X represents large interlayer cations, such as K+, Na+, Ca2+, Ba2+, Rb+, Cs+, etc. Y represents octahedral cations, such as Al3+, Fe2+, Mg2+, Cr3+, Ti4+, Fe3+, etc. Z is mainly Si4+ and Al3+; The ideal ratio of the additional anion (OH)- to (O)2- is 2:10, and (OH)- can be replaced by F- and Cl-. Most mica contain 4% to 5% water. Therefore, although they may be called mica, the chemical composition of different mica may be very different, for the purpose of research, we collectively refer to the minerals with this type of characteristics as mica group minerals.