Here we use coupled mineralogy and geochemistry to reveal the complexities of clay diagenesis in the saline, alkaline paleo-waters of Pliocene Paleolake Olduvai, in northern Tanzania. Multiple authigenic phases are distinguished and quantified by XRD and deconvolution of 060 peaks coupled with geochemical (EMP, HRTEM-AEM) and crystallographic (IR) investigation. By assigning geochemistry according to the XRD deconvolution, we identify three principal phases: dioctahedral Al-rich, dioctahedral relatively Fe-rich, and Mg-rich with a trioctahedral component. These have average octahedral structures of Al1.30 FeO0.57 Mg0.13 (Al-rich), Al0.56 Fe0.80 Mg0.62 (Fe-rich), and Al0.09 Fe0.23 Mg2.09 (Mg-rich) per half formula unit. IR data confirm the geochemical assessments of individual phases. The Mg-rich phase either has both dioctahedral and trioctahedral domains within sheets, or a homogeneous octahedral sheet with an intermediate composition. This is, to our knowledge, the first report of such an intermediate octahedral structure in a 2:1 phyllosilicate.
All three phases occur in different areas of the paleolake; their proportions and octahedral compositions are variable. Total Mg content of bulk <0.1µm clays reflects the relative abundance and Mg content of the Mg-rich phase. This helps to identify periods of elevated salinity and alkalinity in the paleolake. In addition to shedding light on the crystal chemistry of authigenic 2:1 clay minerals, this demonstrates the paleolimnologic importance of discriminating multiple authigenic clay phases in lacustrine deposits.