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Glauconite and Fe-smectite, which can be distinguished by their peculiar morphology and stacking sequences, coexist in the Galapagos Spreading Centre hydrothermal mounds. Analytical electron microscopy (AEM) data suggest that Fe is entirely in octahedral sites in Fe-smectite whereas glauconite is K-rich with Fe in tetrahedral and octahedral sites, However, the MOssbauer spectra, recorded at various temperatures for samples containing both smectite and glauconite, were satisfactorily analysed with three overlapping doublets corresponding to Fe in octahedral sites. The contradictory results obtained with the two methods are explained by the presence of small particles of iron oxide intimately associated with glauconite. These particles were detected in M6ssbauer spectra obtained at 77 K and 4-2 K and were observed by transmission electron microscopy. Iron oxide is a secondary phase formed by alteration of smectite. These data are in good agreement with the hypothesis that the smectite-glauconite reaction, which occurs at 30 m and low temperature in the Galapagos hydrothermal mounds, is a dissolution-precipitation process, dissolution of Fe-rich smectite being followed by precipitation of glauconite and iron oxides.Green clays of hydrothermal origin have been reported in many oceans Thompson et al., 1985;Singer et al., 1984). These deposits are generally found near mid-ocean ridges. They consist principally of Fe-rich clays and their formation is explained by advection of, and precipitation from, low-temperature hydrothermal fluids that circulate through the oceanic crust. In the Galapagos Spreading Centre (GSC), green clay deposits are located at 20 km on the south flank of the ridge and are intercalated with pelagic calcareous sediments (Fig. 1). They form mounds approximatley 30 m high. Their mechanism of formation has been studied using several approaches. The most recent study (Buatier et al., 1989) utilized transmission electron microscopy (TEM) and showed that a transition from smectite to a mica-like mineral occurs at approximately 30 m below the sea water-sediment interface. However, the compositions of these clay minerals and therefore the nature of the chemical reaction which takes place during early diagenesis are still unknown. The aim of the present study is to characterize precisely the minerals of these hydrothermal deposits in order to understand the chemical exchange that occurs at the phase transition. Analytical electron microscopy (AEM) and M6ssbauer spectroscopy were used to characterize the chemical compositions of the clay minerals and thus to determine the valence and sites for Fe in the clay structure. The chemical data were interpreted on the basis of structural data obtained using selected area electron diffraction (SAED) and oblique texture diffraction (OTED) on the same sample.9 1993 The Mineralogical Society

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