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

Abstract

Quantitative phase analyses of carbonate rocks containing Mg-rich calcite and non-stoichiometric dolomite by the Rietveld method yielded improved results when the substitutions are refined for either minerals. The refinement is constrained by the c-axis of the lattice for both minerals using the formula c = −1.8603 nMg + 17.061 for calcite, where nMg is the molar fraction of Mg replacing Ca, and c = 16.0032 + 0.8632∆n Ca for dolomite, with ∆n Ca being the excess Ca in its B site. The one-step procedure was implemented into the Topas software and tested on twenty-two carbonate rock samples from diverse geological settings, considered analogues to petroleum system lithotypes of the pre-evaporite deposits of Southeastern Brazil. The case study spans over a wide range of calcite and dolomite compositions: up to 0.287 apfu Mg in magnesian calcite, and Ca in excess of up to 0.25 apfu in non-stoichiometric dolomite, which are maximum substitutions the formulas support. The method overcomes the limitations for the quantification of minerals by stoichiometry based on whole-rock chemical analysis for complex mineralogy and can be employed for multiple generations of either carbonate. It returns the mineral quantification with unprecedented detailing of the carbonates' composition, which compares very well to spot analysis (both SEM-EDS and EMPA) if those cover the full range of compositions. The conciliation of the quantification results based on the XRD is also excellent against chemical analysis, thermogravimetry, and carbon elemental analysis.

The argonaut constructs its shell via physical self-organization and coordinated cell sensorial activity

“…The results of the Rietveld analysis ( Table S1 ) show well-defined changes in calcite unit cell parameters with respect to pure inorganic calcite, which can be explained by both the substitution of Ca 2+ by Mg 2+ ions and the incorporation of organic molecules into the calcite crystal structure ( Pokroy et al., 2006 ). In particular, the reduction in calcite unit cell parameters observed in the sample treated at 400°C (Δ a = −0.0432 ± 3 × 10 −4 Å = −0.865%; Δ c = −0.19678 ± 9 × 10 −4 Å = −1.153%), when the influence of organic matter is negligible, can be explained by a 10.4% (molar) substitution of Ca 2+ by smaller Mg 2+ ions ( Dos Santos et al., 2017 ). The expansion of calcite unit cell parameters (Δa = 0.107%, and Δc = 0.082%) observed in the untreated shell mineral compared with the sample treated at 400°C (no organic matter) may result from the incorporation of organic shell components into the calcite crystal structure.…”

Onshore groundwater spring carbonate mounds to lacustrine microbialites, the perplexing record of a transitional Great Salt Lake carbonate shoreline at Lakeside, Utah

“…X-ray diffraction (XRD) was made on 62 selected samples at CETEM (Bruker-AXS D4 Endeavour, using Ni-filtered CoKα, 40 kV, 40 mA) radiation, primary aperture 0.6 mm and primary and secondary 2.5° soller slits; 2θ range from 4° to 105° with 0.02° step size and 1 s acquisition time per step with LynxEye detector, and sample rotation 15 rpm; quantitative phase analysis (QPA) by fundamental parameters Rietveld method (Bruker-AXS Diffrac. TOPAS software, v.6) following Santos et al (2017) and Neumann et al (2018).…”

Mineralogy and chemistry of a new halloysite deposit from the Rio de Janeiro pegmatite province, south-eastern Brazil

“…The quantitative mineral analysis was performed with the Bruker-AXS TOPAS v5 software, sourcing structures from the ICDD database (2019) and the Crystallography Open Database (COD). The mineral structures used were montmorillonite (COD 900277; Viani et al, 2002), muscovite (ICDD 04-015-8226; Brigatti et al, 2008), kaolinite (ICDD 00-014-0164 BISH; Goodyear and Duffin, 1961), quartz (ICDD 00-046-1045; Kern and Eysel, 1993), microcline (ICDD 00-019-0926; Technisch Physiscge Dienst, 1966), albite (ICDD 00-019-1184; Technisch Physiscge Dienst, 1967), calcite (Modified; Santos et al, 2017), dolomite (Modified; Santos et al, 2017) and alunite (COD 9012350; Schukow et al, 1999). The abbreviations for names of rock-forming minerals were based on Whitney & Evans (2010) and Warr (2020).…”