COMPOSITION AND STRUCTURE OF LOW-TEMPERATURE NATURAL CARBONATES OF THE CALCITE-DOLOMITE SERIES
Г€.P. Solotchina, P.A. Solotchin
Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia
Keywords: кристаллохимия, карбонаты, ряд кальцит-доломит, рентгенография, ИК спектроскопия, озерные осадки, палеоклимат, crystal chemistry, carbonates, calcite-dolomite series, X-ray diffraction, IR spectroscopy, laсustrine sediments, paleoclimate
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 δ
18O and δ
13С isotopes, and other methods are applied. By modeling the complex XRD profiles by means of the Pearson VII function Mg-calcites with different Mg concentrations and Сa-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 Cu
K
a = 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-Са dolomite as the extreme member of the Mg-calcite 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.
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