils Tower in Wyoming (a phonolite porphyry intrusion). Search alkaline volcanic fields, plugs, and dome complexes.
Frequently asked questions
How do you identify phonolite?
Phonolite is a fine-grained greenish to bluish-gray volcanic rock containing alkali feldspar and feldspathoids (nepheline or leucite); a thin slab rings with a clear tone when struck.
Why does phonolite ring when struck?
Its dense, fine-grained, often platy structure allows thin slabs to resonate with a clear metallic clink when tapped, which is how the rock earned its name meaning "sounding stone."
Phonolite vs trachyte — what's the difference?
Both are feldspar-rich alkaline volcanics, but phonolite is silica-undersaturated and contains feldspathoids like nepheline, while trachyte is silica-saturated and lacks them.
Phonolite vs basalt — how can I tell them apart?
Basalt is dark, dense, and pyroxene-rich with no feldspathoids and does not ring, whereas phonolite is lighter greenish-gray, less dense, and rings when a slab is struck.
Where is phonolite found?
It forms in alkaline, silica-poor volcanic provinces such as the East African Rift, the German Eifel, the Canary Islands, and as the intrusion forming Devils Tower in Wyoming.