Stevensite in the Tertiary lacustrine basin of the Jbel Rhassoul in Morocco is used as fuller's earth. Observations of the structure and textures of the deposits provide evidence of diagenetic replacement of sedimentary lacustrine dolomite by stevensite. Carbon and oxygen isotopic compositions confirm the precipitation of primary sedimentary dolomite and the diagenetic formation of secondary stevensite. According to thermodynamic calculations, stevensite is stable at the expense of dolomite in a silica-rich and CO 2 -poor environment. Under such conditions, destabilized detrital illite, chlorite, and palygorskite release Al, Fe, Li, and trace elements, which are incorporated into the structure of stevensite. Compared with other lacustrine stevensite occurrences, the Jbel Rhassoul deposit seems to correspond to a new mode for the formation of diagenetic stevensite after dolomite.