Pyric (US) or porphyritic texture is an igneous rock texture with two distinctive crystal sizes, i.e., bimodal crystal sizes. One set of crystals is visibly larger than the other.
These rocks will have large, often well-formed crystals called phenocrysts surrounded by a finer-grained matrix known as groundmass.
What makes the phenocrysts and groundmass phases depend on the composition of magma or lava and is determined by Bowen’s reaction series.
Phenocrysts have euhedral grains and are sometimes subhedral. Euhedral refers to well-formed crystals with sharp, easy-to-recognition faces, while subhedral aren’t so well-formed.
These crystals often have well-formed crystals since they grew in the melt unimpeded or without restriction from neighboring crystals. When so when phenocrysts are very large, they are known as megaphenocrysts.
Most form or crystallize at high temperatures, but not always. Similarly, some form earlier, but again, not always.
Usually, phenocrysts up to 50% of aphanitic or even glassy melt. Beyond this, they would prevent, immobilize and retard extrusive flow.
Some phenocrysts can show zoning – normal, discontinuous, oscillatory, etc.- while others are rimmed and when so large.
On the other hand, the matrix or groundmass consists of smaller grain sizes. It can have a fine-grained, medium-grained, or coarse-grained texture.
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Note: If phenocrysts that enclose smaller crystals of another mineral form a poikilitic texture. The phenocrysts will be called oikocrysts.
Related terms
Some of the terms closely associated or related to porphyritic texture include the following:
1. Vitrophyric texture
Vitrophyric texture forms in igneous rocks in which large, well-formed crystals are sent in a glassy matrix or groundmass. It is like a porphyritic texture, but the groundmass is glassy, not crystalline.
2. Aphyric texture
Aphyric texture refers to igneous rock with a single-grain size. Such stones are rare and are not porphyritic or don’t have phenocrysts.
3. Microporphyritic
Microporphyritic are igneous rocks whose porphyritic texture is only seen under a microscope. They differ from mega porphyritic or simply porphyritic, where the texture is visible in hand specimens.
4. Hiatial porphyritic
Hiatial porphyritic is a porphyritic texture with a huge difference in the sizes of the phenocrysts and groundmass.
In such rocks, the larger, well-formed crystals are enormous compared to the size of the matrix or groundmass grains.
5. Cumulophyric texture
Cumulophyric texture forms when phenocrysts of different mineral grains cluster or adhere to each other, a process called synneusis in the matrix.
It results in lower energy due to less crystal-melt interfacial energy than scattered crystals, like surface tension in liquids.
6. Glomeroporphyritic texture
Glomeroporphyritic texture forms when phenocrysts of the same mineral cluster or aggregate together or adhere to each other in the groundmass.
7. Seriate texture
The seriate texture is a texture in igneous rocks where rocks have crystals that gradually increase in size. It differs from porphyritic, where grains have two distinctive sizes.
Porphyry and porphyritic texture
The term porphyry is a noun. It describes igneous rocks with large crystals or phenocrysts in a fine-grained matrix or groundmass.
Porphyry is from Ancient Greek from the word porphura or porphyra, which means purple dye or the gastropod the dye is derived from. It was coined by Pliny the Elder to describe the source of Egypt-sourced purplish construction stone.
Pliny the Elder (Gaius Plinius Secundus) was a respected Roman author, naval army commander, and naturalist.
Originally, porphyry referred to the purple or red porphyritic dacitic rock mined in Egypt. This rock, called red, purple, or Roman Imperial porphyry, was highly valued during antiquity as a dimensional stone and was used to make sculptures among the Greeks, Egyptians, and Romans.
However, today, it describes any igneous rock with large mineral grains set in a finer-grained matrix. Leonhard was the first porphyritic term in this sense for the first time in 1823.
You can prefix porphyry with rock names or compositions, such as andesite or granite porphyry. Also, you can prefix it with phenocrysts, such as feldspar-quartz or feldspar porphyry, or with porphyry with a textural term, such as rhomb porphyry.
