Physics
Scientific paper
Feb 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52..365s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 52, Issue 2, pp.365-374
Physics
2
Scientific paper
Fine-grained sepiolite (Mg 2 Si 3 O 7.5 (OH) · 3H 2 O) and kerolite (Mg 3 Si 4 O 10 (OH) 2 · H 2 O) samples from lacustrineplaya sediments at Amboseli, Kenya, were used in dissolution experiments with five different solutions at 25°C for time periods approaching 10 years. The hydrolysis reactions sepiolite + 4 H + = 2 Mg 2+ + 3 SiO 2 ( aq ) + 5.5 H 2 O kerolite + 6 H + = 3 Mg 2+ + 4 SiO 2 ( aq ) + 5 H 2 O have estimated log K values of 15.76 ± 0.18 and 25.79 ± 0.24, respectively (error estimate 1 s.d.). The estimate for kerolite hydrolysis is a minimum value due to the possibility of kerolite beginning to transform to a more stable phase during the experiments. The corresponding values of the and are -4,624,340 and -5,736,700 J mol -1 , respectively, relative to the data sources in the text. These values replace preliminary results reported earlier after approximately one year of dissolution time in the experiments. The sepiolite log K is in good agreement with the value of 15.86 predicted by et al. ( Amer. J. Sci. , 1973) from measurements at higher temperatures. Under earth-surface conditions, kerolite is metastable with respect to sepiolite in solutions at equilibrium or super-saturated with respect to quartz. However, the solubility difference is minor, suggesting that reaction kinetics exert a major control in determining which mineral forms in the sedimentary environment.
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