In situ micro-FTIR spectroscopic investigations of synthetic ammonium phengite under pressure and temperature

Abstract

Abstract. Phengite is known to be an important mineral in the transport of alkalis and water to upper mantle depths. Since ammonium (NH4+) can substitute for K+ in K-bearing minerals, phengite is thus a potential host to transport nitrogen into the mantle. However, the temperature and pressure conditions at which devolatilisation of NH4-bearing phengite occurs are not well constrained. In this study, NH4-phengite (NH4)(Mg0.5Al1.5)(Al0.5Si3.5)O10(OH)2 was synthesised in piston-cylinder experiments at 700 ∘C and 4.0 GPa. Its devolatilisation behaviour was studied by means of in situ micro-FTIR (Fourier transform infrared) spectroscopy under low and high temperatures from −180 up to 600 ∘C at ambient pressure using a Linkam cooling–heating stage and pressures up to 42 GPa at ambient temperature in diamond anvil cell (DAC) experiments. In addition to these short-term in situ experiments, we performed quenched experiments where the samples were annealed for 24 h at certain temperatures and analysed at room conditions by micro-FTIR spectroscopy. Our results can be summarised as follows: (1) an order–disorder process of the NH4+ molecule takes place with temperature variation at ambient pressure; (2) NH4+ is still retained in the phengite structure up to 600 ∘C, and the expansion of the NH4+ molecule with heating is reversible for short-term experiments; (3) kinetic effects partly control the destabilisation of NH4+ in phengite; (4) ammonium loss occurs at temperatures near dehydration; (5) NH4+ in phengite is apparently distorted above 8.6 GPa at ambient temperature; and (6) the local symmetry of the NH4+ molecule is lowered/descended/reduced by increasing pressure (P) or decreasing temperature (T), and the type and mechanism of this lowered symmetry is different in both cases. The current study confirms the wide stability range of phengite and its volatiles and thus has important implications for the recycling of nitrogen and hydrogen into the deep Earth. Moreover, it is considered as a first step in the crystallographic determination of the orientation of the NH4+ molecule in the phengite structure.

Nitrogen Retention in Feldspar: Implications for Nitrogen Transport in Subduction Zones

“…The turning point at 500°C for ammonium v 3 band in feldspar is consistent with that in phengite, suggesting hydrogen bonding (N–H…O) between ammonium and the lattice formed at temperatures above 500°C. On the other hand, the higher‐wavenumber shifting of ammonium band with increasing pressure suggests pressure‐induced strengthening of N–H bonding, which is in agreement with previous observations for buddingtonite and phengite at high pressures (Abdel‐Hak et al., 2020; Vennari et al., 2017). The ammonium v 3 band in buddingtonite linearly shifts to higher wavenumbers with increasing pressure to 29.8 GPa (Vennari et al., 2017).…”

“…The above results show that no matter at elevated temperatures or at high pressures, ammonium bands broaden, which is consistent with those in buddingtonite and phengite at high temperatures or under compression (Vennari et al, 2017;Y. Yang et al, 2017;Abdel-Hak et al, 2020). It is generally accepted that ordering of ammonium ions within crystals will lead to band sharpening, which has been seen in the ammonium spectra of NH 4 Br under compression (Reed & Williams, 2006).…”

“…The discontinuity in the wavenumber evolution with pressure was interpreted by the authors to variations of ammonium symmetry. Therefore, both the two studies suggested no interactions of ammonium and the framework for buddingtonite and phengite at high pressures (Abdel‐Hak et al., 2020; Vennari et al., 2017), which is opposite to the behavior of ammonium at high temperatures. In contrast to the pressure‐induced constant higher‐wavenumber shifting of ammonium v 3 band in buddingtonite and phengite, the ammonium band of the feldspar in this study shifts to higher wavenumbers with increasing pressure to 15 GPa, and the evolution goes on a reverse trend above 15 GPa.…”

“…They concluded no hydrogen bonding involved and that ammonium acted as a guest ion during the compression. For phengite, the ammonium v 3 band also shifts to higher wavenumbers, but with different gradients at pressures lower and higher than 8.6 GPa (Abdel‐Hak et al., 2020). The discontinuity in the wavenumber evolution with pressure was interpreted by the authors to variations of ammonium symmetry.…”