On the other hand, the word porphyritic or phyric is an adjective from the noun porphyry. It describes igneous rocks with modal crystal size.
You can use it as a suffix to rock name/composition or phenocrysts. For instance, you can have porphyritic andesite or feldspar. Here, andesite is a compositional or rock name, while feldspar only tells you that the rock has large crystals of feldspar.
How does porphyritic texture form?
Porphyritic rocks form from two-stage cooling when crystallization starts outside the eutectic composition or when minerals crystallize and nucleate at different rates. Also, this texture can result in sudden loss of water-rich fluids or volatiles.
Here is more on how this texture forms:
1. Two-stage cooling history (polygenetic)
Porphyritic texture can form from two-stage cooling histories, i.e., a slow cooling deep inside the Earth’s crust and a faster one on or near the Earth’s surface.
A slow cooling stage deep inside the Earth’s crust will cause a low degree of supersaturation or small undercooling. This will favor crystal growth over nucleation.
Therefore, you will have a few sparse crystals that will grow large. This will form the large, well-formed grains or phenocrysts since the grains are sparse, and crystals will grow unimpeded inside mostly liquid magma.
On the other hand, the faster cooling stage will happen when magma with phenocrysts extrudes or erupts. This will cause quick cooling that will cause a high degree of supersaturation or supercooling.
The high degree of supersaturation or supercooling will favor more nucleation, allowing many crystals to form but with little time or space to grow large.
Therefore, the uncrystallized magma will crystallize and solidify to form the finer-grained groundmass during the fast-cooling stage. This groundmass will surround earlier formed phenocrysts.
Two-stage cooling forms porphyritic rocks, primarily volcanic and subvolcanic rocks. Subvolcanic or hypabyssal rocks form at depths less than two kilometers from the surface.
Lastly, while given as the most common cause of porphyritic rocks, two-stage cooling doesn’t account for how this texture forms in plutonic magma chambers. In these chambers, cooling is more or less uniform.
2. Crystallization outside eutectic composition
Magma melt can start crystallizing some minerals outside the eutectic composition. Eutectic composition refers to a homogenous melt whose melting point is lower than any of its constituents.
This happens because different minerals can undercool to different extents since undercooling depends on their melting point, which varies. Therefore, minerals can crystallize sequentially at their melting point.
This deviation from eutectic composition will cause some crystals exceeding the eutectic mixture to form earlier before the melt reaches a eutectic composition. These crystals will be larger and well-formed.
For instance, anorthite and plagioclase can crystalize during the cooling process before the melt has a eutectic composition. Thus, they will form large, well-formed crystals that grow unimpeded.
Diopside and other minerals can also crystallize when the melt reaches a eutectic composition. However, mineral grains that form at eutectic composition will not grow large or be as well-formed due to limited space, i.e., they bump into each other.
Therefore, you will have large, earlier well-formed phenocrysts formed outside eutectic composition set in finer-grained groundmass formed at eutectic composition.
3. Different nucleation and crystal growth rate
Nucleation and crystal growth depend on the surface energy of minerals, the number of faces involved, diffusion rate, undercooling degree, and crystal structures. These factors can vary even within the same magma body.
For example, porphyritic phaneritic granitoids like granites, alkali feldspar tonalites, or granodiorites form large, euhedral alkali feldspar phenocrysts due to a lower nucleation rate above solidus temperature than other constituent minerals.
Usually, granitic and granodiorite magmas will reach alkali feldspar and quartz saturation only a few to tens of degrees above solidus temperature.
At this point, more than half of the magma volume may still be in melt due to the high quartz and alkali feldspar solubility.
Thus, alkali feldspar will have enough space to grow large crystals as they grow from only a few sparse nuclei.
Experiments have confirmed that alkali feldspar nucleates more slowly than quartz and plagioclase in granitic magmas.
This confirms that larger crystals didn’t necessarily form earlier. They could nucleate less but grow faster.
