Granite porphyry describes granite rocks with large crystals in a finer-grained matrix but with the same mineralogy and chemistry as ordinary granite rocks. So, porphyry (pronounced as POR-fə-ree) only indicates the texture. A more accurate name for these rocks is porphyritic granite.
The formation of granite porphyry suggests two-stage cooling. An earlier, slow cooling forms the larger crystals before the remaining magma moves to the final places it cooled and crystallized, forming the finer-grained matrix.
Today, I will discuss porphyritic granite, a felsic or silica-rich intrusive or plutonic igneous rock, including its formation, composition, and uses.
What is granite?
Granite is a coarse-grained intrusive igneous rock made mainly of quartz and felspar (alkali felspar and plagioclase). However, it has smaller amounts of other minerals like mica and amphibole.
This hard rock (Mohs hardness of 6-7) forms deep inside the Earth’s crust, where cooling is slow. The slow cooling rates allow the coarse grains to form, which you can see with an aided eye.
Granite is a felsic (silica-rich), light-colored rock that comes in pink, white, and shades of gray and black. Usually, most rocks have 2-4 shades of these colors.
Did you know that granite is the most common intrusive rock on the Earth’s surface and forms in continental crusts as batholiths or dykes and sills? Also, its composition is similar to rhyolite, its extrusive equivalent.
What is a granite porphyritic granite?
Porphyry is a textural term for igneous rocks that describes the presence of consistently larger crystals embedded in a finer-grain matrix.
The larger crystals are known as phenocrysts. If the phenocrysts are very large (>5mm), they are known as megaphenocrysts, and smaller (0.03mm to 0.3mm) are microphenocrysts.
On the other hand, the finer-grained matrix is known as groundmass. Groundmass may be coarse-grained (phaneritic), fine-grained (aphanitic), or even glassy or microcrystalline. For granite porphyry, it is coarse-grained.
In most cases, a porphyritic granite rock will have pinkish-to-whitish potassium feldspar phenocrysts set in a phaneritic groundmass. However, there could be other phenocrysts, such as quartz, plagioclase, biotite, etc.
On the other hand, the groundmass may have quartz (grayish blebs), sodium plagioclase (cream), and black amphibole (black). Also, it may have mica (biotite or muscovite) or hornblende that occur in typical granitic magma but in small amounts.
Please note that granite porphyry mineral composition (mineralogy) and chemistry are the same as typical granite rocks, i.e., they have potassium felspar, sodium plagioclase, quartz, amphibole, and sometimes minerals like hornblende, biotite, or muscovite but in small amounts.
Lastly, it is not just granitic rocks that form the porphyritic texture. Most of the other igneous rocks do, popular among them diorite, andesite, rhyolite, or basalt porphyry.
Naming these rocks
Sometimes, you may find names like biotite, alkali, or red granite porphyry. Biotite, red, or alkali tells you more about the color of or which forms the dominant phenocrysts. For instance, alkali means the phenocrysts are K-felspar (potassium).
How are porphyritic granite rocks formed?
Granite porphyry indicates that cooling occurred in two stages inside the Earth’s crust. The larger crystals formed at an earlier stage and had a lot of time to grow large before the remaining magma, the present large crystals, moved to the final place of crystallization.
One possible explanation for this texture formation is that granitic magma cooled much slower when the phenocrysts formed. It could be deeper inside the Earth’s crust. So, they had enough time to fractionally crystallize (starting with one with the highest melting point) and grow, forming larger crystals.
After the phenocrysts formed, the remaining magma and formed crystals were brought to a place where cooling was not as slow and crystallized. Since cooling was not as slow, the groundmass couldn’t form as large crystals. However, it gave time for considerable crystal growth, i.e., the groundmass is coarse-grained.
Occurrence
Porphyritic granite occurs in the same places as ordinary granite rocks, i.e., on dykes and batholiths. They also form a large part of the continental crust.
Some of the geological units in the US where you can find porphyritic granite rocks include Pembine-Wausau terrane (Wisconsin), Milford-Dedham Zone (Massachusetts), Lowrys (South Carolina), Coys Hill (Massachusetts), and Johnson Peak (California).
Uses of granite porphyry rocks
Since porphyry doesn’t affect the chemistry of mineralogy, you can use it like ordinary granite. Some of the uses of porphyritic granite include dimensional stone for paving, curbing, landscaping, and making buildings or monuments.
Also, when polished, these rocks will make slabs, tiles, countertops, stair treads, flooring tiles, bathroom sinks, window sills, etc.
References
- Granite (porphyritic). (n.d.). Oxford University. Retrieved August 30, 2022, from https://www.earth.ox.ac.uk/~oesis/rocks/ign1.html
- Department of Geological Sciences. (n.d.). Porphyritic granite. East Carolina University Retrieved August 30, 2022, from https://geology.ecu.edu/geol1501/igneous/porphyritic-granite/
- Porphyry (geology). (2022, August 6). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Porphyry_(geology)&oldid=1102772210