Nacrite in the Lodeve Permian Basin: a TEM and fluid-inclusion study

Nacrite, a two-layer polytype of kaolin (space group Cc) generally formed a t high temperature, was found in the Lodeve Basin, France. It occurs in dol omite cavities in the Cambrian basement of the Permian basin, as authigenic , euhedral, up to millimetre-sized crystals, associated with barite deposit s of hydrothermal origin. Formation of the mineral deposits is attributed t o subbasinal fluid discharge and trapping during a Late Permian extensional regime. Textural relations suggest that nacrite and barite formed contempo raneously. The combination of transmission electron microscopy (TEM) images and electr on diffraction patterns (SAED) confirm the two-layer periodicity of nacrite . In low-magnification images, nacrite displays a lamellar structure define d by a high density of planar defects parallel to (001). Each lamella is fr om 1.4 to 600 nm thick. SAED and high-resolution images imply that these pl anar defects are "twin" boundaries. Fluid inclusions occurring in barite are mostly one-phase, but coexist with rare two-phase inclusions. Tee-melting temperatures indicate the presence of a high-salinity brine (up to 25 wt% NaCl equiv.), whereas heating runs i ndicate formation at low temperature, approximately 80 to 100 degrees C. Ox ygen isotope data for nacrite are consistent with that low temperature rang e. These data imply that the occurrence of nacrite rather than other kaolin -family polytypes can not generally be used as a qualitative geothermometer that implies formation at relatively high temperatures.