When liquid rock (magma) cools slowly beneath the surface, it forms intrusive igneous rocks with large, well-formed crystals. These crystals are known as phaneritic crystals.

Here's why:

* Slow Cooling: The slow cooling process allows atoms in the magma to move more slowly and arrange themselves into organized, repeating patterns.

* Large Crystals: The slower cooling gives ample time for the crystals to grow larger.

* Well-Defined Shapes: The undisturbed environment allows the crystals to develop distinct, geometric shapes.

Some examples of common intrusive igneous rocks with phaneritic crystals include:

* Granite: Contains quartz, feldspar, and mica crystals.

* Gabbro: Contains plagioclase feldspar and pyroxene crystals.

* Diorite: Contains plagioclase feldspar and hornblende crystals.

In contrast, when magma cools quickly above the surface (extrusive igneous rocks), it forms smaller, less well-defined crystals, known as aphanitic crystals.