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 was first described by Finnish Jakob Sederholm in 1891.[1] Since then southern Finland's Rapakivi formations have been the type locality of this type of granite.[2]
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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 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 .[3]
Vorma (1976) states that rapakivi granites can be defined as:[4]
Latter definition by Haapala & Rämö states:[6]
Rapakivi granites are type-A granites, where at least in larger associated batholites have granites with rapakivi structures.
Rapakivi is the material used in Åland's Middle Age stone churches.
Modern building uses of Rapakivi granites are in polished slabs used for covering the buildings, floors or pavements.
In 1770 Rapakivi boulder (Thunder Stone) was used as a pedestal for the Bronze Horseman statue in Saint-Petersburg.