Metasomatism

From Freepedia

Metasomatism is a geologic process where metamorphism causes an alteration in a mineral or rock mass that involves a chemical change of the substance with the addition of material, as when chrysolite (olivine) is converted to serpentine basically by the addition of water. This is contrasted with ordinary metamorphism which varies from simple recrystallization (example: sandstone to quartzite), which is a physical change; to recrystallization with local migration of chemicals resulting in chemical change of the minerals with no external addition of materials. The process of metasomatism adds material to the affected rock body. The added material may come from magmatic fluids, moving connate fluids, or from convective circulating groundwater or seawater.