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S U M M A RY. Analyses o fglauconites and celadonites from continental sedimentary rocks and sea-floor basalts using X-ray diffraction, electron probe microanalysis, infra-red spectroscopy, and X-ray fluorescence spectroscopy are reported. The minerals are shown to be distinct species; each an isomorphous replacement series, glauconite hav-ing an average half unit cell of Ko.85(Fe 3+, A13+)x.34 (Mg 2+, Fe2+)o.66(Si3.76Alo.z4)Olo(OH)2 whereas cela-donite approaches the ideal half unit cell of K(Fe 3+, A13+)(Mg 2+, Fe2+)Si4Oxo(OH)2. Considerable Fe 3+-AI vl interchangeability occurs in the octahedral layer in both minerals and considerable substitution of alum-inium in the tetrahedral l yer of glauconites results in the more disordered IMd type of structure compared with the more highly ordered IM structure of celadonites. Som