Substantial amounts of authigenic clay minerals can accumulate in terrestrial mudstones where the following conditions are met: surface or pore waters are alkaline, aqueous silica activity is high, at least some dissolved magnesium (Mg) is present, and detrital input is relatively low. Availability of Al- or Fe-rich detrital clays likely leads to Mg-rich smectite formation, whereas sepiolite or kerolite is favored in environments with no detrital substrates. Surface waters of the Ngorongoro Crater, Tanzania, provide a good example of alteration of incoming detrital clay minerals into authigenic sediments with strong partitioning of Mg into silicate rather than carbonate phases. Quaternary...