Porphyritic texture describes an igneous rock texture with large crystals embedded in a finer-grained matrix. It can occur in virtually all igneous rocks, i.e., intrusive and extrusive.
This texture indicates two cooling stages, i.e., an initial slower cooling rate (deep in the Earth’s crust) that forms the larger crystals (phenocrysts) and a final that forms the finer-grained matrix or groundmass.
Let us explore more about the definition and formation of porphyritic texture. We will also look at what it indicates or tells us and give you a few examples, like porphyritic andesite, granite, basalt, rhyolite, etc.
What is a porphyritic texture?
The porphyritic texture is an igneous rock texture characterized by well-formed, larger crystals in a finer-grained matrix. The consistently larger crystals are known as phenocrysts, while the finer-grained matrix is known as groundmass.
Given a sample of rock with a porphyritic texture, you will notice consistently larger crystals embedded in a finer-grained or glassy matrix. A thin section will reveal well-formed, larger crystals and finer-grained or glassy matrix.
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A rock can have one or more phenocrysts. Also, if they are larger than 5mm, we call them megaphenocrysts, and if they are 0.03 to 0.3mm, microphenocrysts. These terms will only tell you more about the size of the consistently larger crystals.
On the other hand, the groundmass may be coarse-grained (phaneritic), fine-grained (aphanitic), or glassy, where mineral crystals are not visible.
When the groundmass is aphanitic, the rock may be said to be porphyritic-aphanitic, while porphyritic-phaneritic indicates that the groundmass is coarse-grained or phaneritic.
Igneous rocks showing porphyritic texture are said to be porphyry (a textural term), and porphyritic is an adjective. So, any rock that has this texture is known as porphyritic rock. For example, you will have porphyritic granite or andesite.
As you define porphyritic texture, note that it doesn’t tell you the relative sizes of the large and smaller mineral crystals. Neither does it tell you the mineralogy or chemistry of any rock. It only indicates the presence of two different grain or crystal sizes.
Lastly, to help broaden your understanding of the porphyritic texture, here are a few more definitions you may encounter and what they mean:
1. Bimodal size range
Bimodality or bimodal size range indicates that a given rock has two different crystal sizes, i.e., the larger phenocrysts and groundmass.
2. Seriate and hiatuses
If the porphyritic igneous rocks show a gradual grain size variation or the variation occurs in series, such a rock is known as a seriate. It indicates crystallization of the phenocrysts continued as cooling rates increased until the final groundmass crystallized.
Therefore, you will end up with phenocrysts of varying sizes. This situation is possible when magma slowly moves closer to the ground surface.
On the other hand, if crystal size changes abruptly, you have hiatuses or a hiatal texture. This is a variation of porphyritic texture.
Hiatal texture indicates that the initially nucleated crystals continued growing until the groundmass formed.
Lastly, most igneous rocks show a combination of seriate and hiatal textures.
3. Glomeroporphyritic texture
Glomeroporphyritic texture is a porphyritic texture variant where the larger crystals (phenocrysts) occur in clusters inside a groundmass or finer-grained matrix. Glomerocrysts are aggregated larger crystals made of one mineral but often two or more.
Glomeroporphyritic texture is common in basalt, dacites, and andesites. For example, you will get clusters of plagioclase or pyroxenes in a groundmass.
How do they form? They form by a process “known as synneusis, where the accumulation of crystals occurs by surface tension and fixing by interpenetration due to crystal growth” (Alex Strekeisen, n.d.)
An example of this texture is the plagioclase glomeroporphyroclast in basalt rocks
4. Xenoliths and xenocrysts
Sometimes, in the porphyritic and glomeroporphyritic textures, the larger mineral crystals, i.e., the phenocrysts or glomerocrysts, may be torn from country rock and not originally crystallized from the magma. In such a case, phenocrysts will be known as xenocrysts and glomerocrysts xenoliths.
