Rapakivi is a term that is applied to a very distinctive igneous texture found in some granites, where pink or brown alkali feldspar (K-spar) phenocrysts are rimmed by grey plagioclase feldspar (calcium-rich feldspar), forming circular structures (in cross-section) up to a diameter of 4 cm. These crystals are then surrounded by a finer-grained groundmass, usually hornblende- and/or biotite-bearing feldspar. Granite showing this texture is often refered to as "rapakivi granite".

The processes leading up to the formation of rapakivi are not fully understood. The fact that plagioclase crystals surround the K-spar crystals allude to the fact that the K-spar crystallized first, which seems contrary to the order that minerals crystalize from a melt as stated in Bowen's reaction series. Igneous petrologists suppose that this irregular pattern is caused by the presence of volatiles like water or carbon dioxide in the cooling magma.

Some rapakivi granites exhibit what is called anti-rapakivi texture. This variation occurs when K-spar phenocrysts are rimmed by plagioclase, which is in turn rimmed by another layer of K-spar.

Rapakivi takes its name from a site in eastern Finland, where some of the first examples were found. (According to gn0sis, the literal Finnish translation of "rapakivi" is "mudstone". While this term is genetically inaccurate, we can forgive the Finns, considering that the rock likely got its name before we had an understanding of igneous petrology.) Since it was first cataloged, the texture has been observed in granitic intrusions across the world.


Sources:
Principles of Igneous and Metamorphic Petrology (1990), Anthony R. Philpotts, 498 pp., Prentice Hall
Ketilidian structure and the rapakivi suite between Lindenow Fjord and Kap Farvel, South-East Greenland - http://www.geus.dk/publications/review-greenland-99/gsb186p50-59.pdf
xrefer - http://www.xrefer.com/entry/618324

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