Aphanite and phanerite are two different forms of rocks. Among the various different types of rocks found in various parts of the world, there are many that are similar to each other but have slight differences. Not always are these differences easy to detect; some rocks show no difference in appearance. However they have been given separate names and fall under different categories for some reason. A lot of similar characteristics does not conclude that any two rocks are exactly the same. There could be differences between them at the atomic level (or the molecular level) which are not visible to the naked eye. When we talk about aphanitic rocks or phaneritic rocks, once again the similarities would make you think that there are different names for the same thing. But as we will now make clear, the two are indeed different from each other.
The word aphanitic, which is used as an adjective to describe aphanites, is a term that is used for certain forms of igneous rocks that are very fine grained. Indeed, they are so fine that their components (mineral crystals) cannot be detected by the human eye as long as some magnifying instrument has not been used. The geological texture that is specific to these aphanitic rocks is due to rapid cooling of these rocks in hypabyssal (shallow subsurface) or volcanic environments. In contrast to this, the term phaneritic is normally used to refer to rocks that are also igneous but are not as finely grained as aphanites. They are what is known as rock grain size. This means that the size of the matrix grains in these rocks are quite large and unlike aphanites, they can be noted and distinguished by the naked eye. No magnifying instrument is needed.
Moreover, as opposed to aphanitic rocks, the texture of phaneritic rocks does not result from rapid cooling but from gradual cooling. The magma cools down slowly deep under the ground in what is termed as plutonic environment. The texture is similar to metamorphic rocks.
As we have seen, the two main differences are in the size of the crystals and the quickness with which cooling takes place before they are formed. These two things are actually interrelated. If the magma slows down in a longer period of time, the crystals that are formed are larger as they take time to form and can gain a considerable size. On the other hand, when the magma cools rapidly, the crystals that are formed form very quickly and are hence very small textured. This process of cooling and forming crystals is known as crystallization and it is what determines the end product.
Saying that some crystals are small and some are large is somewhat of a vague difference. We have to be more precise if we have the two at hand and need to tell them apart. Rocks that are aphanitic have a grain size below 1 millimetre. On the other hand, phaneritic rocks have grains that are coarse and the grain size is usually between 1 millimetre and 10 millimetres.
Summary of differences expressed in points
- The word, Aphanitic, that is used as an adjective to describe aphanites, is a term used to describe certain forms of igneous rocks that are very fine grained, so fine that their components (mineral crystals) cannot be detected by the human eye as long as some magnifying instrument has not been used; the term phaneritic is normally used to refer to rocks that are also igneous but are not as finely grained as aphanites
- Aphanites have crystals that are very finely grained; phaneritic rocks, however, have crystals that are coarse grained
- Rocks that are aphanitic have a grain size below 1 millimetre; phaneritic rocks have grains that are coarse and the grain size is usually between 1 millimetre and 10 millimetres
- The texture of phaneritic rocks results from gradual cooling, the magma cools down slowly deep under the ground in what is termed as plutonic environment; the texture of aphanitic rocks results from rapid cooling
- The grain size and cooling intervals are related; If the magma slows down in a longer period of time, the crystals that are formed are larger as they take time to form and can gain a considerable size (phanerites); when the magma cools rapidly, the crystals that are formed form very quickly and are hence very small textured (Aphanites)