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Rapakivi granite is an Igneous rock intrusive rock and variant of alkali feldspar granite. It is characterized by large, rounded crystals of orthoclase each with a rim of oligoclase (a variety of plagioclase). Common mineral components include hornblende and biotite. The name has come to be used most frequently as a textural term where it implies plagioclase rims around orthoclase in plutonic (intrusive) rocks. Rapakivi is a Finnish language compound of "rapa" (meaning "mud" or "sand", while rapautua means "to erode") and "kivi" (meaning "rock"), because the different heat expansion coefficients of the component minerals make exposed rapakivi crumble easily into sand. Tietoaineistot – maaperäkartan käyttöopas – rapautuminen – GTK
Rapakivi was first described by Finnish people petrologist Jakob Sederholm in 1891."Ueber die finnländischen Rapakiwigesteine Since then, southern Finland's rapakivi granite have been the type locality of this variety of granite." 3000 miljoonaa vuotta, Suomen Kallioperä " Finnish geological society, 1998, chapter 9, . Language: Finnish.
Rapakivi granites have formation ages from Archean to recent and are usually attributed to anorogenic plate tectonics. They have formed in shallow (a few km deep) sills of up to 10 km thickness.
Rapakivi granites are often found associated with intrusions of anorthosite, norite, charnockite and mangerite. It has been suggested that the entire suite results from the fractional crystallization of a single parental magma.Zhang, S-H., Liu, S-W., Zhao, Y., Yang, J-H. Song, B. and Liu, X-M. The 1.75–1.68 Ga anorthosite-mangerite-alkali granitoid-rapakivi granite suite from the northern North China Craton: Magmatism related to a Paleoproterozoic orogen. Precambrian Research, 155, 287–312.
Rapakivi is high in fluoride, ranging 0.04–1.53%, compared to other similar rocks at around 0.35%. Consequently, groundwater in rapakivi zones is high in fluoride (1–2 mg/L), making the water naturally fluoridated. Some water companies actually have to remove fluoride from the water.
The uranium content of rapakivi is fairly high, up to 24 ppm. Thus, in rapakivi zones, the hazard from radon, a decay product of uranium, is elevated. Some indoor spaces surpass the 400 Bq/m3 safety limit.Valmari, T., Arvela, H., ja Reisbacka, H. 2012. Radon in Finnish apartment buildings. Radiation Protection Dosimetry, 152, 146–149.Weltner, A., Mäkeläinen, I., ja Arvela, H. 2002. Radon mapping strategy in Finland. In: International Congress Series 1225, 63–69.
A more recent definition by Haapala & Rämö states:
Rapakivi granites are type-A granites, where at least in larger associated have granites with rapakivi structures.
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