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. Since then, southern Finland's rapakivi granite intrusions have been the type locality of this type of granite.
Read more about Rapakivi Granite: Occurrence, Formation, Morphology, Use As A Building Material
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