Coarse-grained Phaneritic Rocks:

* Phaneritic means "visible" and refers to rocks where the individual mineral grains are large enough to be easily seen with the naked eye.

* These grains are typically greater than 1 millimeter in diameter, allowing for easy identification and analysis.

* Examples include granite, gabbro, and diorite.

Fine-grained Aphaneritic Rocks:

* Aphaneritic means "invisible" and describes rocks where the mineral grains are too small to be distinguished without a magnifying glass or microscope.

* These grains are less than 1 millimeter in diameter, and often appear as a homogeneous mass to the naked eye.

* Examples include basalt, rhyolite, and andesite.

Here's a simple way to remember the difference:

* Phaneritic rocks have visible grains (think of "phan" as sounding like "fan" - you can see the individual grains like you can see the individual blades of a fan).

* Aphaneritic rocks have invisible grains (think of "aphan" as sounding like "a-fan" - you can't see the individual grains, just like you can't see individual blades on a fan from afar).

The size of the mineral grains in igneous rocks is primarily determined by the rate of cooling of the magma or lava. Slower cooling allows for larger mineral crystals to form, resulting in phaneritic rocks. Faster cooling leads to smaller crystals, resulting in aphaneritic rocks.