Describe the main process of foliation development in a metamorphic rock such as schist.

Metamorphism is the process of transformation of one rock type into another due to changing temperature, pressure, and other conditions within the Earth's crust. Foliation is a particular characteristic of metamorphic rocks, resulting from the preferred alignment of minerals along specific planes which give the rock a layered or banded appearance.

The process of foliation development begins with the parent rock, which can be any rock type (igneous, sedimentary, or another metamorphic rock). In the case of schist, the common parent rocks are shale, clay-rich sedimentary rocks, and mafic igneous rocks.

As the parent rock gets subjected to increasing temperature and pressure during metamorphism, the minerals within the rock begin to recrystallize and grow larger. This process, known as "neocrystallization," involves the breakdown of the original minerals and the formation of new, more stable minerals.

As the rock is subjected to differential stress from the changing conditions, minerals within the rock start to re-orient themselves parallel to the direction of the least principal stress. This preferred alignment is a direct result of the internal deformation taking place in the rock due to the applied stress. In response, minerals with sheet or plate-like crystal structures, such as micas and chlorite, preferentially align themselves to form planar surfaces called foliation planes.

The continued increase in temperature and pressure leads to the further growth and alignment of plate-like minerals such as micas, which progressively dominate the rock's mineralogy. This process results in the development of a parallel arrangement of the minerals called "schistosity." Schistosity is a type of foliation specific to schist.

As the parent rock undergoes further metamorphism and foliation development, it transforms into a foliated metamorphic rock called schist. Schist is characterized by its strong foliation, the presence of platy or elongated minerals, and the ability to be easily split along the foliation planes. In summary, the main process of foliation development in schist involves the following steps: 1. The parent rock is subjected to increasing temperature and pressure. 2. Minerals within the rock recrystallize and grow larger (neocrystallization). 3. Plate-like minerals preferentially align themselves parallel to the least principal stress direction. 4. Schistosity, a type of foliation, develops due to the continuous growth and alignment of minerals. 5. The parent rock is transformed into the foliated metamorphic rock schist.