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Nacrite in the Lodève Permian Basin

Martine D. Buatier^*,(), Dimitri DENEELE^*, Michel DUBOIS^*, Jean-Luc POTDEVIN^* and Michel LOPEZ^*

^* Université des Sciences et Technologies de Lille, UFR Sciences de la Terre, UMR 8577, Sédimentologie et Géodynamique, 59655 Villeneuve d'Ascq, France
New address: Université de Franche-Comté, UFR Sciences et Techniques, Département Géosciences, 16 Route de Gray, 25030 Besançon, France. e-mail: martine.buatier{at}univ-fcomte.fr

e-mail: martine.buatier{at}univ-fcomte.fr

Nacrite, a two-layer polytype of kaolin (space group Cc) generally formed at high temperature, was found in the Lodève 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 (001). Each lamella is from 1.4 to 600 mm 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% NaCl 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 polytypes can not generally be used as a qualitative geothermometer that implies formation at relatively high temperatures.

Key-words: nacrite, kaolin polytype, twin intergrowth, TEM, barite, fluid inclusions, stable isotopes, Lodève.

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