In situ micro-FTIR spectroscopic investigations of synthetic ammonium phengite under pressure and temperature
2020
Abstract. Phengite is known to be an important mineral in the
transport of alkalis and water to upper mantle depths. Since ammonium
( NH 4 + ) 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 NH 4 + molecule takes place with
temperature variation at ambient pressure; (2) NH 4 + is still retained in the phengite
structure up to 600 ∘ C, and the expansion of the
NH 4 + molecule with heating
is reversible for short-term experiments; (3) kinetic effects partly control
the destabilisation of NH 4 + in
phengite; (4) ammonium loss occurs at temperatures near dehydration; (5) NH 4 + in phengite is apparently
distorted above 8.6 GPa at ambient temperature; and (6) the local symmetry of the NH 4 + 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
NH 4 + molecule in the phengite
structure.
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