The geochemical origin of sepiolite and kerolite at Amboseli, Kenya

A massive white sepiolite deposit at Amboseli precipitated from magnesium and silica released during the ground water dolomitization of an earlier lacustrine sepiolite. Kerolite has since formed in proximity to the massive sepiolite as an alteration product of sepiolite and as a ground water precipitate when the pH is below 8. Authigenic sepiolite also occurs in the overlying younger sediments. Kerolite is likely to occur but has not yet been positively identified. Presently, ground waters within the Amboseli Basin are supersaturated with respect to sepiolite and kerolite. This supersaturation results from the weathering of alkaline olivine basalts on the edge of the basin. The precipitation of sepiolite and/or kerolite does not control ground water compositions in the basin. These reactions are slow compared to other aqueous-mineral reactions such as those maintaining carbonate mineral equilibria. Equilibrium between disordered-dolomite and calcite buffers the log a Mg2+/(a H+)2 as a function of log f CO 2 in ground waters in the proximity of the massive sepiolite. This reaction can help explain the presence of sepiolite associated with dolomites in other near-surface deposits besides Amboseli.