5. Vitrophyric texture
A vitrophyric texture is where the porphyritic extrusive or volcanic rock has larger crystals set in a glassy groundmass. Such a rock is known as vitrophyre. A good example is Pitchstone porphyry.
7. More definitions of porphyritic texture
We have given what porphyritic texture means. Here is what a few other sources say it is:
- “Porphyritic texture is an igneous rock texture in which large crystals are set in a finer-grained or glassy groundmass. Porphyritic textures occur in coarse, medium, and fine-grained igneous rocks” (Alex Strekeisen, n.d.)
- “A porphyritic texture displays minerals in two distinct size populations: one or more minerals are consistently larger than the rest of the minerals in a rock.” (Department of Geology & Planetary Science, n.d.)
Porphyritic texture occurrence
Intrusive, including subvolcanic or hypabyssal rock, and extrusive (volcanic) igneous rocks can have a porphyritic texture. Therefore, nearly all igneous rock can show some extent of porphyritic.
However, the intrusive, subvolcanic (formed closer to the Earth’s surface, i.e., 2 km 1.2 miles or less) and extrusive rocks will have different groundmass grain sizes.
For instance, the porphyritic volcanic texture will have large crystals in a glassy, partially cryptocrystalline, non-crystalline, or amorphous groundmass. This happens because the rapid cooling doesn’t give lava time to form crystals.
On the other hand, plutonic or intrusive porphyry rocks will have large crystals in a granitic or finely crystalline matrix. Why? Because the rate of cooling is much slower here.
Lastly, the groundmass of subvolcanic will have intermediate or medium grains whose size ranges between extrusive and intrusive rocks.
How do porphyritic textures form?
The formation of porphyritic texture in igneous rocks indicates a two-stage cooling history. An initial, slower cooling rate during which phenocrysts form, followed by a final faster cooling stage that forms the groundmass.
Also, there must have been magma movement after the phenocrysts formed, resulting in a change in cooling rates.
Let us see how phenocrysts and groundmass form:
1. Phenocrysts form deep within the crusts.
Phenocrysts formed deep within the Earth’s crust, where cooling was slow. This slow rate of cooling allows crystals to grow and become larger. These larger crystals form by fractional crystallization, where minerals with the highest melting point will start crystallizing and segregating from the remnant or residual melt.
Also, since phenocrysts form first, they have a well-formed euhedral crystal shape. However, they may also be anhedral-shaped. For instance, “andesitic rocks consist of large euhedral and anhedral phenocrysts of plagioclase, alkali feldspar, pyroxenes, biotite, and quartz in a fine-grained groundmass of feldspar.” (Zauyah et al., 2018).
The other explanation for subhedral to anhedral crystals is if the residual melt has minerals that solidify at a single temperature (eutectic point). Such a melt will fill the remaining space, limiting the size and shape of phenocrysts.
A possible scenario associated with phenocrysts formation is cumulates. Cumulates form when already formed; larger crystals float if they have a lower density than the remaining melt or settle at the bottom, which is denser.
However, cumulates may not often form. Why? Because the lava interruption during an eruption would mix them. Also, if the densities of melt or phenocrysts are comparable, they will remain trapped inside the rock.
Another possible explanation of the origin of phenocrysts, especially in plutonic rocks, is the low nucleation just above the solidus temperature and not groundmass constituents.
2. Groundmass forms at the final crystallization stage
The finer-grained or glassy matrix forms in the final stage when the remnant, uncrystallized magma, or lava moves near or on the Earth’s surface, where cooling is faster.
The faster cooling doesn’t allow time for crystals to form, grow, or become larger. If it is so rapid, you will have a glassy or cryptocrystalline structure. If slower, finer grains.
3. Bottom line
Porphyritic textures in igneous rocks formed when slowly cooling and crystallizing magma (with phenocrysts) deep inside the Earth’s crust suddenly moves closer to or erupts on the Earth’s surface. The remaining uncrystallized magma or lava melt will cool quickly, forming a groundmass that embeds the already formed phenocrysts.
