Well-crystallized, relatively pure 1 M illite occurs in clay fractions of Cambro-Ordovician sandstones from the Sinclair Mediterranean Petroleum Company Rhourde el Baguel No. 1 well, Algeria. This is the first report of such a clay (well-crystallized 1M, relatively pure) from a sedimentary rock. Preliminary data show that the chemical composition of this clay approaches that of "ideal" muscovite. Petrographic evidence and the lack of a terrigenous source for 1M illite strongly indicate a diagenetic origin. It is suggested that this clay is the result of post-depositional, in-place alteration of feldspars in the presence of saline formation waters.