Quarzporphyr

Quarzporphyr, or quartz porphyry, is a porphyritic igneous rock characterized by conspicuous quartz phenocrysts embedded in a fine-grained groundmass. The quartz crystals are typically well-formed and range from millimeters to a centimeter or more in size, contrasting with the much finer matrix surrounding them.

The groundmass of a quarzporphyr commonly consists of felspathic minerals, such as plagioclase and alkali feldspar, with minor dark minerals like biotite or hornblende. The overall chemical composition is felsic to intermediate, and the presence of quartz phenocrysts is the defining feature rather than a single fixed silica content. Accessory minerals such as zircon, apatite, or magnetite may occur but are usually rare.

Formation of quarzporphyr involves two-stage crystallization. A silica-rich magma begins to crystallize deeper in crustal sheets or shallow chambers, forming quartz phenocrysts, followed by rapid emplacement and cooling that produces a fine-grained groundmass. This texture indicates a history of slow early crystallization with later rapid cooling during extrusion or shallow intrusion.

Geographically widespread, quartz porphyries occur in various continental-margin and intra-cratonic settings where silica-rich magmas have evolved. They are described in many geological literature as quartz-porphyritic rocks and are sometimes encountered in quarrying or decorative stone contexts, though not all varieties see widespread commercial use. The term Quarzporphyr is common in German-language geology, while the English equivalent remains quartz porphyry.