The aphanitic texture is an igneous rock texture characterized by very small, invisible mineral crystals indistinguishable from an unaided eye. You must prepare a thin section and use a polarizing petrological microscope to see the minerals. An electron microbe or microscope will also work.
This texture is also known as fine-grained, and rocks that have this texture are aphanites. Basalt, rhyolite, and andesite are examples that best describe rocks that exhibit texture.
Rocks with a coarse grain or larger crystals visible to the naked eye, like granite, gabbro, and diorite, are known as phaneritic. Other common igneous rock textures are porphyritic, glassy, pyroclastic, and pegmatic.
The formation of aphanitic igneous rocks indicates fast cooling near or on the Earth’s surface. However, note that it is only a textural term. So, it doesn’t give you mineralogy or rock chemistry, and rocks with this texture may be mafic, felsic, or intermediate.
Since individual mineral crystals are invisible in the field, rocks with this texture may be classified as light (felsic), intermediate, or dark (mafic) colors.
Learn more about aphanitic rock textures. We will define and discuss the formation and examples of igneous rocks with this texture. Of course, it occurs mainly in extrusive but may also occur in subvolcanic intrusive rocks.
What is the aphanitic texture?
Aphanitic texture describes igneous rocks with small or fine mineral crystals you cannot see or distinguish by the naked eye or a hand lens. Such rocks are fine-grained and need a petrographic microscope to see or identify individual mineral crystals.
Rocks with this texture are known as aphanites. The term comes from an Ancient Greek word αφανης (aphanḗs), which means invisible to denote their fine grains that are invisible to an unaided eye.
Examples of aphanitic rocks are basalt, rhyolite, and andesite. And since aphanitic is a textural term, these can be mafic, felsic, or intermediate. It all depends on their mineral composition.
When defining aphanitic rocks, bear in mind that these rocks are crystalline (made of crystals or holocrystalline). However, in most instances, an aphanitic stone may have some glass or even slightly larger crystals. We will still consider it aphanitic if it doesn’t the large or glassy components don’t account for more than 5 to 10%.
Also, talking about aphanitic texture, you may come across microcrystalline and cryptocrystalline. Some people use them interchangeably, but they have a slight difference.
Microcrystalline indicates tiny crystals you cannot see with an unaided eye, but you can easily see and distinguish mineral crystals with a petrographic microscope.
On the other hand, cryptocrystalline refers to “the texture of a rock consisting of crystals that are too small to be recognized and separately distinguished even under the ordinary microscope (although the use of the electron microscope may show crystallinity)” (American Geoscience Institute, 2018).
In simpler words, cryptocrystalline represents a much smaller crystal size than microcrystalline. However, both describe aphanitic texture.
Lastly, you must prepare a thin section in a laboratory to see crystals in aphanites. Afterward, you will view it with an electron microbe, polarizing petrographic microscope, or even an electron microscope. You cannot see crystals with a hand lens.
Aphanitic texture grain size
Aphanitic rocks will have grain sizes less than 1/16 mm or 0.0025 inches, with some sources putting the size at 1/10mm or 0.004 inches (Encyclopedia Britannica, 1999).
The size you cannot see with the naked eye will be smaller than 1/10mm. There exists no agreed maximum size for aphanitic rocks.
Lastly, an aphanitic rock may have some glass or slightly larger crystals in most instances. We will still consider it aphanitic if it doesn’t the large or glassy components don’t account for more than 5 to 10%.
How are aphanitic rocks formed?
Igneous rocks with aphanitic texture indicate a rapid cooling history, meaning that these rocks form near or on the Earth’s surface.
Therefore, aphanitic rocks form when lava (or magma if subvolcanic) cools fast on or near the Earth’s surface. The fast cooling doesn’t give enough time for crystals to grow large. Therefore, the rocks will end up with very fine crystals.
Sometimes, you may end with porphyritic aphanitic rocks characterized by large crystals in a fine-grain matrix. It indicates that the larger crystals had formed during an eruption or magma movement to or near the Earth’s surface.
Also, you may end up with fine-grained or aphanitic rocks with pits or holes (vesicles). It forms in magma with dissolved gasses. These gases will form gas bubbles responsible for the holes or vesicles on extrusion. Good examples include scoria and pumice.
Lastly, if lava is quenched so fast, you will have a glassy, vitreous texture that lacks crystals. Such a texture is what holohyaline describes.
Are aphanitic rocks intrusive or extrusive?
Aphanitic texture is an extrusive or volcanic igneous rock texture. However, in some cases, it can be intrusive. For instance, subvolcanic or hypabyssal rock may have this texture.
Why would subvolcanic rocks have an aphanitic texture? Because cooling is relatively fast in their forming environments, i.e., at depths less than 1.2 miles (2 km) from the Earth’s surface, such as in sills, dikes, or laccoliths.
However, in most instances, subvolcanic will be porphyritic with a fine-grained cryptocrystalline groundmass. Others will have medium-grain sizes, and those very shallow or close to the Earth’s surface will have an aphanitic texture.
Examples of aphanitic rocks
The most obvious examples of aphanitic rocks are basalt, rhyolite, and andesite. However, there are many others.
Here are some examples of aphanites.
1. Basalt – a fine-grained mafic rock
Basalt is a common dark-gray to black mafic (rich in iron and magnesium) fine-grained volcanic igneous rock. It comprises mainly plagioclase felspar, olivine, hornblende, and biotite with less than 20% quartz.
Basalt, the gabbro’s volcanic equivalent, forms mainly from the lava flow and sometimes in sills and dykes.
2. Rhyolite – a felsic rock
Rhyolite is light-colored (gray, pink, or reddish) felsic (silica-rich) aphanitic extrusive igneous rock. It mainly has quartz, plagioclase, sanidine, and smaller amounts of biotite and hornblende.
This rocks from very viscous lava and is the intrusive equivalent of granite, a reason some people call it aphanitic granite. Besides being fine-grained, it may be porphyritic – have large phenocrysts in a fine-grain groundmass.
3. Andesite – intermediate aphanitic rock
Andesite is an intermediate (composition between felsic and mafic) fine-grained volcanic rock whose minerals lie between basalt and granite. This bluish-gray or dark-gray to gray rock is the extrusive equivalent of diorite, and some people call it a gabbro aphanitic rock. However, it is not as dark as basalt.
4. Dacite
Dacite is gray to whitish (light-colored) volcanic rock. It has a composition between rhyolite and andesite. Specifically, this rock has less plagioclase than rhyolite and more quartz than andesite.
While it may be aphanitic, dacite is mostly porphyritic. Its composition is mainly quartz and plagioclase, with a low composition of alkali metal oxides.
5. Pumice
Pumice is a fine-grained or glassy vesicular extrusive rock that forms from an explosive volcanic eruption. This felsic rock may be white, cream, gray, tan, black, or green-brown and has a low density.
6. Scoria
Scoria is a black, purplish red, or dark-brown mafic vesicular volcanic rock with an aphanitic to glassy texture. It resembles pumice rock but has larger vesicles.
References
- American Geoscience Institute. (2018, October 8). Cryptocrystalline. https://www.americangeosciences.org/word/cryptocrystalline
- Igneous rock – Granularity. (1999, July 26). Encyclopedia Britannica. Retrieved September 03, 2022, from https://www.britannica.com/science/igneous-rock/Granularity
- Appalachian State University. (n.d.). Classification of Igneous Rocks. Retrieved September 3, 2022, from http://www.appstate.edu/~abbottrn/rck-id/ignchrt.html
- Aphanite. (2022, July 3). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Aphanite&oldid=1096283574