What is the main difference between schist and gneiss?

The main difference between schist and gneiss lies in the degree of foliation, which refers to the alignment and layering of minerals within the rock. Schist typically exhibits well-defined foliation, characterized by a shiny appearance due to the presence of significant amounts of mica, giving it a pronounced layered structure. The minerals tend to align parallel to one another, resulting in a texture that reflects the intense pressure and temperature conditions it experienced during metamorphism.

On the other hand, gneiss demonstrates a more pronounced banding structure, where there are distinct alternating layers or bands of different mineral compositions. This banding is generally coarser and less regular compared to the foliation found in schist. The greater degree of metamorphic processes has caused the minerals in gneiss to segregate into bands, making it more apparent than the foliation seen in schist.

While mineral composition, color and shade, and type of formation can vary between the two, the defining feature that helps distinguish schist from gneiss is indeed the degree of foliation, reflecting their respective metamorphic histories.