RAPAKIVI  GRANITES

 

The rapakivi granites are very odd, but very visually distinctive varieties of granite.  They are reported to have the same chemistry and mineralogy as ordinary granites, but they are strongly porphyritic, and have large spheroidal K-feldspar masses.  Igneous petrologists usually consider the spheroidal shapes to be the result of magmatic corrosion of the original K-feldspar crystal shapes.

 

The Baltic Shield (Fennoscandian Shield) of southern Finland has some spectacular, world-class, Precambrian-aged rapakivi granites that are quarried for use as decorative stones.

 


 

Baltic Brown Granite - a variety of rapakivi granite called wiborgite, characterized by having large K-feldspar spheroids rimmed (usually) by greenish plagioclase feldspar.  Wiborgite is composed of K-feldspar, plagioclase feldspar, quartz, biotite mica, and hornblende amphibole.  This rock comes from the Wiborg Batholith (a.k.a. Wibork Rapakivi Granite Batholith, Wiborg Massif, Vyborg Batholith), which dates to the late Paleoproterozoic (about 1.615-1.645 billion years).  This sample comes from the Husu Quarry, near Ylmaa in southern Kymi Province, southern Finland.  Baltic Brown Granite is also produced from the Parkkola Quarry.

 


 

Baltic Red Granite - another rapakivi granite from southern Finland, also from the Paleoproterozoic Wiborg Batholith (1.615-1.645 billion years).  This variety of rapakivi granite is called pyterlite.  Pyterlite is characterized by having large K-feldspar spheroids that usually lack rims of greenish plagioclase feldspar.  So, wiborgite has common plagioclase rims, and pyterlite commonly lacks plagioclase rims.  The pyterlite sample shown here has one K-feldspar spheroid at the upper left with a greenish-brown plagioclase feldspar rim.  Pyterlite is composed of reddish K-feldspar, greenish-brown plagioclase feldspar, dark-grayish quartz, and black biotite mica.  This rock comes from the Pyterlahti Quarry, near Virolahti, near the Russian border, in far-southern Kymi Province, far-southern Finland.  Baltic Red Granite is also produced from the Lypsyniemi Quarry.

 


 

 

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