What does felsitic mean?
Felsitic is an adjective used in geology to describe rock material that consists of, or resembles, felsite — a pale, extremely fine-grained igneous rock made up largely of quartz and feldspar. The word most often qualifies the groundmass, or matrix, of volcanic rocks such as rhyolites and certain lavas, whose individual crystals are too small to be seen without a microscope. A felsitic texture is therefore dense, compact, and essentially stony in appearance, sometimes likened to unglazed porcelain. The term derives from felsite, itself built on the German word Fels, meaning rock. Because it belongs to the specialised vocabulary of petrography, felsitic rarely strays outside scientific contexts, appearing in field descriptions, thin-section analyses, and geological literature. For readers of earth-science writing, it signals precisely that a rock is both chemically silica-rich and microscopically fine-grained.
Composed of or relating to felsite, a fine-grained igneous rock consisting mainly of quartz and feldspar.
"The hand specimen was identified as felsitic porphyry from the old quarry workings."
Chiefly used in petrographic description of volcanic and hypabyssal rocks.
Having the dense, fine-grained texture characteristic of felsite, especially as applied to a rock groundmass.
"The lava's felsitic groundmass indicated rapid cooling at or near the surface."
Felsitic rocks look almost like unglazed porcelain under the microscope — so smooth and fine-grained that early geologists thought they were stone that had somehow melted into glass.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of felsitic
Felsitic derives from felsite, a term introduced by German geologists in the early nineteenth century, formed from the German Fels meaning 'rock', combined with a mineral-suffix element. English adopted the word alongside other German contributions to petrology during the same period, when systematic rock classification was developing rapidly. It shares this Germanic root with feldspar (formerly also spelled felspar), another cornerstone term of mineralogy describing the most abundant mineral group in the Earth's crust.
Related word forms
How felsitic is actually used
A technical term confined almost entirely to petrology and geology; it appears in academic writing, field reports, and textbooks rather than general prose. It carries no figurative or informal use.
Easily confused with felsitic
Feldspathic means containing or relating to feldspar specifically, whereas felsitic refers to felsite or its characteristically fine-grained, compact rock texture.