FEATURES OF ELEMENTAL ANALYSIS OF DOLOMITES OF THE LOWER PERMIAN DEPOSITS BY X-RAY FLUORESCENCE SPECTROSCOPY

Abstract

In this work, using the methods of scanning electron microscopy (SEM), X-ray phase analysis (XRF), X-ray fluorescence spectrometry (XFS) and thermogravimetric analysis (TGA), the physicochemical properties of dolomite rocks selected from one well at different depths of occurrence were studied. The SEM method shows that, depending on the depth of occurrence, dolomite is characterized by both ordinary crystals of rhombohedral morphology, the size of which varies from 50 to 80 microns along the diagonal axis, and unusual shapes, in the form of hexagonal plates up to 1 microns thick and 1 to 10 microns in diameter. It has been found that dolomite plates in places form spherical aggregates ranging in size from 20 to 40 microns. According to the RF analysis data, it was found that in samples of dolomite rock (of ordinary and unusual shape), after calcination at 1000 °C for 5 hours, there is a twofold increase in the molar stoichiometric ratio of Mg and Ca, compared with the initial (non-calcined) state of dolomite. By the XRD method, it was found that during calcination of dolomite samples, a phase transformation (CaMg(CO3)2 → CaO + MgO + 2CO2) is observed, in which lime (CaO) and periclase (MgO) phases are formed. Precision SEM studies have shown that periclase nanocrystals, whose size varies from 100 nm to 300 nm, are evenly distributed on the surface of larger lime fragments. As a result of numerous measurements of the X-ray spectra of the RF, it was found that the overlap of lime fragments with numerous periclase nanocrystals is the cause of a significant overestimation of the magnesium content after calcination of dolomite samples. It is concluded that in order to correctly conduct elemental analysis and obtain adequate information about the weight fraction of rock-forming oxides in dolomite samples, they must first be dissolved in hydrochloric acid. The method of sample preparation proposed by the authors for the correct elemental analysis of dolomite rocks has been tested on numerous samples, which showed convincing convergence of the data obtained by XRF (decomposition into rock-forming oxides) and XRF analysis.