Phonolite

Phonolite is a fine-grained extrusive igneous rock composed of alkali feldspar (sanidine) and feldspathoid minerals such as nepheline or leucite, with essentially no quartz. It is the volcanic equivalent of nepheline syenite.

Its name comes from Greek for 'sounding stone' (and the English 'clinkstone') because thin, dense slabs ring with a metallic tone when struck. It is usually grey, greenish-grey, or brownish and often platy.

Phonolite is characteristic of strongly alkaline, silica-undersaturated volcanism and is relatively uncommon compared with basalt or rhyolite.

Phonolite forms from highly evolved, alkali-rich and silica-poor magmas, typically generated by extensive fractional crystallization of alkaline basaltic magma or by low-degree partial melting in the mantle. The silica deficiency causes feldspathoids to crystallize instead of quartz.

It erupts as lava flows, domes, and intrusions in continental rift zones and ocean-island volcanoes. Famous occurrences include the Devils Tower in Wyoming (a phonolite porphyry), the East African Rift volcanoes, the Canary Islands, Tenerife, and the volcanic regions of the Czech Republic and Germany.

Look for a dense, fine-grained grey to greenish-grey rock that is often platy and may ring like a bell when a thin slab is tapped. Small glassy sanidine or square leucite crystals may be visible.

The ringing sound, platy fracture, and lack of quartz are good clues. Confirming the feldspathoids (nepheline, leucite) usually needs a hand lens or lab analysis.

Look-alikes include trachyte (similar but with little feldspathoid and no ring as reliably), basalt (darker, denser, no feldspathoid sound), and andesite. Composition often requires thin-section study.

Phonolite is used as crushed aggregate, railway ballast, and road stone, and the platy varieties have been used as roofing and paving slabs. Some phonolite is ground for use as a soil conditioner and slow-release potassium source in agriculture, and as a raw material in glass and ceramics.

Geologically it is significant as a marker of alkaline, silica-poor magmatism and forms dramatic landforms like Devils Tower. It has no gemstone use, though distinctive specimens and 'clinkstone' that rings are valued by collectors.