rapakivet

Rapakivi is a type of granite characterized by its distinctive texture and mineral composition. It is an intrusive igneous rock, meaning it formed from the slow cooling of magma beneath the Earth's surface. The name "rapakivi" comes from Finnish and translates to "crumbly" or "rotten" stone, referring to its tendency to weather more easily than many other granites. This weathering is often due to the alteration of its feldspar minerals.

The most striking feature of rapakivi is its ovoid or rounded phenocrysts of potassium feldspar, often surrounded by a distinct rim of plagioclase feldspar. These large crystals are embedded in a finer-grained groundmass composed of quartz, plagioclase feldspar, biotite, and sometimes amphibole. The color of rapakivi can vary, but it is commonly reddish or pinkish due to the potassium feldspar, although yellowish and grayish varieties also exist.

Rapakivi granites are found in several large intrusions around the Baltic Shield, including in Finland, Sweden, and northwestern Russia. They are also known from other parts of the world, such as Brazil and North America, though the term is most strongly associated with the Fennoscandian occurrences. These formations are typically associated with large anorogenic (non-orogenic) magmatic events, often occurring in rift settings.

Due to their distinctive appearance and relative ease of weathering, rapakivi granites have been used as building and ornamental stones, particularly in historical constructions and monuments. Their unique texture makes them easily recognizable, and their widespread distribution in the Baltic region has made them an important geological marker.