Rapakivi granite is anigneousintrusive rock and variant ofalkali feldspar granite. It is characterized by large, rounded crystals oforthoclase each with a rim ofoligoclase (a variety ofplagioclase). Common mineral components includehornblende andbiotite. 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 aFinnish compound of "rapa" (meaning "mud" or "sand", whilerapautua means "to erode") and "kivi" (meaning "rock"),[1] because the differentheat expansion coefficients of the component minerals make exposed rapakivi crumble easily into sand.[2]
Rapakivi was first described byFinnish petrologistJakob Sederholm in 1891.[3] Since then, southernFinland's rapakivi graniteintrusions have been thetype locality of this variety of granite.[4] In the same publication, Sederholm introduced the termrapakivi texture to describe the occurrence of two generations ofquartz andalkali feldspar crystals in rapakivi granites, the older alkali feldspar crystals beingovoid-shaped.[5]
Finnish Rapakivi granites occur in the Åland, Laitila, Vehmaa, and Wiborgbatholiths. The age of the rapakivi granites of the Wiborg batholith ranges between 1646 and 1627Ma; these granites were intruded into older metamorphic rocks. The Wiborg rapaviki granites consist of 80% wiborgite, and smaller amounts of pyterlite,porphyritic rapakivi granite, even-grained rapakivi granite, andaplitic rapakivi granite.[8]
Rapakivi granites have formation ages from Archean to recent and are usually attributed toanorogenictectonic settings. They have formed in shallow (a few km deep)sills of up to 10 km thickness.[citation needed]
Rapakivi is enriched in K, Rb, Pb, Nb, Ta, Zr, Hf, Zn, Ga, Sn, Th, U, F andrare earth elements, and poor in Ca, Mg, Al, P and Sr. Fe/Mg, K/Na and Rb/Sr ratios are high. SiO2 content is 70.5%, which makes rapakivi an acidic granite.[11]
Rapakivi is high influoride, 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.[11][12]
The uranium content of rapakivi is fairly high, up to 24 ppm. Thus, in rapakivi zones, the hazard fromradon, a decay product of uranium, is elevated. Some indoor spaces surpass the 400 Bq/m3 safety limit.[13][14]
Most (but not all) orthoclase crystals haveplagioclase rims (wiborgite orviborgite type, named after the city ofVyborg)[16]: 157
Orthoclase andquartz have crystallized in two phases, early quartz is in tear-drop shaped crystals (pyterlite type, named after the location ofPyterlahti).[16]: 134 [17]
A more recent definition by Haapala & Rämö is:[18]Rapakivi granites areA-type granites that, at least in the larger batholiths, have rapakivi texture.
Rapakivi is the material used inÅland'smediaeval stone churches.[19] In 1770, a rapakivi granitemonolith boulder, the "Thunder Stone", was used as thepedestal for theBronze Horsemanstatue inSaint Petersburg, Russia. Weighing 1,250tonnes, this boulder is claimed to be the largest stone ever moved by humans.[20] Modern building uses of rapakivi granites are in polished slabs used for covering buildings, floors, counter tops or pavements. As abuilding material, rapakivi granite of the wiborgite type is also known as "Baltic Brown".[21][22]
^Some geologists of the first half of the 20th century regarded the rapakivi granites as "granitized"Jotnian sediments, an idea which is now discredited.[10]
^Haapala, I.; Rämö, O.T. (1992). "Tectonic setting and origin of the Proterozoic rapakivi granites of southeastern Fennoscandia".Transactions of the Royal Society of Edinburgh: Earth Sciences.83 (1–2):165–171.doi:10.1017/s0263593300007859.S2CID129835203.
^Teixeira, Wilson; D'Agrella-Filho, Manoel S.; Hamilton, Mike A.; Ernst, Richard E.; Girardi, Vicente A.V.; Mazzucchelli, Maurizio; Bettencourt, Jorge S. (2013). "U–Pb (ID-TIMS) baddeleyite ages and paleomagnetism of 1.79 and 1.59 Ga tholeiitic dyke swarms, and position of the Rio de la Plata Craton within the Columbia supercontinent".Lithos.174:157–174.Bibcode:2013Litho.174..157T.doi:10.1016/j.lithos.2012.09.006.hdl:11380/887090.
^Bettencourt, J.S.; Tosdal, R.M.; Leite, W.B.; Payolla, B.L. (1999). "Mesoproterozoic rapakivi granites of the Rondoˆnia Tin Province, southwestern border of the Amazonian craton, Brazil — I. Reconnaissance U–Pb geochronology and regional implications".Precambrian Research.95 (1–2):41–67.Bibcode:1999PreR...95...41B.doi:10.1016/S0301-9268(98)00126-0.
^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.
^abRämö, T., Haapala, I. ja Laitakari, I. 1998. Rapakivigraniitit – peruskallio repeää ja sen juuret sulavat. In: Lehtinen, M., Nurmi, RA., Rämö, O.T. (Toim.), Suomen kallioperä – 3000 vuosimiljoonaa. Suomen geologinen seura. Gummerus kirjapaino, Jyväskylä. 257–283.
^Lahermo, P.; Sandström, H.; ja Malisa, E. (1991). "The occurrence and geochemistry of fluorides in natural waters in Finland and East Africa with reference to their geomedical implications".Journal of Geochemical Exploration.41 (1–2):65–79.doi:10.1016/0375-6742(91)90075-6.
^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.
^Vorma A., 1976. On the petrochemistry of rapakivi granites with special reference to the Laitila massif, southwestern Finland. Geological Survey of Finland, Bulletin 285, 98 pages.
^abLe Maitre, R. W., ed. (2002).Igneous Rocks — A Classification and Glossary of Terms. Cambridge: Cambridge University Press.ISBN978-0-521-66215-4.
^Haapala, I.; Rämö, O.T. (1992). "Tectonic setting and origin of the Proterozoic rapakivi granites of southeastern Fennoscandia".Transactions of the Royal Society of Edinburgh: Earth Sciences.83 (1–2):165–171.doi:10.1017/s0263593300007859.S2CID129835203.
Media related toRapakivi granite at Wikimedia Commons