Rapakivi Granite
“The granite with feldspar inside feldspar”
Rapakivi granite is a distinctive variety of granite characterized by large, rounded alkali feldspar megacrysts (wiborgite texture) mantled by a thin rim of sodium-rich plagioclase. These 'mantled feldspars' formed through complex magma mixing or pressure changes in the magma chamber.
alkali feldspar (mantled megacrysts), plagioclase (rim and groundmass), quartz, biotite, hornblende, fluorite
Vyborg (Russia/Finland — type locality), Åland Islands (Finland), Fennoscandian Shield, Amazonia (Brazil), Ukraine, Labrador (Canada)
6-7
Rapakivi means 'rotten rock' in Finnish because the mantled feldspars weather in a peculiar way — the plagioclase rim decomposes faster than the core, causing the rock to crumble into a gritty sand.
Test Your Knowledge
What is the geological significance of the plagioclase rim (mantle) surrounding alkali feldspar megacrysts in rapakivi granite?
Related Specimens
Want to identify rocks yourself?