Summary: Analyses of glauconites 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 having an average half unit cell of K_0·85(Fe³⁺, Al³⁺)_1·34 (Mg²⁺, Fe²⁺)_0·66(Si_3·76Al_0·24)O₁₀(OH)₂ whereas celadonite approaches the ideal half unit cell of K(Fe³⁺, Al³⁺)(Mg²⁺, Fe²⁺)Si₄O₁₀(OH)₂. Considerable Fe³⁺Al^vi interchangeability occurs in the octahedral layer in both minerals and considerable substitution of aluminium in the tetrahedral layer of glauconites results in the more disordered 1Md type of structure compared with the more highly ordered 1M structure of celadonites. Some mixed layer glauconite-smectites and celadonites were also examined and could be distinguished from true glauconites and celadonites by chemical analysis, XRD, and IR techniques. It is proposed that the terms ‘glauconite’ and ‘celadonite’ should be used only for those minerals containing less than 5% interlayering.