Abstract

The ammonium analogues of the high-pressure potassium-bearing silicate phases K-hollandite, K-Si-wadeite, K-cymrite, and phengite were synthesized in the system (NH4)2O(-MgO)-Al2O3-SiO2-H2O [N(M)ASH] using multi-anvil and piston-cylinder equipment. Syntheses included NH4-hollandite (NH4AlSi3O8) at 12.3 GPa, 700 °C; NH4-Si-wadeite [(NH4)2Si4O9] at 10 GPa, 700 °C; NH4-cymrite (NH4AlSi3O8·H2O) at 7.8 GPa, 800 °C; and NH4-phengite [NH4(Mg0.5Al1.5)(Al0.5Si3.5)O10(OH)2] at 4 GPa, 700 °C. Run products were characterized by SEM, FTIR, and powder XRD with Rietveld refinements. Cell parameters of the new NH4 end-members are: a = 9.4234(9) Å, c = 2.7244(3) Å, V = 241.93(5) Å3 (NH4-hollandite); a = 6.726(1) Å, c = 9.502(3) Å, V = 372.3(1) Å3 (NH4-Si-wadeite); a = 5.3595(3) Å, c = 7.835(1) Å, V = 194.93(5) Å3 (NH4-cymrite). NH4-phengite consisted of a mixture of 1M, 2M1, 2M2, 3T, and 2Or polytypes. The most abundant polytype, 2M1, has cell dimensions a = 5.2195(9) Å, b = 9.049(3) Å, c = 20.414(8) Å, β = 95.65(3)°, V = 959.5(5) Å3. All unit-cell volumes are enlarged in comparison to the potassium analogues. Substitution of NH4 for K does not cause changes in space group. NH4 incorporation was confirmed by the appearance of NH4-vibration modes ν4 and ν3 occurring in the ranges of 1397–1459 and 3223–3333 cm−1, respectively.

Ammonium in eclogite facies metasediments is mainly bound in micas and concentrations may reach up to a few thousand parts per million. It can be stored to greater depths in high-pressure potassium silicates during ongoing subduction. This possibly provides an important mechanism for nitrogen and hydrogen transport into the deeper mantle.

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