Introduction
At the shown coordinates, there is a large section of Rapakivi granite as cladding of the side: of the building.
Granite
Granite is an intrusive igneous rock which means it was formed deep underground during the cooling of magma. In general, the slower the magma cools, the bigger the mineral crystals are. Granites are rich in different types of minerals. These include:
Feldspar (Felsic)
Quartz
Granite is phaneritic which means that its individual crystals are large enough to be seen by the naked eye. It is also holocrystalline meaning that it is completely made up of mineral crystals
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 (a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group) and have large spheroidal K-feldspar masses (an abundant rock-forming mineral typically occurring as colourless or pale-coloured crystals and consisting of potassium). Igneous petrologists (people who study how rocks are formed) usually consider the spheroidal shapes to be the result of magmatic corrosion (A process of re-solution in which an early formed phase later becomes corroded as the result of some change in the conditions affecting the solubility of the phase) of the original K-feldspar crystal shapes.
Types of Rapakivi
I’m going to describe 2 types of Rapakivi, one of which features at GZ
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 (different types of minerals)
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.
Logging Tasks
- Please look at the Rapakivi Granite featured at GZ. Do you think is is Wyborgitic or Pyterlitic? How did you come to this conclusion?
- Please measure the diameter of the largest crystal you can find.
- Please describe the feel of the rock.
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