Rapakivi granite
Encyclopedia
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
-biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
containing large rounded crystals of orthoclase
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
mantled with oligoclase
Oligoclase
Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite and anorthite . The albite:anorthite molar ratio ranges from 90:10 to 70:30.Oligoclase is a high sodium...
. The name has come to be used most frequently as a textural term where it implies plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
rims around orthoclase in plutonic rocks. Rapakivi is Finnish
Finnish language
Finnish is the language spoken by the majority of the population in Finland Primarily for use by restaurant menus and by ethnic Finns outside Finland. It is one of the two official languages of Finland and an official minority language in Sweden. In Sweden, both standard Finnish and Meänkieli, a...
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
Jakob Sederholm
Jakob Johannes Sederholm was a Finnish petrologist most associated with his studies of migmatites.Troubled by illness throughout his life, Sederholm originally chose to study geology to allow him to work outdoors...
in 1891. Since then southern Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
's Rapakivi formations have been the type locality
Type locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
of this type of granite.
Occurrence
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, parts of the Baltic Shield
Baltic Shield
The Baltic Shield is located in Fennoscandia , northwest Russia and under the Baltic Sea. The Baltic Shield is defined as the exposed Precambrian northwest segment of the East European Craton...
, southern Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
, southern Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
and China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
. Most of these examples are found within Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
metamorphic
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
belts, although both Archaean and Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
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 orogenyOrogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
. They have formed in shallow (a few km deep) sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
s of up to 10 km thickness.
Rapakivi granites are often found associated with intrusions of anorthosite
Anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
, norite
Norite
Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbro without thin section study under the petrographic microscope...
, charnockite
Charnockite
Charnockite is applied to any orthopyroxene-bearing granite, composed mainly of quartz, perthite or antiperthite and orthopyroxene , as an end-member of the charnockite series.-Charnockite series:...
and mangerite
Mangerite
Mangerite is a plutonic intrusive igneous rock, that is essentially a hypersthene-bearing monzonite. It often occurs in association with norite, anorthosite, charnockite and rapakivi granite in Proterozoic metamorphic belts....
. It has been suggested that the entire suite results from the fractional crystallization
Fractional crystallization (geology)
Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the...
of a single parental magma .
Morphology
Vorma (1976) states that rapakivi granites can be defined as:- OrthoclaseOrthoclaseOrthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
crystals have rounded shape - Most (but not all) orthoclase crystals have plagioclasePlagioclasePlagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
rims (type Vyborgite, named after the city of VyborgVyborgVyborg is a town in Leningrad Oblast, Russia, situated on the Karelian Isthmus near the head of the Bay of Vyborg, to the northwest of St. Petersburg and south from Russia's border with Finland, where the Saimaa Canal enters the Gulf of Finland...
) - Orthoclase and quartzQuartzQuartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
have crystallized in two phases, early quartz is in tear-drop shaped crystals (type Pyterlite, named after the location of Pyterlahti).
Latter 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
Rapakivi is the material used in Åland's Middle AgeMiddle age
Middle age is the period of age beyond young adulthood but before the onset of old age. Various attempts have been made to define this age, which is around the third quarter of the average life span of human beings....
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.