The phaneritic texture is one of the igneous rock textures. It describes rocks with large crystals visible to the naked eye, i.e., you can see and distinguish minerals with an unaided eye. Other common textures are aphanitic, glassy, pyroclastic, pegmatitic, porphyritic, etc.
The occurrence of a phaneritic [pronounced făn′ə-rĭt′ĭk] texture indicates a slower, longer cooling history of magma. Such a slow cooling rate happens deep in the Earth’s crust, making crystals grow larger, typical of intrusive rocks. Also, prevailing conditions must favor the rate of crystal growth over nucleation.
Beyond the above basic explanation, there is much more to know about formation, typical grain sizes, and examples of rocks with this texture. Also, we will give you a comparison with the aphanitic texture.
What is the meaning of phaneritic texture?
Phaneritic texture describes the texture of igneous rocks with large mineral crystals visible by the unaided eye, meaning you can distinguish the minerals present with the naked eye. Such a texture is also considered coarse-grained and is typical of intrusive or plutonic rocks like granite, gabbro, or diorite.
Phaneritic is an adjective formed from the prefix phaner. Phaner originates from the Greek name phanaros, which denotes visible. In our case, visible refers to the fact that the crystals are visible by the eye, i.e., the minerals are megascopically crystalline.
.jpg){kind=link}
The other thing to note is that the phaneritic texture is holocrystalline. A holocrystalline rock is entirely crystalline and not glassy (holohyaline) or a mixture of glass and crystals hypocrystalline.
That is not all. Phaneritic igneous rocks with phenocrysts are said to be porphyritic or what is known as porphyry. This texture describes the presence of consistently larger crystals in a finer-grained matrix. The finer-grained matrix may be phaneritic, aphanitic, or glassy.
Lastly, the European Commission – Joint Research Centre excludes coarse-grained igneous rocks with exotic minerals. Examples include more than 10% leucite, kalsilite, melilite, or more than 50% carbonates like Kimberlite, Lamproite, Carbonatite, Kalsilitic, or Melilitic rocks.
What does phaneritic texture indicate and form?
The phaneritic texture indicates or implies a few things about rocks’ origin and cooling history with this texture.
First, phaneritic textured rocks formed via slow cooling for a long time. The slow cooling allowed for crystals to form and grow larger. Such slow cooling conditions are typically deep within the Earth’s crust, such as in an underground magma chamber, meaning these rocks are intrusive by nature, i.e., erosion and other factors may have brought them to the surface.
Otherwise, if crystallization occurs closer to or on the Earth’s surface, the faster cooling rates will not allow larger crystals to grow. If cooling is so rapid, you will end up with fine-grained (aphanitic) crystals or glassy.
Secondly, the crystal growth rate was faster than nucleation (the process by which solid mineral crystals start forming). If nucleation rates were higher, crystals would limit each other’s growth, and you would end up with many smaller crystal sizes.
Thirdly, the relative grain size of the phaneritic rock may help indicate the depth of crystallization. Larger-grained rocks indicate they crystallized at greater depth than those with smaller grains.
The other thing is that minerals that crystallize fast will have larger, well-formed crystals than those that form in the final crystallization stage. Why? Because they have more space to grow without any limitations. This is not true for the crystals that form last as they will compete for space, filling any available spaces, including irregular ones.
To conclude, phaneritic textured rocks are intrusive igneous rocks formed deep inside the Earth’s crust, with slow cooling rates. This slow cooling rate favored larger crystal growth and slower nucleation.
Phaneritic rock grain size – 1/16 mm to 3 cm
A possible question is what grain size phaneritic or coarse-grained textured rocks have. Unfortunately, there is no unanimity on the exact size. So, the sizes may vary from one country or source to another.
For instance, Appalachian State University notes that the grain size of phaneritic rocks is 1/16mm to 3 cm (0.0025 to 1.18 inches). Any rock with most grains larger than 3 cm is said to have a pegmatitic texture.
If we go by the smallest grain an eye can see, the smallest crystals in a phaneritic igneous rock texture will be about 0.1mm or 1/10mm.
When giving an average grain size, consider most of what crystals measure. For instance, if a granite rock sample has 90% grains measuring 1cm and there are smaller or slightly larger ones, the size will be 1cm.
Lastly, a rock with a phaneritic texture may have grains whose sizes vary or are nearly equal (equigranular) as with gabbro. It still qualifies if the size is within the range above.
Subvolcanic or hypabyssal rock textures
Subvolcanic rocks are intrusive igneous rocks formed closer to the Earth’s surface, usually at 2km (1.2 miles) or less. Their texture is between plutonic and volcanic igneous rocks, and they commonly form on sills, laccoliths, and dykes.
