Sediment cores were studied from the oldest yet known, newly revealed, deepest area of the Olduvai paleolake in northern Tanzania as part of the Olduvai Gorge Coring Project (OGCP). Illite was the predominant mineral identified in the X-ray Diffraction (XRD) analyses of 112 samples from Core 2A (between fifth and FLK faults) at 156 - 215.3 meters below the surface (mbs). The illite was aluminous based on the (060) d-spacing values of 1.511Ǻ, confirming the dioctahedral phase. This aluminous illite is in contrast to the well-known magnesium-rich illite in younger deposits, and the magnesian illite was deposited in a less saline Olduvai paleolake >2 Ma ago.f That lake water composition contrasted with the hypersaline, hyper alkaline lake waters known from the geochemical and mineralogical analyses of the younger units of Olduvai Beds I and II. The octahedral composition of illite (Mg/(Al+Fe) varied from 0.09 to 0.24. A lower MgO anomaly was observed in the claystone geochemistry restricted to the studied interval.