Shallow metamorphic rocks are formed by slight metamorphism of clayey, silty sedimentary rocks, intermediate-acid tuff rocks, and sedimentary tuffs. Black or gray-black. The lithology is dense, with plate-like cleavage developed.
There are often a small amount of sericite and other minerals on the board surface, which makes the board surface slightly shiny.
There is no obvious recrystallization. Under the microscope, some non-uniformly distributed quartz, sericite, chlorite and other mineral crystal grains can be seen, but most of them are cryptocrystalline clay minerals and carbonaceous and iron powders. With variable redundant structure and spot-like structure.
