Rapakivi granite

Rapakivi granite is an igneous intrusive 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 granite
Baltic Brown Granite

Igneous rock
Composition
Primary	Alkali feldspar; Quartz
Secondary	Biotite; Plagioclase; Hornblende

Rapakivi was first described by Finnish petrologist Jakob Sederholm in 1891.^[3] Since then, southern Finland's rapakivi granite intrusions have been the type locality of this variety of granite.^[4]

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,^[5] Rondônia, Brazil^[6]) 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

Finnish Rapakivi granites occur in the Åland, Laitila, Vehmaa, and Wiborg batholiths. The Wiborg batholith dates from 1900-1830 Ma. The age of the Wiborg granites range from 1646 to 1627 Ma, and consist of 80% wiborgite, and smaller inclusions of pyterlite, porphyritic rapakivi granite, even-grained rapakivi granite, and aplitic rapakivi granite.^[7]

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.^{[citation needed]}

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.^{[8][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.^[10]

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.^[10][11]

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.^[12][13]

Rapakivi type wiborgite

Petrography

Rapakivi type pyterlite

Vorma (1976) states that rapakivi granites can be defined as:^[14]

• Orthoclase crystals have rounded shape
• Most (but not all) orthoclase crystals have plagioclase rims (wiborgite or viborgite type, named after the city of Vyborg)^[15]: 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).^{[15]: 134 [16]}

A more recent definition by Haapala & Rämö states:^[17]

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.^[18] 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.^[19] 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".^[20][21]

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.^[9]