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Minerals 2017, 7(9), 164; doi:10.3390/min7090164

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

CETEM—Centre for Mineral Technology, Division for Technological Characterisation, 22461-908 Rio de Janeiro, Brazil
Museu Nacional, Universidade Federal do Rio de Janeiro, 20940-040 Rio de Janeiro, Brazil
Author to whom correspondence should be addressed.
Received: 3 July 2017 / Revised: 1 September 2017 / Accepted: 4 September 2017 / Published: 8 September 2017
(This article belongs to the Special Issue Fundamentals and Frontiers in Mineralogy)
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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 + 0.8632ΔnCa for dolomite, with ΔnCa 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. View Full-Text
Keywords: carbonates; magnesian calcite; non-stoichiometric dolomite; Rietveld method; X-ray diffraction carbonates; magnesian calcite; non-stoichiometric dolomite; Rietveld method; X-ray diffraction

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dos Santos, H.N.; Neumann, R.; Ávila, C.A. Mineral Quantification with Simultaneous Refinement of Ca-Mg Carbonates Non-Stoichiometry by X-ray Diffraction, Rietveld Method. Minerals 2017, 7, 164.

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