Experimentally determined K-NH4 partitioning between feldspars, muscovites and aqueous chloride solutions

Experiments using the „exchange synthesis“ technique were performed at 400-600 °C / 400 MPa and at 500-600 °C / 1500 MPa, respectively and yielded a variety of solid solutions (ss) along both, the (K-NH4)feldspar and the (K-NH4)-muscovite joins. Different analytical methods including Rietveld refinement of XRD spectra, EMP analysis and IR spectroscopy were used on the synthesised solid solutions for comparison. First, emphasise was laid on a thorough characterisation of the NH4-endmembers buddingtonite NH4[AlSi3O8] and tobelite NH4Al2[AlSi3O10](OH)2. Buddingtonite as well as the synthesised (K-NH4)feldspar ss are isostructural to sanidine (space group C2/m). The size of the corresponding unit-cells depends on the NH4 concentration in feldspars. With increasing NH4 content the unit-cells expand especially with regard to lattice parameter a. EMP analyses of (K-NH4)-feldspars demonstrate that a linear correlation between a and NH4 content exists. This dependence was used to derive X K fsp = [K/(K+NH4)] values for solid solutions along the buddingtonite – K-feldspar join. The detection limit for NH4 by EMP analyses using a standard equipped Cameca SX50 was estimated to 0.8 wt% NH4. In tobelite and synthesised (K-NH4)-muscovites mica polytype 1M (space group C2/m) is dominating. Lattice constant c of the corresponding unit-cell increases linearly with increasing NH4 content and was used to determine X K musc values for the muscovite ss. Additionally, ratios of the N-H stretching band at 1430 cm and the OH-band at 3635 cm obtained from IR spectra allowed for calculation of X K musc . Both methods show an excellent agreement within the standard deviation 2σ( X K musc ) = 0.06. The analytical data demonstrates that a continuous substitution of NH4 for K exists for the (K-NH4)feldspar and the (K-NH4)-muscovite series, respectively. However, at 600 °C / 400 MPa and high NH4 content in the starting mixture, no feldspar ss with low X K fsp were obtained due to preferred formation of muscovite ss and quartz from feldspar stoichiometry. This indicates that NH4-rich muscovite is more stable under elevated temperatures than NH4-rich feldspar. The (K-NH4) concentration in feldspar and muscovite solid solutions along with the (K-NH4) content of the coexisting fluids determined by ion chromatography yielded the (K-NH4) distribution behaviour. In all experimental runs ammonium preferentially fractionates into the fluid phase. No temperature dependence exists, whereas with increasing pressure from 400 to 1500 MPa the (K-NH4) partitioning between solids and fluids slightly decreases. For low NH4 concentrations, relevant for rock-forming processes, constant partition coefficients D NH solid fluid 4 − = X NH solid 4 / X NH fluid 4 are derived. They are 0.22 ± 0.08 and 0.25 ± 0.07, respectively for the feldspar-fluid series at 400-600 °C / 400 MPa and 500-600 °C / 1500 MPa. For the muscovite-fluid series D NH solid fluid 4 − values are 0.19 ± 0.10 and 0.26 ± 0.08, respectively at 400-600 °C / 400 MPa and 500-600 °C / 1500 MPa. These experimental results fit very well with natural observations of ammonium release from garnet phyllites and garnet-phengite schists during metamorphism (Mingram and Bräuer, 2001). Loss of nitrogen as NH4 from metasediment sequences during prograde metamorphism is possible and may be related to the fluid-solid fractionation behaviour observed within this study. The obtained data also indicates that synthesised (K-NH4)-feldspars incorporate more NH4 in comparison to (K-NH4)-muscovites. This is especially seen in the range of X K fsp between 0.1 and 0.9 and could explain, why natural buddingtonites found so far are richer in NH4 than natural tobelites.