The rocks are comprised of cracks and fractures in between the sediments and mineral grains that have the capacity to store fluid in them. This fluid is generally the water that contains various types of dissolved ions. These fluids in the rocks are very much important because the chemical reactions that take place include the change in mineral from one type to another new type. This can highly increase the amount of dissolved ions that are dissolved and transported by the water. This alteration of rock that occurs chemically due to these fluid refers to the process which is commonly known as the metasomatism. Whereas, the rock alters mineralogically due to the increasing temperature and pressure condition, that changes the grade of rocks.
This is how metamorphism alters the rock chemically in comparison to mineralogically.
