Micas : structure, chemistry, petrogenesis


Chemistry: The basic mica formula is:

W2 Y(4-6) Z8 O20 (OH)4
where W represents the contents of largest, 12-fold co-ordinated, sites which may be occupied by K, Na and Ca (more rarely Ba), Y (Mg, Fe2+, Fe3+, Al, Ti) is the content of the octahedral sites, and Z (Si,Al) is the content of the tetrahedral sites. The white micas have only four octahedral sites occupied, whereas the "dark micas" (biotites) have six octahedral sites occupied. The generic terms di-octahedral mica for the "white mica" and tri-octahedral mica for "dark mica" relate to the number of occupied octahedral sites on the basis of 10 oxygens.

The composition of Fe-Mg white micas, phengite, leucophyllite, and celadonite can be related to muscovite by the tschermakite and Fe3+Al-1 substitutions.
muscovite K2Al6Si6O20(0H)4   
paragonite NaK-1 Na2Al6Si6O20(0H)4
margarite CaAlK-1Si-1 Ca2Al8Si4O20(0H)4
phengite MgSiAl-2 K2MgAl4Si7O20(0H)4
leucophyllite MgSiAl-2 K2Mg2Al2Si8O20(0H)4
celadonite MgSiAl-2   
  Fe3+Al-1 K2Mg2Fe3+AlSi8O20(0H)4