Abstract
The determination of the degree of Mg-Ca order in the dolomite structure is crucial to better understand the process or processes leading to the formation of this mineral in nature. I01.5/I11.0 intensity ratios in the X-ray powder diffractograms are frequently measured to quantify dolomite cation order in dolomites. However, the intensity of diffraction peaks can be affected by factors other than the Mg-Ca distribution in the dolomite structure. The most relevant among these factors are (i) deviations from the ideal dolomite stoichiometry, and (ii) the partial substitution of Mg and Ca atoms by Fe, Mn, and Zn impurities. Using the VESTA software, we have constructed crystal structures and calculated I01.5/I11.0 ratios for dolomites with Mg:Ca ratios ranging from 0.5 to 1.5, and with Fe, Mn, and Zn contents up to 30%. Our results show that both deviations from dolomite ideal stoichiometry and the presence of impurities in its structure lead to a significant decrease in I01.5/I11.0 intensity ratios, an effect which must be considered when cation orders of natural dolomites from different origins are compared.
Highlights
Dolomite, CaMg(CO3)2, is an abundant carbonate mineral with a structure characterized by a highly ordered alternation of magnesium and calcium layers along the c axis
Given that changes in the XMg in the dolomite structure have a clear effect on diffraction patterns [8], the degree of the cationic order of dolomites can be conveniently evaluated from X-ray powder diffractograms
One method to proceed is to calculate the ratio of the intensities of a selected ordering peak and the nearest non-ordering peak to the selected ordering peak [8,9,10,11]
Summary
CaMg(CO3), is an abundant carbonate mineral with a structure characterized by a highly ordered alternation of magnesium and calcium layers along the c axis. At temperatures below ~100 ◦C, the formation of dolomites with such a high cation order cannot occur by direct precipitation from aqueous solutions, but it requires a Mg-Ca ordering process or processes after first crystallisation [1,2,3]. The investigation of both the mechanisms and kinetics of dolomite ordering process in nature first requires to accurately quantify the degree of Mg-Ca order within the dolomite structure. Known as superstructure peaks or superstructure reflections, must fulfill the following condition: h0.l and 0k.l with l odd (e.g., 00.3, 10.1, and 01.5 peaks) [12]
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