Rapakivi granite is an igneousintrusive 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 compound of "rapa" (meaning "mud" or "sand", while rapautua means "to erode") and "kivi" (meaning "rock"),[1] because the different heat expansion coefficients of the component minerals make exposed rapakivi crumble easily into sand.[2]
Rapakivi granites have formation ages from Archean to recent and are usually attributed to anorogenictectonic 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 and rare 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.[9]
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.[9][10]
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.[11][12]
Rapakivi type wiborgite
Petrography
Rapakivi type pyterlite
Vorma (1976) states that rapakivi granites can be defined as:[13]
Most (but not all) orthoclase crystals have plagioclase rims (wiborgite or viborgite type, named after the city of Vyborg)[14]: 157
Orthoclase and quartz have crystallized in two phases, early quartz is in tear-drop shaped crystals (pyterlite type, named after the location of Pyterlahti).[14]: 134 [15]
A more recent definition by Haapala & Rämö states:[16]
Rapakivi granites are type-A granites, where at least in larger associated batholites have granites with rapakivi structures.
Use as a building material
Rapakivi is the material used in Åland's mediaeval stone churches.[17] In 1770, a rapakivi granite monolith boulder, the "Thunder Stone", was used as the pedestal for the Bronze Horsemanstatue in Saint Petersburg, Russia. Weighing 1,250 tonnes, this boulder is claimed to be the largest stone ever moved by humans.[18] Modern building uses of rapakivi granites are in polished slabs used for covering buildings, floors, counter tops or pavements. As a building material, rapakivi granite of the wiborgite type is also known as "Baltic Brown".[19][20]
Notes
^Some geologists of the first half of the 20th century regarded the rapakivi granites as "graniticized" Jotnian sediments, an idea which is now discredited.[8]
^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.
^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.
^Walter Wahl: Die Gesteine des Wiborger Rapakiwigebietes. Fennia, Band 45/20, Helsingfors (Tilgmann) 1925, p. 24
^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.
