This is an urban EarthCache. Please be respectful to others, on this busy street.
🤔 So what are we here to look at?
We are here to look at Rapakivi Granite. Rapakivi granites are most famous from SE and SW Finland because this is where they were first explored in connection with building and decorative stone production.
🤔 Granite is an igneous rock.
Granite is a common type of igneous rock. Igneous rock is one of the three main rock types , the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.
➡️ Rapakivi granite.
This a term used for a variety of granite, which is characterized by the presence of large ovoid crystals of orthoclase which are usually mantled by plagioclase. Orthoclase and Plagioclase are both types of feldspar.
➡️ Feldspar.
Feldspar is the name of a group of silicate minerals, that make up a large proportion of the earths crust.
➡️ Orthoclase.
Orthoclase is a potassium feldspar. Orthoclase typically has a pinkish hue.
➡️ Plagioclase.
Plagioclase is a sodium and calcium feldspar, it is often white to grey.
🤔 Crystal size.
Crystal size depends largely upon the cooling history of the granite but also on the presence of volatiles such as water, fluorine and carbon dioxide.
➡️ Rapakivi granites contain two generations of crystals:
⚪️Firstly, phenocrysts, representing the early stage of magma crystallization in deep magma chambers and reservoirs.
⚪️ Secondly, much smaller, anhedral minerals that crystallized at the final intrusion level.
➡️ A phenocryst is a crystal in an igneous rock, the crystal being larger than the other crystals that the rock is made of.
➡️ Anhedral means crystals which do not have well defined edges.
🤔 Mantled Texture.
Rapakivi granite is characterised by crystals that have a mantle. Basically this is an outer covering formed of plagioclase.
🤔 Formation of Rapakivi granite texture.
There are three proposed mechanisms for the formation of Rapakivi texture.
➡️ Mixing of two magmas of different composition.
A catalyst for stimulating the changes leading to rapakivi granite, may be the influx of ‘fresh’ mafic magma from below, upsetting the status quo of the felsic magma.
🟥 Mafic.Mafic minerals are those rich in iron and magnesium, and are typically dark in colour.
🟥 Felsic. This refers to silicate minerals that are rich in elements like silicon, oxygen, aluminum, sodium, and potassium. Felsic minerals tend to have a lighter colour.
🌋Magma. Magma is the molten or semi-molten natural material from which all igneous rocks are formed.
➡️ Crystallisation of granite, and marked pressure release.
Some scientists propose that rapakivi textures are formed when hot and dry A‐type granite magma (∼780 °C) containing quartz and potassium‐feldspar phenocrysts is transported sub‐adiabatically (without temperature change) from intermediate depths (15‐20 km), where potassium‐feldspar and quartz are stable, to upper crustal levels. During this ascent there is no change in magma chemistry and very little change in temperature but a large decrease in pressure, so that conditions become such that potassium‐feldspar and quartz are resorbed while plagioclase feldspar becomes stable and precipitates. This would explain the ovoid shape notable in rapakivi granite.
➡️ Synneusis.
This is the drifting together and systematic attachment of crystals in the melt—of plagioclase crystals at the surface of potassium‐feldspar phenocryst during magma flow and/or crystal sinking. This process requires the potassium‐feldspar to move into plagioclase‐rich (mafic) melt batches in order for the plagioclase to form pure and complete mantles around the potassium‐feldspar.
🤔 The above talks a lot about large oval crystals (ovoids) of orthoclase being mantled by plagioclase. So what does this mean? Well, in the picture below, you can see a white ovoid, which is surrounded by a greeny gray cover. The white oviod is the orthoclase. The mantle of plagioclase is the greeny gray cover . Basically one mineral is surrounded by another, think of biscuit covered in the chocolate, in laymans language, the biscuit is the orthoclase, and the plagioclase is the chocolate.
🤔 Viborgitus versus Pyterlitic.
Granites where plagioclase mantles are present are described as viborgitic. Granites where plagioclase mantles are absent are described as pyterlitic.
This being an EarthCache, in order to log it, I ask that you answer some questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.
1. Looking at the Rapakivi granite, would you describe this as Viborgitic or pyterlitic? What is the rationale for your answer?
2. What type of rock is rapakivi granite?
3. Please explain what features a rapakivi granite crystal has, making reference to the terms described above.
4. Please draw a rapakivi granite crystal with your geocaching name on it, and take a photograph of yourself or your device looking up the High Street.