Rapakivi granite

Rapakivi granite is a hornblende-biotite granite containing large round crystals of orthoclase each with a rim of oligoclase (a variety of plagioclase). The name has come to be used most frequently as a textural term where it implies plagioclase rims around orthoclase in plutonic rocks. Rapakivi is Finnish for "crumbly rock", because the different heat expansion coefficients of the component minerals make exposed rapakivi crumbly.[1]

Rapakivi from a moraine in Northern Germany.

Rapakivi was first described by Finnish petrologist Jakob Sederholm in 1891.[2] Since then, southern Finland's rapakivi granite intrusions have been the type locality of this variety of granite.[3]

Occurrence
Eroded rapakivi granite in Finland

Rapakivi is a fairly uncommon type of granite, but has been described from localities in North and South America (Illescas Batholith, Uruguay,[4] Rondônia, Brazil[5]) parts of the Baltic Shield, southern Greenland, southern Africa, India and China. Most of these examples are found within Proterozoic metamorphic belts, although both Archaean and Phanerozoic examples are known.

Formation

Rapakivi granites have formation ages from Archean to recent and are usually attributed to anorogenic tectonic settings. 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.[6][note 1]

Geochemistry

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.[8]

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.[8][9]

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.[10][11]

Rapakivi type wiborgite

Petrography
Rapakivi type pyterlite

Vorma (1976) states that rapakivi granites can be defined as:[12]

  • Orthoclase crystals have rounded shape
  • Most (but not all) orthoclase crystals have plagioclase rims (wiborgite or viborgite type, named after the city of Vyborg)[13]: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).[13]:134[14]

A more recent definition by Haapala & Rämö states:[15]

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.[16] In 1770, a rapakivi granite monolith boulder, the "Thunder Stone", was used as the pedestal for the Bronze Horseman statue in Saint Petersburg, Russia. Weighing 1,250 tonnes, this boulder is claimed to be the largest stone ever moved by humans.[17] 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".[18][19]

Notes
  1. Some geologists of the first half of the 20th century regarded the rapakivi granites as "graniticized" Jotnian sediments, an idea which is now discredited.[7]

References
  1. Tietoaineistot - maaperäkartan käyttöopas - rapautuminen - GTK
  2. "Ueber die finnländischen Rapakiwigesteine
  3. "3000 miljoonaa vuotta, Suomen Kallioperä" Finnish geological society, 1998, chapter 9, ISBN 952-90-9260-1 . Language: Finnish.
  4. 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. doi:10.1016/j.lithos.2012.09.006.
  5. 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: 41–67.
  6. 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.
  7. von Eckermann, Harry (1939). "The Weathering of the Nordingrå Gabbro". Geologiska Föreningen i Stockholm Förhandlingar. 61 (4): 490–496. doi:10.1080/11035893909444616.
  8. Rämö, 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.
  9. 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: 65–79. doi:10.1016/0375-6742(91)90075-6.
  10. Valmari, T., Arvela, H., ja Reisbacka, H. 2012. Radon in Finnish apartment buildings. Radiation Protection Dosimetry, 152, 146-149.
  11. Weltner, A., Mäkeläinen, I., ja Arvela, H. 2002. Radon mapping strategy in Finland. In: International Congress Series 1225, 63-69.
  12. 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.
  13. Le Maitre, R. W. (editor) (2002). Igneous Rocks — A Classification and Glossary of Terms. Cambridge: Cambridge University Press. ISBN 978-0-521-66215-4.CS1 maint: extra text: authors list (link)
  14. Walter Wahl: Die Gesteine des Wiborger Rapakiwigebietes. Fennia, Band 45/20, Helsingfors (Tilgmann) 1925, p. 24
  15. 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: 165–171. doi:10.1017/s0263593300007859.
  16. Eckerö church, Retrieved 2012-10-19.
  17. Adam, Jean-Pierre (1977). "À propos du trilithon de Baalbek: Le transport et la mise en oeuvre des mégalithes". Syria. 54 (1/2): 31–63. doi:10.3406/syria.1977.6623.
  18. North Carolina Museum of Natural Sciences blog, Retrieved 2012-10-19.
  19. "Baltic Brown". Finnish Natural Stones. Finnish Natural Stone Association. Retrieved 16 June 2017.
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