felsiset

Felsiset is the Finnish term for felsite, a fine-grained, silica-rich igneous rock. In geology, felsites are light-colored rocks that crystallize from silica-rich magmas or form from rapid cooling of volcanic material, resulting in minimal crystal growth and a predominantly glassy or microcrystalline texture.

Texture and mineralogy: Felsites are typically microcrystalline or glassy and may show afelsitic texture, where crystals are too small to identify with the naked eye. They are commonly composed mainly of quartz and alkali feldspar, with minor plagioclase and little to no ferromagnesian minerals. Silica content is high, often over 70 percent by weight.

Formation and occurrence: Felsites form by rapid cooling of silica-rich magma, such as rhyolitic or dacitic compositions, or by the consolidation of volcanic ash and pumice into fine-grained rocks. They occur in volcanic arcs and other continental crust settings, often in lava domes, flows, or tuff deposits.

Classification and usage: In modern petrology, the term felsite is used less for routine naming and more as a descriptive term. More specific names—such as rhyolite, dacite, or quartz-rich glassy rocks—are preferred when mineralogy and chemistry are known. The adjective felsitic describes the texture of very fine-grained, glassy, or cryptocrystalline rocks.