caa a22 4500 465791077 CHVBK 20180323112024.0 cr unu---uuuuu 170327e19900701xx s 000 0 eng 10.1007/BF01575625 doi (NATIONALLICENCE)springer-10.1007/BF01575625 A re-examination of the role of hydrogen in Al−Si interdiffusion in feldspars [Elektronische Daten] [Colin Graham, Stephen Elphick] Recent experimental studies have shown that the rates of Al−Si order-disorder and interdiffusion in alkali feldspars at high pressures under dry conditions increase dramatically in the approximate pressure range 7-14 kb, depending on temperature and feldspar composition (Goldsmith 1987, 1988). Enhancement of Al−Si interdiffusion rates is ascribed to the involvement of hydrogen, but the species of hydrogen involved is undetermined. A simple kinetic analysis of the data of Goldsmith (1987) on disordering of dry albite at 800°-950° C and 6-24 kb in the solid media press is consistent with the NaCl pressure cell acting as a proton donor by enhancing dissociation of water in the pressure medium, generating a high $$a_{H^ + } $$ in the experimental environment. The rate constant for disordering of albite is found to increase linearly with the estimated experimental $$a_{H^ + } $$ and with the density of aqueous salt solution, implicating H+ as the rate-enhancing species. Further experimental studies confirm the importance of $$a_{H^ + } $$ . At 16 kb and 850° C, dry albite in sealed Pt capsules in a NaCl cell containing tantalum powder (which reduces H2O to H2) remains highly ordered over the same time that complete disordering would occur in the absence of Ta. H2 cannot therefore be the rate-enhancing species. At 1 kb and 850° C, the extent of Al−Si disorder in albite in direct contact with various NaCl−H2O solutions increases from partially disordered for pure H2O to completely disordered for saturated aqueous NaCl solution, giving strong support to the proton model. SIMS scanning ion imaging of albite run products demonstrates conclusively that solution-reprecipitation is not responsible for enhanced disordering rates. Results of disordering experiments in the solid media apparatus cannot be duplicated in Ar gas media internally-heated pressure vessels, even with the same experimental configuration around the albite-bearing capsules, due to the different proton-buffering capacities of the solid and gas media apparatus. Springer-Verlag, 1990 Graham Colin Department of Geology and Geophysics, University of Edinburgh, West Mains Road, EH9 3JW, Edinburgh, Scotland aut Elphick Stephen Department of Geology and Geophysics, University of Edinburgh, West Mains Road, EH9 3JW, Edinburgh, Scotland aut Contributions to Mineralogy and Petrology Springer-Verlag 104/4(1990-07-01), 481-491 0010-7999 104:4<481 1990 104 410 https://doi.org/10.1007/BF01575625 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/BF01575625 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Graham Colin Department of Geology and Geophysics, University of Edinburgh, West Mains Road, EH9 3JW, Edinburgh, Scotland aut NATIONALLICENCE 700 1- Elphick Stephen Department of Geology and Geophysics, University of Edinburgh, West Mains Road, EH9 3JW, Edinburgh, Scotland aut NATIONALLICENCE 773 0- Contributions to Mineralogy and Petrology Springer-Verlag 104/4(1990-07-01), 481-491 0010-7999 104:4<481 1990 104 410 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer