Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Watenphul, A. Articles by Heinrich, W. Search for Related Content GeoRef GeoRef Citation

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

Deutsches GeoForschungsZentrum (GFZ), Section 4.1, Telegrafenberg, 14473 Potsdam, Germany

Correspondence: ^* E-mail: watenphul{at}gfz-potsdam.de

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(Mg_0.5Al_1.5)(Al_0.5Si_3.5)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) ų (NH₄-hollandite); a = 6.726(1) Å, c = 9.502(3) Å, V = 372.3(1) ų (NH₄-Si-wadeite); a = 5.3595(3) Å, c = 7.835(1) Å, V = 194.93(5) ų (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) ų. 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 ₄ and 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 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.

Key Words: Hollandite • wadeite • cymrite • phengite • ammonium • high-pressure synthesis • nitrogen cycle

This article has been cited by other articles:

				K.-F. Li, K. Pahlevan, J. L. Kirschvink, and Y. L. Yung Atmospheric pressure as a natural climate regulator for a terrestrial planet with a biosphere PNAS, June 16, 2009; 106(24): 9576 - 9579. [Abstract] [Full Text] [PDF]