Magnesium quantification in calcites [(Ca,Mg)CO3] by Rietveld-based XRD analysis: Revisiting a well-established method

Abstract

Mg-calcites commonly occur in natural environments, with Mg-contents ranging between about 0 and 32 mol% (referring to MgCO 3 ). Often, different Mg-calcite phases occur within the same sample. The Mg-content in calcites permits the classification of the diagenetic environment (marine vs. meteoric), or the reconstruction of paleotemperatures from skeletal remains. Since the 1960s, there have been published various calibrations for Mg determination in calcites based on XRD measurements. Recently, this method has come to be superseded by wet chemical, laser ablation, and microprobe analysis, due to their higher accuracy and/or higher sample resolution of these latter methods.This study presents a new calibration for the Mg determination in calcites using XRD measurements analyzed by means of the Rietveld refinement method. The calibration is based on lattice parameters and exhibits a reliable Mg-determination accuracy of more than 0.8 mol% between 0 and 15.5 mol%. The incorporation of the calibration into the Rietveld refinement software TOPAS permits a fast, standardized workflow. This, in turn, enhances the user-friendliness and the reproducibility of results, as well as allows the simultaneous analysis of multiple Mg-calcites in a sample. Mixtures of two Mg-calcites were analyzed using this new method. The Mg-content of two co-occurring Mg-calcite phases can be reliably quantified providing the Mg-calcites differ by at least 4.9 mol% and the minor Mg-calcite phase makes up more than 2.7 wt%. If the Mg-calcite phases differ by 3.4 mol% or less, the reliable identification of different Mg-calcite phases is questionable, due to the fact that differences involved here are too small. If such XRD patterns are refined with only one Mg-calcite phase, then the systematic misfit between measured and refined pattern can be used for the identification of a second Mg-calcite phase down to a difference in the Mg-content of about 3 mol%. However, a reliable phase quantification for these patterns cannot be achieved.

Mineral Quantification with Simultaneous Refinement of Ca-Mg Carbonates Non-Stoichiometry by X-ray Diffraction, Rietveld Method

“…After retrieving the refined lattice parameters and plotting them against the chemical analyses of his samples, they used a calibration curve based on cell volume to calculate the Mg and Ca site occupancy, adjusting these values in the software for phase quantification. The results obtained by Titschack et al [32] are compared to those of the present work in Figure 9. Both of these studies display very close results, with similar correlation coefficients.…”

“…Titschack et al [32] also used a fundamental parameters approach to the Rietveld method for magnesian calcite quantification, starting refinement with the structure defined by Paquette & Reeder [42], with molar fraction of 0.129 MgCO3. After retrieving the refined lattice parameters and plotting them against the chemical analyses of his samples, they used a calibration curve based on cell volume to calculate the Mg and Ca site occupancy, adjusting these values in the software for phase quantification.…”

“…Both of these studies display very close results, with similar correlation coefficients. Rather than dynamically constraining the Mg-for-Ca substitution, Titschack et al [32] applied a twostep refining routine, i.e., refining lattice parameters, using them to determine substitution, and then adjusting the occupancy in the structure for further operations. The authors also discuss the possibility of multiple calcite compositions in the samples, and use their method to quantify the (simulated) binary mixtures.…”

“…Our data, based mostly on data of Goldsmith et al [23] and Zhang et al [38], extends the linear relation continuously up to 0.287 apfu of Mg. Comparison of the results of our study with different calibrations [22][23][24]26,27,30,32,42] for determination of the molar fraction of MgCO 3 in calcite shows that the new calibration is in agreement with the trend of the former publications (Figure 10), except for the c/a relation, as already observed by Titschack et al [32]. Some authors validate this linear relationship only up to 0.18 apfu of Mg [20,27], stating that the relation is not linear any more above this content.…”

“…Another paramount necessity is using the structure files that really describe the refined minerals. For mixed crystals, as in samples studied here, choosing adequate structures was not an easy task, and was used to require a multistep approach as the one adopted by Titschack et al [32] for magnesian calcite. If solid solutions are a part of samples with complex mineralogy, the initial cell parameters refinement without constraints on occupancy can easily retrieve approximate values, leading to wrong occupancy.…”