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Stoichiometric Characterization of Dolomites by Cell and Rietveld Refinements (Middle Triassic, French Jura): A New Approach
(1) Sedimentology & Stratigraphy, IFP, Rueil-Malmaison, France.
By cation substitutions in the dolomite crystal lattice through dissolution-precipitation reactions, the stoichiometry of the dolomite (%CaCO3), and therefore its stability/reactivity, is bound to change. Reaching stable forms dolomites are more resistant to further changes and may retain their reservoir properties. This contribution presents a new approach using the X-ray diffractometry (XRD) technique coupled with cell and Rietveld refinements on dolomitized rock samples, which enables in one analysis with few powder material to achieve mineralogical quantification, crystallographic investigation, and assessment of dolomite stoichiometry.
Core-samples were investigated from Chatelblanc 1 well, representing the stratigraphic contact of the Upper Muschelkalk and Lettenkohle Formations (Middle Triassic). XRD analyses were combined to classical petrographic and geochemical data (e.g. major/trace element composition, stable oxygen and carbon isotopes). The investigated sequence covers an interval about 10m thick, consisting of limestones (mudstones and bioclastic wacke/grainstones), dolostones and evaporites.
Microscopic examination (transmitted light optical and cathodoluminescence) resulted in identifying (i) a calcitic and/or dolomitic bulk-rock matrix; (ii) three phases of calcite cements (aragonite replacement; bioclasts filling; fractures filling) and, (iii) three phases of dolomite cements (early planar-e; planar-e filling bioclasts; planar-s fracture filling). Stable isotopes results may reveal heating associated to burial during the emplacement of cements (δ18O-calcite: -6.65 to -0.25‰ VPDB; δ18O-dolomite: -8.27 to -1.89‰ VPDB). Our new analytical approach concerning dolomite stoichiometry analyses was achieved in two phases. First, an abacus - with crystallographic data on dolomites - was built from a comprehensive literature review. Such data comprise cell parameters related to the lattice Ca percentage of various dolomites. Then, using the dolomite cell parameters determined by cell refinement and this abacus, dolomite stoichiometry could be calculated. This approach resulted in recognizing distinct groups of dolomites based on their stoichiometric values (while they show similar petrographic textures) in the investigated Triassic sediments. Dolomites with the highest nonstoichiometry (~53% CaCO3) are associated with rocks showing the lowest dolomite abundance, while the more stable (stoichiometric; ~50% CaCO3) with pervasive dolostones.