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.
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.
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 .
Vorma (1976) states that rapakivi granites can be defined as:
- Orthoclase crystals have rounded shape
- Most (but not all) orthoclase crystals have plagioclase rims (type Vyborgite, named after the city of Vyborg)
- Orthoclase and quartz have crystallized in two phases, early quartz is in tear-drop shaped crystals (type Pyterlite, named after the location of Pyterlahti).
A more recent definition by Haapala & Rämö states:
Rapakivi granites are type-A granites, where at least in larger associated batholites have granites with rapakivi structures.
Use as a building material
Modern building uses of rapakivi granites are in polished slabs used for covering the buildings, floors, counter tops or pavements. As a building material, rapakivi granite is also known as "Baltic Brown".
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.
- Tietoaineistot - maaperäkartan käyttöopas - rapautuminen - GTK
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- "3000 miljoonaa vuotta, Suomen Kallioperä" Finnish geological society, 1998, chapter 9, ISBN 952-90-9260-1 . Language: Finnish.
- 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.
- 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.
- 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, pp. 165 - 171.
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