Rapakivi granite

Rapakivi from a moraine in Northern Germany.

Rapakivi granite is a hornblende-biotite granite containing large rounded crystals of orthoclase mantled with oligoclase. 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 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, 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.

Best known occurrence range is from Ukraine, through Finland and Scandinavia, southern Greenland to the Labrador peninsula and on through the North American continent to California.

Formation

Rapakivi granites have formation ages from Archean to recent and are not usually associated with orogeny. 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.[4][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.[6]

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.[6][7]

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.[8][9]

Petrography

Rapakivi type Vyborgite
Rapakivi type Pyterlite

Vorma (1976) states that rapakivi granites can be defined as:[10]

A more recent definition by Haapala & Rämö states:[12]

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 Middle Age stone churches.[13] In 1770 a rapakivi 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.[14] 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 is also known as "Baltic Brown".[15]

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.[5]

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. ↑ 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.
  5. ↑ von Eckermann, Harry (1939). "The Weathering of the Nordingrå Gabbro". Geologiska Föreningen i Stockholm Förhandlingar 61 (4): 490–496. doi:10.1080/11035893909444616. Retrieved 15 August 2015.
  6. 1 2 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.
  7. ↑ 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.
  8. ↑ Valmari, T., Arvela, H., ja Reisbacka, H. 2012. Radon in Finnish apartment buildings. Radiation Protection Dosimetry, 152, 146-149.
  9. ↑ Weltner, A., Mäkeläinen, I., ja Arvela, H. 2002. Radon mapping strategy in Finland. In: International Congress Series 1225, 63-69.
  10. ↑ 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.
  11. ↑ Walter Wahl: Die Gesteine des Wiborger Rapakiwigebietes. Fennia, Band 45/20, Helsingfors (Tilgmann) 1925, p. 24
  12. ↑ 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, pp. 165 - 171.
  13. ↑ Eckerö church, Retrieved 2012-10-19.
  14. ↑ 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.
  15. ↑ North Carolina Museum of Natural Sciences blog, Retrieved 2012-10-19.
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