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Amorphous materials: Properties, structure, and durability

Solubility and solution mechanisms of NOH volatiles in silicate melts at high pressure and temperature—amine groups and hydrogen fugacity

¹ Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20005, U.S.A.
² Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori 682-0193, Japan
³ Geological Faculty, Moscow State University, Moscow 119899, Russia

Correspondence: ^* E-mail: bmysen{at}ciw.edu

The solubility and solution mechanisms of nitrogen in silicate melts have been examined via nitrogen analyses and vibrational spectroscopy (Raman and FTIR). Pressure (P), temperature (T), hydrogen fugacity (f_H2), and silicate melt composition (degree of melt polymerization) were independent variables in experiments in the 1–2.5 GPa pressure and 1300–1500 °C temperature ranges. The f_H2 was controlled at values defined by the magnetite-hematite (MH), Mn₃O₄-MnO (MM), NiO-Ni (NNO), magnetite-wustite (MW), and iron-wustite (IW) buffers together with H₂O.

The nitrogen solubility ranges from about 1 to about 5 mol%, calculated as N, with X_N/P > 0 and X_N/f_H2 > 0. The /f_H2(X_N/P) is also positive. Raman and FTIR spectroscopic data are consistent with solution mechanisms that involve reduction of nitrogen with increasing f_H2. At low f_H2 [f_H2(MH) and f_H2(MM)], nitrogen is dissolved in melts only as molecular N₂. At f_H2(NNO) and f_H2(MW), there is partial reduction of nitrogen to form N₂, NH₂⁺ complexes and molecular NH₃ in the melts, whereas at the highest f_H2(IW), only molecular NH₃ and NH₂^– groups can be identified. OH groups are also formed whenever there is reduction of nitrogen from N₂. Solution in silicate melts of reduced, NH-bearing species results in silicate melt depolymerization. At f_H2(NNO) and f_H2(MW), depolymerization occurs via H⁺ interaction with oxygen and NH₂⁺ groups serving as network-modifier. Under more reducing conditions, oxygen is replaced by NH₂^– groups. Solution of reduced nitrogen in silicate melts causes depolymerization of their structure. This implies that melt properties that depend on silicate structure depend on redox conditions.

Key Words: NOH volatiles • melt • structure • spectroscopy