The methods of identifying the I & S granite types in the manual sample are discussed in our discussion.
I-Type Granite: High content of biotite and amphibole
S-type granite: Al-silicates such as muscovite, cordierite or minerals from the Al2SiO5 group.
In many cases, however, it is difficult to distinguish between the types in the field.
As a rough guide in the field, the S type granites commonly contain two mica types (muscovite and biotite), because they are more Al2O3 rich than I type granites. They also commonly lack hematite dusting of the potassic feldspars (i.e. they are usually creamy white, rather than pink).
Here are some field identification techniques that can help you distinguish between I-type and S-type granite:
Color: I-type granite is generally light-colored, while S-type granite can have a more varied color range, including pink, grey, and red.
Texture: I-type granite is typically fine-grained and has a uniform texture, while S-type granite can have a more heterogeneous texture with larger crystals.
Mineral content: I-type granite is rich in potassium feldspar and plagioclase feldspar, while S-type granite is rich in sodium feldspar.
Here is some additional details inculding some of previoys answer’s too;
There are a few key ways to recognize I-type and S-type granite in the field. One way is to look at the mineral composition of the rock. I-type granites typically have a higher proportion of alkali feldspar and plagioclase feldspar, while S-type granites have a higher proportion of muscovite and biotite. Another way to tell the difference is to look at the texture of the rock. I-type granites tend to have a coarse-grained texture, while S-type granites tend to have a finer-grained texture.
Another way to recognize I-type and S-type granite in the field is by observing the rock's color. I-type granites tend to be lighter in color, typically shades of white and light gray, while S-type granites tend to have darker shades of gray, black, and even purple hues.
Additionally, you can observe the form of the granite. I-type granite forms in a large-scale plutonic environment and tends to have large, visible crystals. S-type granite forms in a smaller scale and tends to have more subtle, finer crystals.
Another way to tell the difference is to observe the rock's weathering characteristics. I-type granite is typically more resistant to weathering, and therefore will have sharper edges and cleaner surfaces, while S-type granite is more susceptible to weathering and will have more rounded edges and weathered surfaces.
Lastly, you can conduct a geochemical analysis, such as X-ray fluorescence (XRF) or a portable X-ray diffraction (pXRD) to determine the mineralogy and chemical composition of the granite, which can help to confirm whether it is an I-type or S-type granite.
I tell/told my scholars to do not move the laboratory in the field and waste your precious time with useless efforts. Determine in the field everything that you can identify and capable of determining in the field, size, dip, strike, etc. with the unarmed eye or the hand lens. Take reliable samples well located according to altitude , latitude and longitude and leave the rest of the job for the desk and laboratory. This is in my opinion "economic geology sensu lato"
The subdivision of granite sub-types in the filed which in practice needs the application of mineralogy and geochemistry is such a case in point.
After having made your decision in the study indoor take your reference samples go outdoor and map the area until you get in touch with another species which you have not yet categorized and re-start the process from the beginning.
HGD
One more clue. If you have aeromagnetic data, I type granites tend to contain magnetite, while S type granites contain ilmenite.
In majority of the cases, I-type granites show 'mag highs' whereas the S-type granites show 'mag lows'. Somewhere I read, is it true??
The S type granites do not show mag lows, they just do not show a magnetic signature. So if they have intruded magnetic rocks they will look like a low, but if they have intruded non-magnetic rocks they will look just like the non-magnetic rocks.
It may be worth getting acquainted with the rocks around the granite body. For example, are there andesites or diorites with gabbro. These rocks, together with granites, are often I-type
My experience with S type granites suggests they are much more radioactive. Often associated with uranium deposits. So if you have a scintillometer try it out near springs. The water soluble radioactive salts should accumulate there. That was my experience in Colorado. There were pods of highly radioactive minerals in the Silverplume Granite…a two mica granite.
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