On the condition of thermal contact metamorphism, clay minerals in coal are changed into andalusite. 无烟煤中红柱石是煤中粘土质矿物经热接触变质作用形成的。
Medium-rank bituminites are formed by pneumato-hydrothermal metamorphism on the basis of burial metamorphism, and it is partial. 深大断裂带及其附近局部发育的中级烟煤是在深成变质基础上,叠加了高温气液热变质作用而形成。