Famous Rock from Finland

The fascinating thing about geology is that a stone can make a place famous on a global scale. The the country of Finland and especially its southern region was made famous by a specific granite with unique feature. We are talking here about rapakivi granite. With this Earthcache I want to bring to your attention special features of the rapakivi granite, such as round crystals and plagioclase mantle, features that make this type of granite unique. 

Rapakivi

Before any geological surveys were carried out, local people in southern Finland noticed the distinctive, unique structure of the local granite rocks. The large, round, reddish crystals contained in the black matrix may have resembled the rock contained in the mud. "Rapa" means mud and "kivi" means stone. Thanks to folklore, we know that the name "rapakivi" has existed in ordinary Finnish for centuries. The first written record dates to 1694.

Rapakivi is quite rare. The presence of the plagioclase mantles that surround the round crystals of potassium feldspar are such a unique and rare features that we can say with certainity that the slabs on the facade of the Fitzwilliam Hotel in Dublin come from the south of Finland. 
These features testify to very specific conditions in the magma chamber where these rocks were formed. In this case, we are dealing with several  overlapping geological processes that occurred during the formation of these igneous rocks. Let's learn about these processes one by one. 

Source: Own photo, own work.

Round crystal shapes

The formation of round potassium feldspars has long been a mystery because these minerals only grow with straight edges. However, when the temperature of already cooled and partially crystalized magma suddenly increases or the pressure in the magma chamber decreases as a result of rising magma, the potassium feldspar crystals will lose their straight edgdes. In such a scenario the potassium feldspar crystals begin to melt on the outside. First they become round, and start shrinking. They can even shrink to such an extend until they completly dissolve. If the melting and shrinking process of the crystals is interrupted, they will remain round.
Such partial melting is a result of pressure reduction and occurs only in granitic magma, which contains very little amount of water, higher amounts of fluorine and carbon dioxide. Such composition of magma is very rare and unique.

Plagioclase mantle

Likewise, the formation of circular plagioclase mantles around the round potassium feldspar crystals remained a geological mystery for a very long time. Rapakivi magma allows the formation of plagioclase rings only at the end of the crystallization process. Water-poor granite melt has a fundamentally different crystallization process than ordinary granites, whose magma usually contains more water.
Because quartz and alkali feldspar melt as the pressure of the water-poor melt increases, plagioclase continues to crystalize due to its unique proprty under these condistions. It continues to crystallize in small amounts. This is the reason why the plagioclase mantle rings can be observed on the outside of the alkali feldspar.

Tasks/Questions:
1. Find a round potassium feldspar crystal with a plagioclase mantle, place a coin next to it, take a picture of it and add it to the log.
2. Carefully examine the rapakivi slabs at the facade of the Fitzwilliam Hotel. What is the largest potassium feldspar crystal you have found?
3. Estimate how much potassium feldspar makes up the rapakivi granite on the facade of the hotel? What percentage is black matrix?
4. What does the composition of the granite magma need to be in order to rapakivi granite to form?
 

Resources:
https://www.visitfinland.com/en/product/c766617b-1533-4565-999a-3d938160cd93/pyterlahti-granite-quarry/
https://link.springer.com/chapter/10.1007/978-3-031-18861-9_6
THE WIBORG RAPAKIVI BA THOLITH AND ASSOCIATED ROCKS IN SOUTH-EASTERN FINLAND, Matti Vaasjoki and Tapani Rämö, 1989