Ammonium muscovite, NH4Al2AlSi3O10(OH)2, and ammonium phlogopite, NH4Mg3AlSi3O10(OH)2, have been synthesized hydrothermally at gas pressures of 2 kilobars and temperatures between 550° and 730°C. Both micas are stable only in environments of high ammonia fugacity. Ammonia or nitrogen, or both, are released by thermal decomposition, cation exchange, or oxidation. The ammonia : nitrogen ratio in the gas depends primarily on the hydrogen fugacity and the temperature of the environment. Calculations show that, even in a predifferentiated Earth, nitrogen may have predominated. The total amount of nitrogen present on the surface of Earth could be accounted for by the decomposition of a layer of ammonium muscovite 170 meters thick.