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 YlŠmaa 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.