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

Abstract

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 CuK _α = 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.