This mainly depends on the cooling rate of the magmatic rock during deposition. Quickly cooling magmas tend to produce fine crystalline rock while slowly cooling magmas tend to produce coarse graines rocks. Effusive lavas can produce glasslike rocks like obsidian.

Thanks, Christian

You mentioned the igneous rock, as we know, rock types can be divided into three types: igneous rock, sedimentary rock and metamorphic rock, what about other two kinds of rocks.

For example, we also know granite is slowly cooling magmas, however, still, we can find some articles, in which granite are thought to be coarse-grained.

Regards

This is right. Granite is a coarse grained rock.

The grain size of sedimentary rocks can vary over a wide range (from fine grained clay rocks to coarse grained conglomerates). I don't know if you can call clayrocks "nanocrystalline". That would mean that you have grain sizes of 10^-9 m. This is the typical range for small pores in a clayrock. Additionally it is difficult to define the grain size in clayrocks because the single clay crystals are "lumped together" to bigger aggregates.

In the case of metamorphic rocks you also have a wide range of grain sizes. Here the grain size depends on p-T-conditions, recrystallisation processes etc. But I don't know if there are nanocrystalline metamorphic rocks. If there are such rocks they are an exception and should be pretty rare.

Thanks, Christian

what you say make sense.

Regards