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Abstract Nacrite, a two-layer polytype of kaolin (space group Cc) generally formed at high temperature, was found in the Lodeve Basin, France. It occurs in dolomite cavities in the Cambrian basement of the Permian basin, as authigenic, euhedral, up to millimetre-sized crystals, associated with barite deposits of hydrothermal origin. Formation of the mineral deposits is attributed to subbasinal fluid discharge and trapping during a Late Permian extensional regime. Textural relations suggest that nacrite and barite formed contemporaneously. The combination of transmission electron microscopy (TEM) images and electron diffraction patterns (SAED) confirm the two-layer periodicity of nacrite. In low-magnification images, nacrite displays a lamellar structure defined by a high density of planar defects parallel to (00 I). Each lamella is from 1.4 to 600 nm thick. SAED and high-resolution images imply that these planar defects are "twin" boundaries. Fluid inclusions occurring in barite are mostly one-phase, but coexist with rare two-phase inclusions. Ice-melting temperatures indicate the presence of a high-salinity brine (up to 25 wt% NaCI equiv.), whereas heating runs indicate formation at low temperature, approximately 80 to 100°C. Oxygen isotope data for nacrite are consistent with that low temperature range. These data imply that the occurrence of nacrite rather than other kaolin-family poly types can not generally be used as a qualitative geothermometer that implies formation at relatively high temperatures

Abstract Authigenic euhedral kaolins of nano-to-millimetre size were found in dolomite cavities in the Cambrian basement of the Lodève Basin, France. X-ray diffraction, DTA and infrared-spectroscopy data demonstrate that nacrite is the only kaolin phase present in the cavities. Nacrite is associated with authigenic quartz, barite, dolomite and calcite. Petrographic data indicate a contemporaneous growth of nacrite and quartz after dolomite. In some cavities, barite and nacrite crystallized together. The crystallization of quartz, nacrite and barite could be the result of fluid influx and trapping in a karstified basement during the Late Permian extensional tectonic regime that affected the Lodève Basin.

Sediments and basalts were drilled along an E-W transect in the eastern flank of the Juan de Fuca ridge during the ODP Leg 168. In situ temperature measurements and chemical analyses of the sediment pore waters were also realized. The preliminary results of the Leg show that the basement relief controls the fluid circulation and that a continuous sediment layer provides a hydrologically resistive barrier to fluid exchange. Fluid discharge is restricted to basement outcrops. Lateral fluid flow over long distances (100 km) is also demonstrated.