Evidence of an authigenic origin for palygorskite in a Middle Eocene carbonate sequence from North Qatar.

Abstract

The "Simisma Member" from the Ras Laffan area in North Qatar is mainly composed of dolomites of shallow marine origin. Clay minerals contained in the dolomite (average 4%) are mainly palygorskites. There is no indication of the presence of any transitional forms between palygorskite and other clay minerals. Delicate dense aggregates of the palygroskite fibres form matted films covered fine-grained dolomites. The long palygorskite fibres are also bridged and fill pore spaces between the coarse dolomite crystals. The presence of well preserved long fibres indicates that the palygorskite could not have been transported; they are authigenic and must have been formed directly by precipitation from solutions rich in Al, Si and Mg. The elemental compositions of the studied palygorskite are similar to previously published data. However, the high concentrations of Ca and Mg indicate that the palygorskite samples are mixed with carbonate minerals. The relatively high ^0 values (average + 18.7) of the palygorskites support the hypothesis that the palygorskite has been formed from solutions of high salinity. Meanwhile, the ^°0 contents of dolomites (range from -i-0.3 to +2.7%o PDB) and late sparry calcite crystals (range from -5.7 to -11.8%o PDB) argue against the formation of dolomite, calcite and palygorskite from the identical fluid.