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A rapakivi granite from southern Finland. 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 Wiborg Batholith (a.k.a. Wiborg Rapakivi Granite Batholith, Wiborg Massif, Vyborg Batholith) in the Baltic Shield (Fennoscandian Shield).
