Rapakivi granite

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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, mostly found in southeastern Finland. It can also be found e.g. on Åland, in Laitila in southwestern Finland, Karelian Isthmus in Russia, some parts of Sweden (all are parts of the Baltic Shield), as well as the Llano Uplift in Texas, United States.

Rapakivi is the material used in Åland's Middle Age stone churches.