Gneiss is considered a foliated metamorphic rock. This means it has a layered or banded appearance, which is a result of the high-grade metamorphic processes it undergoes. During metamorphism, the original minerals in the rock become re-crystallized under heat and pressure, leading to the segregation of different mineral types. This process gives gneiss its characteristic bands of light and dark minerals.
The foliation in gneiss is typically well-defined, with distinct layers that can easily be seen. These layers are formed from the alignment of mineral grains, particularly feldspar and quartz, which are predominant in gneiss. Foliated rocks like gneiss often exhibit a planar structure, allowing them to split or fracture along these layers.
In contrast, nonfoliated rocks, such as marble or quartzite, do not show any layering or banding and tend to have a more uniform appearance. Therefore, gneiss is clearly classified as a foliated rock due to its unique structure and formation process.