Mineralogical and geochemical characteristics of Middle and Upper Riphean limestones (Southern Urals)
DOI:
https://doi.org/10.17308/geology/1609-0691/2026/1/58-72Keywords:
limestones, Mg-calcites, unit cell parameters, Avzyan Formation, Katav Formation, Minyar Formation, Bashkir MeganticlinoriumAbstract
Introduction: the precipitation of calcite-dolomite carbonates is determined by the physical and chemical conditions of their formation. The purpose of this research is to identify the causes of variations in the mineralogical, geochemical, and crystallochemical characteristics of calcite and their relationship to the sedimentation conditions of limestone in different-aged Middle and Upper Riphean Formations on the western slope of the Southern Urals.
Methodology: the limestones of the Avzyan Formation of the Middle Riphean and the Katav, Inzer, and Minyar Formations of the Upper Riphean were sampled from several natural outcrops in the central and southwestern parts of the Bashkir Meganticlinorium. The chemical composition of limestone was determined by X-ray fluorescence (X-Calibur spectrometer) analysis. Electron microscopic study was performed on a TescanVega 4 Compact scanning electron microscope with an energy-dispersive analyzer. The crystallochemical features of calcite were determined by X-ray phase (DRON-4 and XD3 diffractometers) analysis. The mineralogical composition of limestone was determined using powder X-ray diffraction and thermogravimetric analysis (Q-1500 derivatograph).
Results and discussion: in limestones, the features of the crystallochemical structure of calcite are associated with the presence of Mg, Fe, and Mn. The reliability of the results is confirmed by the similarity of the d104 interplanar distance and the a and c parameters of the calcite unit cell, which depend on the concentration of impurity elements. According to the amount of magnesium included in the crystal structure (MgCO3 ≤ 2.26 wt. %), the calcite of the studied formations is classified as low-magnesian. The Mg content in these formations is controlled primarily by the sedimentation conditions, rather than by post-sedimentary processes and associated transformations.
Conclusion: variations in the amount of Mg in calcite from limestones in the Middle and Upper Riphean formations most likely reflect variations in its initial concentrations in the sedimentary basin. The highest concentrations of MgO are found in the Avzyan and Katav limestones, which may be due to the evaporite-like sedimentary environments during the Avzyan and Katav times. The identified features require further, more detailed confirmation using not only mineralogical-geochemical and crystallochemical data, but also parallel isotopic data.
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