These rocks have an intermediate or medium grain size and porphyritic texture. Examples include diabase, diorite, and microgranite. How to classify them depends on their grain size.
Examples of phaneritic rocks
Most of the phaneritic rocks are intrusive or plutonic rocks. Considering their composition, these rocks can be felsic, intermediate, or mafic.
Here are some notable examples:
1. Granite
Granite is a popular, light-colored, coarse-grained intrusive igneous rock composed mainly of quartz and felspar (alkali feldspar and plagioclase) and smaller amounts of mica, amphibole (hornblende) and other mafic minerals.
This felsic (silica-rich) rock is usually gray, whitish, red, or pink and has few, scattered, darker or nearly black mafic mineral crystals. The quartz and felspar interlock and have nearly equal crystal sizes (equigranular).
2. Gabbro
Gabbro is a dark-colored (dark-gray to greenish-black) phaneritic plutonic rock composed mainly of plagioclase feldspar and pyroxene. However, it has other minerals like olivine, hornblende, etc., and it is the intrusive equivalent of basalt.
In terms of composition, this rock is mafic, i.e., it is high in magnesium and iron with relatively high amounts of pyroxene, olivine, amphibole, and mica.
3. Diorite
Diorite is a medium- to coarse-grained, intermediate (falls between felsic and mafic) intrusive igneous rock with dark and white crystals. However, it may have a green, gray, dark-gray, or bluish tint.
This rock is the plutonic equivalent of andesite, a volcanic rock.
4. Peridotite
Peridotite is a name for a dense, dark-colored, ultramafic (has less than 45% silica) phaneritic intrusive igneous rock. It comprises primary olivine with other minerals like pyroxene or amphibole.
This ultramafic rock is relatively high in magnesium and has considerable iron. Its colors are yellowish or greenish-gray to nearly black, the plutonic equivalent of komatiites.
5. Syenite
Syenite is a light-colored (pink, grey, or sometimes brick red) intermediate, coarse-grained plutonic igneous rock whose composition is like granite but is low (<5%) or lacks quartz. This rock has mainly orthoclase feldspar and smaller amounts of mica, augite, magnetite, and plagioclase.
6. Dunite or olivinite
Dunite is a yellowish-green, ultramafic, phaneritic intrusive igneous rock with more than 90% olivine. This rock may have other minerals, including chromite, magnetite, pyroxene, and pyrope.
7. Granodiorite
Our final example of phaneritic intrusive igneous rock is granodiorite, a grayish-white rock. It has a mineral composition that lies between granite and diorite. Also, it has more orthoclase feldspar compared to plagioclase feldspar.
Aphanitic vs. phaneritic rock textures
Phaneritic and aphanitic rocks are igneous textures in mafic, felsic, or intermediate rocks. Also, both are crystalline textures (made of crystals). The main difference between these textures is the size of grains, which is influenced by formation.
Phaneritic rocks have coarse grains, more than 0.1 mm, that you can see and distinguish with the naked eye, while phaneritic textured rocks have fine-grain size, less than 0.1 mm, and you cannot see or distinguish with an unaided eye.
Below is a summary of other differences between phaneritic igneous and aphanitic textured rocks.
| Phaneritic | Aphanitic | |
| Grain size | They have larger or coarse mineral crystals visible and distinguishable by an unaided eye. | They have small mineral crystals not visible to the naked eye. You need a microscope or a lens to see and distinguish their minerals. |
| Cooling rate | These rocks form from a slower cooling rate, allowing crystals to form and become larger. | Aphanitic rocks form from a rapid cooling rate that prevents crystals from growing large. |
| Where they form | Form deeper in the Earth’s crust | Form closer to the Earth’s surface. |
| Types of rocks | Mainly intrusive igneous rocks | Mainly extrusive igneous rocks |
| Examples | Granite, gabbro, and diorite | Basalt, rhyolite, and andesite |
References
- Appalachian State University. (n.d.). Classification of Igneous Rocks. Retrieved September 3, 2022, from http://www.appstate.edu/~abbottrn/rck-id/ignchrt.html
- igneous rock – Granularity. (1999, July 26). Encyclopedia Britannica. Retrieved September 03, 2022, from https://www.britannica.com/science/igneous-rock/Granularity
- The University of Utah. (n.d.). Cell Size and Scale. Retrieved September 03, 2022, from https://learn.genetics.utah.edu/content/cells/scale/
- European Commission – Joint Research Centre. (n.d.). Exotic composition igneous rock. INSPIRE. Retrieved September 3, 2022, from https://inspire.ec.europa.eu/codelist/LithologyValue/exoticCompositionIgneousRock