Rapakivi

rapakivi [¦rä·pə′kē·vē]

(petrology)

Granite or quartz monzonite characterized by orthoclase phenocrysts mantled with plagioclase. Also known as wiborgite.

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Rapakivi 

a granite of porphyroid structure with round phe-nocrysts of pink orthoclase surrounded by a white or light green shell of oligoclase. The granite consists of orthoclase (about 40 percent), oligoclase (about 20 percent), black mica (about 8 percent), idiomorphic quartz (about 30 percent), and secondary minerals (about 2 percent, including amphibole, orthite, sphene, diopside, magnetite, and apatite). Rapakivi is usually brownish pink or reddish; occasionally it is greenish or almost black.

The granite is common in Finland and Sweden. In the USSR it is found in the Karelian ASSR, in Leningrad Oblast (near the city of Vyborg), and in the Ukraine (near the village of Goro-dishche in Cherkassy Oblast). Rapakivi is a valuable building and facing stone. In Leningrad the columns of St. Isaac’s Cathedral are made of the granite, as is the facing of the banks of the Neva River.