Which one of the following statements about metamorphic rocks is not correct?

🔑Key Points:
-Metamorphic rocks are those rocks that have undergone changes in mineralogy, texture, and/or chemical composition as a result of changes in temperature and pressure.
-The original rock may have been an igneous, sedimentary, or metamorphic rock.

✏️The pressure and heat that drive metamorphism are consequences of three forces:
-the internal heat of the Earth.
-weight of the overlying rock.
-horizontal or tectonic forces that cause the rocks to deform
-Deformation of some rocks such as shale and clay-rich sandstone (greywacke) produces textural changes involving the formation of flat or wavy parallel planes within the metamorphosed rock. These flat or wavy planes are referred to as foliation.
-Foliation forms when pressure squeezes the flat or elongate minerals within a rock so they become aligned.
-These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied. 
-Where the foliations develop into broad mineral bands, the metamorphic rock is extremely hard. 
-Rocks that originally were composed of one dominant mineral are often foliated by metamorphism. 
-Minerals in the original rock that survive metamorphism may rotate during deformation to acquire a preferred orientation parallel with the platy minerals. 
-The mineral crystals don’t have to be large to produce foliation. Slate, for example, is characterized by aligned flakes of mica that are too small to see. 

🛑Important Points:
-Foliation is mainly produced in rocks that contain platy minerals like mica and chlorite.
-These platy minerals generally form when shale and clay-bearing sandstone (greywacke) are metamorphosed.
-As these platy minerals grow, their planes take on a preferred orientation that is usually perpendicular to the main direction of forces squeezing the rock.
-Minerals in the original rock that survive metamorphism may rotate during deformation to acquire a preferred orientation parallel with the platy minerals.
-Three types of foliation are slate, schist, and gneiss.
-Slate is classified as low-grade, thin-layered, and very small mineral grains.
-Schist is classified as medium-grade, thin-layered, and large mineral grains.
-Gneiss is classified as high-grade, thick-layered, and very large mineral grains.