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Application of ion beam analysis (RBS and ERD) to the surface chemistry study of leached minerals

1992; Elsevier BV; Volume: 64; Issue: 1-4 Linguagem: Inglês

10.1016/0168-583x(92)95530-5

1872-9584

G. W. Arnold, Henry R. Westrich, William H. Casey,

Geological and Geochemical Analysis

Labradorite feldspar (Ca0.61Na0.38Al1.61Si2.40O8.00) samples were reacted in solutions having pH values of 1–12 at 45°C. 2.8 MeV He RBS was used to profile the elemental constituents. 24 MeV Si ERD was applied to obtain the H-inventories. For pH < 4, the reacted near-surface region was found to be Si-rich and depleted in Al, Ca, and Na. The H uptake, however, was found to be much less than would be expected for exchange by H as H2O or H3O+. This discrepancy has been interpreted as evidence for the repolymerization of the near-surface region with subsequent loss of H2O. For pH values of 5–12, the dissolution is congruent and there is no apparent H uptake. The porous nature of the leached and repolymerized region has been established by RBS measurements on samples leached in a 0.01N HCl-H216O solution (pH = 2) and then placed in an 18O solution for varying time intervals (total time in both solutions constant at 404.25 h). Enrichment of 18O and depletion of 16O (over a leached depth of about 1000 A) was observed for times greater than 17.5 h in the 18O solution. The formation of a porous layer at low pH values which allows rapid diffusion to the interface has important implications for scenarios of rock-solution interaction.