Original paper

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

Buatier, Martine D.; Deneele, Dimitri; Dubois, Michel; Potdevin, Jean-Luc; Lopez, Michel

European Journal of Mineralogy Volume 12 Number 2 (2000), p. 329 - 340

33 references

published: Mar 29, 2000
manuscript accepted: Oct 28, 1999
manuscript received: Jan 29, 1999

DOI: 10.1127/ejm/12/2/0329

BibTeX file

ArtNo. ESP147051202004, Price: 29.00 €

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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


nacritekaolin polytypetwin intergrowthTEMbaritefluid inclusionsstable isotopesLodève