What's the normal granularity (grain size) of granite at the enhanced geothermal systems (EGS)? Since the grain size is affected by many factors, for example, cooling rate, folded sedimentary rocks ( crustal compression), etc. However, since good locations for EGS are over deep granite covered by a 3–5 kilometres (1.9–3.1 mi) layer of insulating sediments that slow heat loss, I wanna know what's the usual grain size of granite at this condition?
FYI:
https://www.reference.com/science/igneous-rocks-coarse-grained-due-slow-cooling-ad20df1985e52992?qo=contentSimilarQuestions
https://www.reference.com/science/grain-size-granite-d91386458c830e39#
http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/grain-size-igneous-rocks
Granite and basalt are excellent examples of coarsely grained igneous rocks. The coarseness of such igneous rocks is a result of magma becoming trapped beneath sediment and stone, which allows the liquid to cool at a slower rate. Rocks that originate by this slow process are classified as phaneritic. These contrast with aphanitic igneous, which are made up of microscopic grains, and igneous rock that is cooled so quickly it takes on characteristics of glass. The conditions necessary to form phaneritic rock leave it buried beneath the Earth's crust. Over time, wind and water work to erode the mineral material containing this slow-cooled magma, until finally it is exposed to the surface.
Granite typically has a large and coarse grain size with highly visible crystals. It is able to grow large crystals due to being an intrusive igneous rock.
Granite is formed underground from felsic lava over a lengthy period of time, which results in the formation of large mineral crystals. It has a high quartz and feldspar content. The minerals found in granite are typically light-colored and may vary depending on which minerals are prevalent. Trace minerals such as mica and amphiboles also affect the color and composition of the rock. Granite is the most commonly found igneous rock in the continental crust and the Earth's surface.
Since granite are generally thought to be coarse-grained, does that mean the grain size of granite at EGS conditions is between 5 mm to 3 cm?
Hello Ma Zhaoyang:
A simple answer to your question is difficult, because grain size in a granite intrusion is affected by many factors as you mentioned. The volume, emplacement depth, initial water content, and chemical composition, etc., of the granite magma must have influenced its cooling rate on one hand. The thermal conditions of country rocks are also causative factors affecting the supercooling of the magma on the other hand. Growth rate of quartz and feldspars is different, too. It is too complicated to provide a simple answer for your question.
Regards,
Xue-Ming Yang
I think that all types of grains are normal, that will depend of the time the system had to cool down.
Hello~ I also want to know the answer and the google guided me here. This is certainly a good question as hydraulic fracturing behavior of granite is significantly influenced by grain size (Ishida et al., 2000). In case of Pohang EGS site, the largest grain for 4.2 km depth granite cores was 1.2 mm (Kwon et al. 2018) which I think should be classified as fine granite. I am thinking about the similar question cause I recently found the grain size and the mineral type could influence the fracture propagation based on microscopic observation.
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