1. Recrystallization:
* Heat: Metamorphism involves intense heat, which causes the existing minerals in the protolith to become unstable.
* Pressure: Increased pressure also contributes to instability, forcing atoms to rearrange themselves into more stable structures.
* New minerals: This instability leads to the recrystallization of existing minerals into new, more stable mineral forms. For example, clay minerals in shale might recrystallize into mica or garnet under metamorphic conditions.
2. Chemical Reactions:
* Fluid activity: During metamorphism, fluids like water and carbon dioxide can circulate through the rock. These fluids can act as catalysts, promoting chemical reactions between minerals.
* New minerals: These reactions can lead to the formation of new minerals that weren't present in the protolith. For example, limestone might react with fluids to form marble, which contains calcite, a different mineral than the original calcium carbonate.
3. Texture Changes:
* Foliation: The pressure during metamorphism can also cause minerals to align in a specific direction, creating a layered or banded texture called foliation. This is common in metamorphic rocks like schist and gneiss.
* Grain size: The heat and pressure of metamorphism can also change the size and shape of mineral grains. This can make the texture of the metamorphic rock very different from the protolith.
In short: Metamorphism changes the mineral composition of a rock because it provides the energy and conditions necessary for minerals to become unstable, recrystallize, and react to form new minerals. This process results in a rock with a different mineral makeup and often a different texture than its original, unmetamorphosed form.
