Composition and structure of low-temperature natural carbonates of the calcite-dolomite series

Mineralogical and crystallochemical studies of endogenic carbonates from bottom sediments of some shalllow saline lakes of Western Transbaikalia are performed. X-ray diffraction (XRD), IR spectroscopy, electron microscopy, elemental analysis (SR XFA), analysis of stable δ18O and δ13C isotopes, and other methods are applied. By modeling the complex XRD profiles by means of the Pearson VII function Mgcalcites with different Mg concentrations and Ca-excess dolomites are found in the assemblage of carbonate minerals. It is shown that in a series of Mg-calcites there is a gap in the region 2θ CuKα = 30°, which is caused by a transition from the structures representing true solid solutions to the structures of layered “domain” nanosized crystals. The idea of excess-Ca dolomite as the extreme member of the Mgcalcite series is substantiated. The carbonate record containing data on the stratigraphical distribution of endogenic carbonates of the calcite-dolomite series is obtained. The amount and ratio of phases with different Mg concentrations in this series are determined by the value of the Mg/Ca ratio, the total salinity and alkalinity of lake waters in the past, changing in accordance with climate cycles and fluctuations of the lake level.