Ammonium Silicate Stability Relations

A method has been developed to control ammonium fugacity, \(f_{{\text{NH}}_{3}}\), at elevated temperatures and pressures. The method uses an internal nitrogen buffer, the assemblage Cr + CrN, in conjunction with a traditional external hydrogen buffer. In this manner, all gas fugacities in the system N-O-H can be calculated.The Cr + CrN buffer has been applied to study equilibria between buddingtonite (ammonium feldspar), ammonium muscovite, sillimanite, and quartz at a constant gas pressure of 2,000 bars. Two of the five relevant reactions were measured experimentally; from these data, it is possible to calculate isothermal sections at 500, 600, and 700° C.Below 600° C, ammonium muscovite is stable even at extremely low levels of \(f_{{\text{NH}}_{3}}\), while buddingtonite requires \(f_{{\text{NH}}_{3}}\geqq10^4\) bars. Release of NH₃ during progressive metamorphism can be achieved by three processes: thermal decomposition, dehydration, and cation exchange. Within the crust, \(f_{{\text{NH}}_{3}}\) predominates over \(f_{{\text{N}}_{2}}\) by several orders of magnitude; but on the surface, nitrogen released as NH₃ by metamorphism will be oxidized to N₂. Biological materials provide important intermediate storage for nitrogen compounds during the nitrogen cycle.