Examples of rocks showing porphyritic texture or porphyry
Almost all igneous rocks have porphyritic textures, including ultramafic, mafic, intermediate, and felsic. Also, these rocks will have the same chemistry and mineralogy as their non-porphyritic counterparts.
Some of the notable examples include the following:
1. Porphyritic rhyolite
This volcanic igneous rock will have quartz (rounded graying blebs), muscovite, or feldspar phenocrysts in a glassy or fine-grained groundmass.
2. Granite
A porphyritic granite rock will have large potassium felspar crystals (and sometimes biotite, quartz, or plagioclase) in a coarse-grained groundmass.
3. Andesite
Porphyritic andesite rocks have plagioclase, hornblende, or amphibole phenocrysts embedded in a fine-grained groundmass.
4. Basalt
Porphyritic basalt has the same composition and mineralogy except for larger crystals of hornblende, augite, olivine, and plagioclase feldspar (calcium-rich) in a fine-grained groundmass.
5. Pitchstone porphyry
Pitchstone porphyry or vitrophyre is a volcanic glass groundmass rock with large plagioclase, quartz, or feldspar (alkali) crystals. However, it may have fewer large crystals of hornblende or pyroxene large crystals.
6. Lamprophyre
Lamprophyre is an example of hypabyssal, fine-grained dark-colored ultrapotassic rocks with large crystals of amphibole, biotite (black mica), or pyroxene in a groundmass. They occur commonly in small intrusions, stocks, laccoliths, dikes, or lopoliths.
Frequently asked questions
Porphyritic texture has large crystals in a finer-grain or glassy matrix, while pegmatitic texture has unusually coarse-grained (more than .04 inches) or large interlocking crystalline crystals. Pegmatitic texture forms in the last stage of granitic magma crystallization.
Porphyritic textured rocks have crystals of two sizes, i.e., larger crystals set in a groundmass. In contrast, poikilitic is a texture where a large crystal completely encloses smaller crystals of a different mineral.
Porphyritic texture has large crystals embedded in a finer-grained or glassy matrix. In contrast, pyroclastic texture has a mixture of rock fragments, pumice, and volcanic ash formed from violent volcanic explosions.
References
- Nelson, A. S. (2004, February 13). Textures of igneous rocks. Tulane University https://www2.tulane.edu/~sanelson/eens212/textures_igneous_rocks.htm
- Department of Geology & Planetary Science (n.d.), Porphyritic textures with coarse-grained groundmasses. University of Pittsburg, Retrieved August 27, 2022, from https://sites.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures/3PorphyriticCoarseGrained.html
- Alex Strekeisen (n.d.). Porphyritic texture. Retrieved on August 27, 2022, from https://www.alexstrekeisen.it/english/vulc/porphyritic.php
- University of Auckland. (n.d.), Igneous rocks. Retrieved August 27, 2022, from https://rocksminerals.flexiblelearning.auckland.ac.nz/rocks/igneous.html
- pitchstone | natural glass. (1998, July 20). Encyclopedia Britannica. Retrieved August 27, 2022, from https://www.britannica.com/science/pitchstone
- igneous rock – Granularity. (1999, July 26). Encyclopedia Britannica. Retrieved August 26, 2022, from https://www.britannica.com/science/igneous-rock/Granularity
- Alex Strekeisen (n.d.). Glomerophyric texture. Retrieved on August 27, 2022, from https://www.alexstrekeisen.it/english/vulc/glomerophyric.php
- Zauyah, S., Schaefer, C. E., & Simas, F. N. (2018). Saprolites. Interpretation of Micromorphological Features of Soils and Regoliths, 37–57. https://doi.org/10.1016/b978-0-444-63522-8.00003-6
- Porphyritic. (2022, June 14). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Porphyritic&oldid=1093082951