This discussion will look at similarities and differences. We will start by looking at definitions, similarities, and grain sizes. Afterward, we’ll have a part on formation, rock type, and examples.
These are textures and don’t indicate rock mineral composition or chemistry. They tell you more about grain size and formation.
Understanding phaneritic and aphanitic
The phaneritic texture is an igneous rock texture characterized by mineral crystals you can see or distinguish with the naked eye. This texture is also known as coarse-grained and is common in plutonic or intrusive rocks like diorite, gabbro, or granite.
On the other hand, aphanitic textured rocks are igneous rocks with tiny mineral crystals that you cannot see or distinguish with the naked eye. These rocks are also said to be fine-grained and to see the mineral crystals. You must prepare a thin section and use an electron and a petrological polarized microscope or electron.
These terms, aphanitic and phaneritic, are textural and don’t give information on mineralogical composition or chemistry.
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What are their grain sizes?
The grain size of phaneritic textured rock ranges from 1/16mm to 3cm, while that of aphanitic is below 1/16mm. However, Encyclopedia Britannica put the measures for phaneritic and aphanitic textures to 1/10mm to 3cm and below 1/10mm, respectively.
There is no unanimity on the respective sizes of phaneritic and aphanitic grains. The emphasis is on the smallest mineral grain you can see with an unaided eye and what you cannot. Our eyes can see a grain of about 1/10mm.
Also, note that rock with grain sizes larger than 3cm is said to have pegmatitic texture and forms in the last stages of magma crystallization.
Lastly, for an igneous rock to be classified as phaneritic or aphanitic, most grain sizes, i.e., at least 90%, must be within the size range. For instance, a rock with 90% of the mineral crystals below 1/16mm is aphanitic, even if there are a few larger crystals.
How are these textures formed?
Phaneritic texture forms from the slow cooling (allows crystals to grow large) of magma deep in the Earth’s crust, while aphanitic texture forms on or near the Earth’s surface where fast cooling doesn’t allow crystals to grow large.
Therefore, you can tell the cooling history by considering these two textures. In aphanitic texture, the cooling rate was fast, while phaneritic had a slow speed for phaneritic. Also, you can know the depths at which cooling occurred.
Rock origin for these textures
The phaneritic texture is an intrusive igneous rock texture since these rocks form deep inside the Earth’s crust. In contrast, aphanitic is mainly extrusive or volcanic igneous rock. However, some aphanitic textured rocks are intrusive, i.e., hypabyssal, the subvolcanic intrusive rock.
Subvolcanic forms at depths less than 1.2 miles (2 km) from the Earth’s surface, especially in sills, dykes, or laccoliths. They may have an aphanitic texture but, more often, have a porphyritic texture.
However, aphanitic and phaneritic rocks can show porphyry or a porphyritic texture. It only indicates two cooling stages, i.e., slow forms, the larger crystals, and the final faster cooling forms, the finer-grained matrix.
Examples of aphanitic and phaneritic rocks
The typical examples of aphanitic rocks are basalt, rhyolite, and andesite, while those of phaneritic texture are granite, gabbro, and diorite.
If you consider their chemical composition or mineralogy, basalt is the volcanic equivalent of gabbro, granite the equivalent of rhyolite, and andesite the equivalent of diorite.
Granite and rhyolite are light-colored, felsic rocks or silica-rich with main quartz and felspar and smaller amounts of mica, hornblende, etc.
On the other hand, basalt and gabbro are dark-colored mafic rocks (high in iron and calcium) with plagioclase feldspar and pyroxene with smaller amounts of biotite and hornblende. Their quartz level is less than 10%.
Lastly, diorite and andesite are intermediate rocks, i.e., their composition lies between felsic and mafic).
Similarities
We have talked about differences. It is time to look at some similarities between these textures, including:
- Phaneritic and aphanitic rocks are igneous rock textures, meaning they form from solidifying magma or lava (if extrusive).
- Both textures are holocrystalline, i.e., crystalline or have mineral crystals, as contrasted to a non-crystalline-like glassy or vitreous texture.
| Phaneritic rocks | Aphanitic rocks | |
| Definition | Coarse-grained or igneous rocks with large mineral crystals that are visible and distinguishable to the naked eye | Fine-grained or igneous rocks with fine mineral crystals, which you cannot distinguish or see with the naked eye |
| Grain sizes | 0.1mm to 3cm | Less than 0.1mm |
| Cooling history | They form via a slow cooling rate that allows crystals to become larger | A relatively faster cooling rate doesn’t give time for crystals to become larger. |
| Where they form | Deep inside the Earth’s crust, where cooling is slow | On or near the Earth’s surface, where cooling is faster |
| Rock type | Rocks with this texture are plutonic or intrusive igneous rocks | It has mainly volcanic or extrusive igneous rock with some subvolcanic. |
| Examples | Gabbro, diorite, granite, peridotite and syenite. Others are granodiorite and dunite. | Basalt, rhyolite, andesite. Also, pumice and scoria may have this texture but with vesicles. |