Abstract
This study concerns an approach for dolomite quantification and stoichiometry calculation by using X-ray diffractometry coupled with cell and Rietveld refinements and equipped with a newly substantial database of dolomite composition. A greater accuracy and precision are obtained for quantifying dolomite as well as other mineral phases and calculating dolomite stoichiometry compared to the classical “Lumsden line” and previous methods. The applicability of this approach is verified on dolomite reference material (Eugui) and on Triassic (Upper Muschelkalk-Lettenkohle) carbonates from the French Jura. The approach shown here is applicable to bulk dolostones as well as to specific dolomite cements and was combined with petrographical and isotopic analyses.Upper Muschelkalk dolomites were formed during burial dolomitization under fluids characterized by increased temperature and variable isotopic composition through burial. This is clear from their Ca content in dolomites which gradually approaches an ideal stoichiometry (from 53.16% to 51.19%) through increasing dolomitization. Lettenkohle dolostones consist of near-ideal stoichiometric (51.06%Ca) and well-ordered dolomites associated with anhydrite relicts. They originated through both sabkha and burial dolomitization.This contribution gives an improved method for the characterization of different dolomite types and their distinct traits in sedimentary rocks, which allows a better evaluation of their reservoir potential.
Highlights
Ideal dolomite has a crystal lattice consisting of alternating layers of Ca and Mg, separated by layers of CO3, and is typically represented by a stoichiometric chemical composition of CaMg(CO3)2 where calcium and magnesium are present in equal proportions (Reeder, 1990)
This paper presents an empirical calibration for the assessment of stoichiometry and crystal lattice properties of dolomite based on X-Ray Diffractometry (XRD) peaks
This paper documents an approach for the stoichiometry calculation of dolomites based on using linear regressions linking the %Ca and the unit cell parameters values (a = b or c) obtained from a cell refinement on XRD profiles, used for mineralogical quantification
Summary
Ideal dolomite has a crystal lattice consisting of alternating layers of Ca and Mg, separated by layers of CO3, and is typically represented by a stoichiometric chemical composition of CaMg(CO3) where calcium and magnesium are present in equal proportions (Reeder, 1990). Non-stoichiometric dolomite crystals are thermodynamically metastable under sedimentary conditions and more reactive to diagenetic environments relative to “ideal” dolomites (Carpenter, 1980; Land, 1980; Lumsden and Chimahusky, 1980; Hardie, 1987; Vahrenkamp and Swart, 1994; Chai et al, 1995; Budd, 1997) For this reason, a burial trend towards stoichiometry exists (Sperber et al, 1984; Vahrenkamp and Swart, 1994) resulting in an overall reset of stable isotope ratios and trace elemental abundances through recrystallization (Land, 1980; Morse and Mackenzie, 1990). This workflow results in a mineralogical quantification of the coexisting phases in the samples based on the Rietveld refinement
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