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1 University of Poitiers, HydrASA, CNRS UMR 6532, 40 Avenue du Recteur Pineau, 86022 Poitiers cédex, France
2 COGEMA-BUM-DEX, 2 Rue Paul Dautier, 78141 Velizy cédex, France
* E-mail address of corresponding author: patricia.patrier{at}hyrasa.univ-poitiers.fr
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 2M1 polytype. They display pseudohexagonal platy crystals with average diameters ranging from 2 to 10 µm. Their chemical composition is Al-rich (Ca0.01Na0.02K1.72) (VIAl3.82Fe3+0.13Mg0.05Ti0.00Mn0.01)(Si6.27 IV Al1.73)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.
Key Words: Crystal Form • Crystal Structure • Diagenesis • Dickite • DTA Data • FTIR • Illite • Kombolgie • Proterozoic Sandstone • XRD
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