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

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 NH(4)-hollandite (NH(4)AlSi(3)O(8)) at 12.3 GPa, 700 degrees C; NH(4)-Si-wadeite [(NH(4))(2)Si(4)O(9)] at 10 GPa, 700 degrees C; NH(4)-cymrite (NH(4)AlSi(3)O(8)center dot H(2)O) at 7.8 GPa, 800 degrees C; and NH(4)-pheilgite [NH(4)(Mg(0.5)A(1.5))(Al(0.5)Si(3.5))O(10)(OH)(2)] at 4 GPa, 700 degrees 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) angstrom, c = 2.7244(3) angstrom, V 24 1.93(5) angstrom, (NH(4)-hollandite); a = 6.726(1) angstrom, c = 9.502(3) A, V = 372.3(1) angstrom(3) (NH(4)-Si-wadeite); a = 5.3595(3) angstrom, c = 7.835(1) angstrom, V = 194.93(5) angstrom(3)(NH(4)-cymrite). NH(4)-phengite consisted of a mixture of 1M, 2M(1), 2M(2), 3T, and 2Or polytypes. The most abundant polytype, 2M(1), has cell dimensions a = 5.2195(g) angstrom, b = 9.049(3) angstrom, c = 20.414(8) angstrom, beta = 95.65(3)degrees, V = 959.5(5) angstrom(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 nu(4) and nu(3) 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 pails 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.