Computer Science
Scientific paper
May 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.1985s&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 61, Issue 10, p. 1985-1994.
Computer Science
2
Scientific paper
The composition of the near-surface of acid-leached labradorite was investigated by depth-profiling with secondary ion mass spectrometry for H, D, O, Na, Al, Si, Cl, and Ca. Specimens were leached at room temperature at pH 1 for 500 h, and pH 2 and 3 for 1200 h, in solutions acidified with HCl. Complete leaching of Na, Ca, and Al occurred to a depth of 100 nm after 1200 h of reaction at pH 3 and produced sigmoidal depletion/concentration profiles. Depletion depths of these cations are shown to increase further with H+-activity and time. The presented element profiles, therefore, represent non-steady-state conditions. Calcium is removed from between 10 and 30 nm deeper within a single profile than the corresponding Al depletion depth. After removal of Na+ and Ca+, a sequence of reaction steps results in preferential leaching of Al over Si and formation of silanol groups around the tetrahedral vacancy. This is immediately followed by spontaneous condensation of adjacent silanol groups, which eliminates a fraction of oxygen from the leached layer. The O/Si ratio of the residual structure is thus reduced, and a high degree of cross-linkage between Si-tetrahedra is maintained. Throughout the leached layer, the O/Si atomic ratio is reduced from 3.3 (fresh labradorite) to values between 2.5 and 2.0. The leached layer is enriched in H in all specimens, with average concentration plateus of about 7·1021 atoms/cm3, or 11 at%, if the leached layer is assumed to have a density equal to amorphous SiO2. At the time of analysis the leached layer is anhydrous and composed solely of H, Si, and O. The H concentrations are in accord with an O/Si atomic ratio of 2.2 in terms of charge balance. The analytical composition corresponds to SiO1.8(OH)0.4. Good agreement between leaching depths determined by SIMS profiling and as calculated from element release rates during kinetic dissolution experiments suggests that the sites for ion-exchange and depolymerization reactions are uniformly distributed over the entire mineral surface, and that any changes in reactive surface area have been small during these experiments.
Schweda Peter
Sjöberg Lennart
Södervall Ulf
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