Formation of metamorphic biotite by decarbonation

Microcline is not necessarily the source of the K content of metamorphic biotite. The biotite may be formed by decalcitization; the released Ca reacts with Al, derived from sericite and chlorite, to form some calc-alumino-silicate such as plagioclase or epidote. It may also result from the reaction of sericite with the Fe---Mg released by dedolomitization. The biotitic character of meta-calco-pelites from the Belgian Ardenne, increasing with their anorthite content, illustrates this mode of biotite formation.