1. Recrystallization: The original minerals in the granite (quartz, feldspar, mica) are unstable under the increased temperature and pressure conditions of metamorphism. They recrystallize into larger, more stable forms. This results in a more granular texture.
2. Mineral Growth: The minerals grow larger and more elongated, forming distinct bands or layers within the rock. This layering is a key characteristic of gneiss.
3. New Mineral Formation: Some minerals may not be present in the original granite but form during metamorphism. For example, garnet, sillimanite, or kyanite can form depending on the specific conditions.
4. Mineral Orientation: The minerals in gneiss tend to align themselves parallel to the direction of pressure. This alignment contributes to the banded appearance.
5. Mineral Changes: The original minerals can undergo compositional changes. For example, feldspar can be altered to sericite or other clay minerals.
Specifically, the following happens to the main minerals in granite:
* Quartz: Quartz is relatively stable and remains as large, often elongated grains.
* Feldspar: Feldspar can be altered to form sericite or other clay minerals, or it can recrystallize into larger, more stable forms like orthoclase.
* Mica: Mica becomes more abundant and often forms distinct bands, creating the characteristic foliation of gneiss.
* Other minerals: Depending on the metamorphic conditions, other minerals like garnet, sillimanite, or kyanite can form.
In summary, metamorphism of granite into gneiss involves significant changes to the mineral composition and texture. The original minerals are recrystallized, grow larger, and align themselves in response to pressure, creating the characteristic banding of gneiss. New minerals can also form, depending on the specific metamorphic conditions.
