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The aim of this paper was to define the nature and the relative chronology of the diagenetic clay bearing assemblages within sandstones of the Middle Proterozoic Kombolgie formation (Northern Territory, Australia). The detrital minerals of these rocks comprise quartz, accessory zircon, tourmaline, rutile and rare phengitic white micas. Diagenetic features consist of pore-sealing secondary quartz overgrowths, strong compaction shown by interlocked structures and stylolith joints, local hematization and the occurrence of two distinct clay parageneses. Blocky crystals of dickite constitute the earlier diagenetic clays. Their FTIR spectra and their DTA curves, with a sharp dehydroxylation endothermic peak near 680°C, are characteristic of the well-ordered dickite already encountered in many deeply-buried sandstones. Quartz overgrowth may be contemporaneous with the crystallization of dickite. Illite occurred during a subsequent stage as grain coatings and as pseudomorphs of dickite in the residual pores of the sandstones. Illite seems to be contemporaneous with the major deformation features associated with compaction phenomena at the maximal burial conditions experienced by the sandstone formation. These illites are essentially of 2M1polytype. They display pseudohexagonal platy crystals with average diameters ranging from 2 to 10 µm. Their chemical composition is Al-rich (Ca0.01Na0.02K1.72) (${(^{{\rm{VI}}}}{\rm{A}}{{\rm{l}}_{3.82}}{\rm{Fe}}_{0.13}^{3 + }{\rm{M}}{{\rm{g}}_{0.05}}{\rm{T}}{{\rm{i}}_{0.00}}{\rm{M}}{{\rm{n}}_{0.01}})$ (VIAl3.82Fe0.133+Mg0.05Ti0.00Mn0.01) )(Si6.27IVAl1.73)O20(OH)4. These Proterozoic rocks provide a natural reference for the illite end-member occurring as a replacement of kaolin subgroup minerals during burial diagenesis of sandstones The textural properties of the Kombolgie sandstones (absence of fracture network, low porosity, well-developed macroscopic stylolith joints…) and the crystal structure of both the diagenetic dickite and illite would tend to indicate that the Kombolgie sandstones were buried at a depth exceeding 5 km

An X-ray powder diffraction study of a vermiculitized chlorite in an amphibole schist near Limoges, France, shows the following weathering sequence: chlorite → ordered interstratified chlorite/vermiculite → vermiculite. Mössbauer spectroscopy indicates that vermiculitization proceeded by the release of ferrous iron from the 2:1 mica layer of the chlorite. The ferric iron content of the vermiculite product is almost the same as that of the initial chlorite. Infrared spectroscopy and chemical microprobe analyses show that Mg was preferentially extracted from the hydroxide sheet of the chlorite, whereas the Si and A1 contents progressively increased to the point of the formation of a pure dioctahedral aluminous vermiculite. The Si, Al, and Mg removal processes support currently accepted vermiculitization mechanisms, but the behavior of Fe is slightly different. In this weathering sequence, vermiculitization does not appear to have taken place by the oxidation of Fe2+, but rather, by the simultaneous leaching of Fe2+and Mg. L’analyse par diffraction de rayons X d’une chlorite magnésienne, dans une arène d’amphibolite, caractérise la séquence d’altération suivante: chlorite → interstratifié régulier chlorite/vermiculite → vermiculite. La spectroscopie Mössbauer indique que la vermiculitisation opère par libération du fer, à l’état ferreux, hors du feuillet 2:1 de la chlorite. La teneur en fer ferrique de la vermiculite est voisine de celle de la chlorite. Les analyses à la microsonde et en spectroscopie infrarouge montrent que Mg est préférentiellement libéré de la couche brucitique de la chlorite tandis que les teneurs en Si et Al augmentent avec l’altération jusqu’à la cristallisation d’une vermiculite dioctaédrique alumineuse. Les méchanismes de libération de Si, Al, et Mg sont semblables à ceux habituellement admis pour la vermiculitisation, mais le comportement du fer est différent. Dans cette séquence d’altération, la vermiculitisation ne procède pas par oxydation du fer, mais par départ simultané du fer ferreux et du magnésium.