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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 (NH 4 ) 2 O(-MgO)-Al 2 O 3 -SiO 2 - H 2 O [N(M)ASH] using multi-anvil and piston-cylinder equipment. Syntheses included NH4-hollandite (NH 4 AlSi 3 O 8 ) at 12.3 GPa, 700 °C; NH 4 -Si-wadeite [(NH 4 ) 2 Si 4 O 9 ] at 10 GPa, 700 °C; NH 4 -cymrite (NH 4 AlSi 3 O 8 ⋅H 2 O) at 7.8 GPa, 800 °C; and NH 4 -phengite [NH 4 (Mg 0.5 Al 1.5 )(Al 0.5 Si 3.5 )O 10 (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 NH 4  end-members are: a = 9.4234(9) Å, c = 2.7244(3) Å, V = 241.93(5) Å 3  (NH4-hollandite); a = 6.726(1) Å, c = 9.502(3) Å, V = 372.3(1) Å 3  (NH 4 -Si-wadeite); a = 5.3595(3) Å, c = 7.835(1) Å, V = 194.93(5) Å 3  (NH 4 -cymrite). NH 4 -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) Å 3 . All unit-cell volumes are enlarged in comparison to the potassium analogues. Substitution of NH 4  for K does not cause changes in space group. NH 4  incorporation was confirmed by the appearance of NH 4 -vibration modes ν 4  and ν 3  occurring in the ranges of 1397-1459 and 3223-3333 cm -1 , respectively.

High-pressure ammonium-bearing silicates: Implications for nitrogen and hydrogen storage in the Earth’s mantle