In what different ways can the crystallization process be studied and observed?

• experimentally (melt, cool and quench in lab).
• theoretically (physical chemistry).
• use of rock textures/associations.

What controls the grain size of an igneous rock?

• rate of cooling: faster = finer, slower = coarse.
• temperature contrast between igenous melt and surrounding.
• fluid content: wet = coarser, dry = finer.