4. Sudden loss of water-rich fluids
Temperature and pressure can affect nucleation, crystal growth, and size. This can make some crystals larger than others.
For instance, crystallizing magma with large crystals may suddenly lose water-rich fluids or volatiles.
The remaining melt will have less fluids or low water pressure. This will raise the melting temperature, causing the melt to supercool. Supercooling will favor rapid nucleation, forming fine-grained groundmass.
This can happen where crystallizing magma at shallow depths raptures the rocks on their roof. Also, these fluids may escape into fractures in country rock, even in a plutonic environment.
While people may argue this represents a two-stage cooling, it doesn’t. An isothermal drop in water pressure doesn’t represent two-stage cooling but two-level volatile concentration.
Note: Fractional crystallization
Where fractional crystallization happens, some phenocrysts in porphyritic texture can be earlier formed crystals that should have been separated or removed.
Such phenocrysts accumulated during separation from the melt, i.e., remained in the magma, i.e., didn’t sink or float. Therefore, such phenocrysts don’t form from the magma or rocks where they are found.
Porphyritic rock examples
The porphyritic texture is far more common in aphanitic or fine-grained volcanic rocks than coarse-grained plutonic rocks.
However, they are common in subvolcanic or hypabyssal rocks formed less than 2 km from the Earth’s surface.
These rocks’ composition ranges from mafic to felsic. Mafic are basic rocks poor in silica (45-52 wt.% SiO2) rocks rich in iron and magnesium, the darker elements.
On the other hand, felsic rocks are acidic rocks that are silica-rich (> 63 wt. SiO2) and high in lighter elements like oxygen, silicon, aluminum, sodium, and potassium.
Intermediate lie between felsic and mafic in silica, mafic, and felsic element composition.
Some strongly porphyritic rocks are rhyolite, basalt, lamprophyres, and andesite.
Also, others like granite, diorite, gabbro, trachyte phonolite, nepheline syenite, syenite, monzonite, kimberlite, dacite, latite, granodiorite have porphyritic texture.
However, pegmatites are never porphyritic. They only have abnormally large crystals formed during the crystallization of water and other volatile-rich crystals during the last portion of magma melt.
Porphyritic texture significance or importance
The formation of porphyritic rocks, which are the majority, confirms that magmas are never superheated inside the Earth’s crust, and the Earth doesn’t have excess heat. They only slightly deviate from eutectic composition.
Also, phenocrysts help understand how magna evolve. You can know which minerals crystallized before the eruption and that there was some period inside the Earth’s crust before the eruption. An electron microscope can give their composition.
Some porphyrites like red, purple, or Roman Imperial porphyry were once highly valued in building and sculpturing rocks.
Also, rapakivi granite is a famous cladding rock with a porphyritic texture and large, oval, or rounded k-feldspar rimmed by plagioclase.
That is not all. Some porphyritic rocks are associated with mineral ore in molybdenum or copper porphyry deposits. Also, they often have iron sulfide (pyrite), gold, silver, and tin.
These deposits occur in veinlets in mostly granitic, quartz-diorite, and quartz-monzonite porphyritic rocks. Kennecott Copper Mine in Utah is an example of a copper porphyry deposit.
Lastly, the various porphyritic rocks are used as similar aphyric rocks in making aggregate, dimensional stones, landscaping, etc.
Frequently Asked Questions (FAQs)
Yes, but not always. Some less nucleating but fast-growing minerals can form phenocrysts in a groundmass of earlier formed, more nucleating but slow-growing minerals.
For instance, large or megaphenocrysts of K-feldspar common in granitoids like granites, alkali feldspar, and tonalite likely formed late in the crystallization sequence, not early.
The phaneritic texture will have large mineral grains visible and distinguishable with an unaided eye. In contrast, the porphyritic texture has two-grain sizes, larger mineral grains set in a finer-grained matrix. The matrix can be phaneritic or aphanitic (fine-grained).