Alkali carbonatite lavas and ashes that erupted from Oldoinyo Lengai, Tanzania, in June 1993 provide evidence for coexisting carbonatite and silicate magmas. The lavas and ashes contain immiscible silicate spheroids of ijolitic composition, which themselves contain carbonatite segregations. In contrast to earlier, very mobile flows, the 1993 lava flows were very viscous; the viscosity of one crystal-rich flow is within the range for rhyolites. They are also the largest carbonatite flows yet recorded from the volcano.