Physics
Scientific paper
Nov 1968
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1968gecoa..32.1177a&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 32, Issue 11, pp.1177-1182
Physics
2
Scientific paper
A method is presented for measuring the surface area of calcite grains by isotopic exchange with C 14 -labeled CO 2 . Tracer uptake is monitored by back-exchange of the surface activity into C 14 -free CO 2 . C 14 activity is measured by direct liquid scintillation counting of 10 m-mole quantities of CO 2 dissolved in toluene under pressure. At 200°-300°C reversible exchange of annealed calcite samples yields surface areas in good agreement with values determined by grain-size measurements. Exchange surface areas of unannealed grains are threefold higher than these "geometric" values. Although photomicrographs reveal that this excess "area" can be attributed in part to a rugged surface topography, internal surfaces, generated during the crushing of the sample, are also required. A measurable fraction of total exchange activity can only be recovered by dissolving the sample. This residual activity is probably the result of diffusion below the surface layer. The exchange reaction obeys a logarithmic rate law at 75° and 125°C. This suggests that the exchange mechanism involves the chemisorption of CO 2 on a calcite surface of heterogeneous reactivity.
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