The Mineral Character and Geomechanical Properties of the Transitional Rocks from the Mesozoic-Neogene Contact Zone in the Bełchatów Lignite Deposit

Abstract

Purpose Beginning more than 30 years ago, opencast lignite mining in the "Bełchatów" area is an important source of accompanying minerals. Lignite mining in the "Bełchatów" mine yields, on average, more than 35 million tonnes per annum and requires removing more than 110 million cubic meters of overburden. Therefore the mine outside of the main mineral exploitation of lignite, leads to a large-scale economy of accompanying minerals. Part of the minerals are present in the overburden and are exposed on the slopes of the opencast mine; these minerals are selectively exploited in the event of the absence of recipients stored on anthropogenic deposits. The object of this mineralogical-geochemical study is a group of transitional rocks such as opoka-rocks, gaize and marls exposed when contact occurs between Neogene sediments and Mesozoic basement rocks in the "Bełchatów" lignite deposit. In the case of these rocks, during preliminary geological research carried out on the mine, doubt often appear as to their explicit petrographic character and hence their practical use. Advanced mineralogical methods allow mistakes in their identification to be avoided and a geomechanical study indicates possible direction of their practical use. Methods The heterogeneous petrographic character of the examined rocks required the use of a broad research spectrum. The following microscopes were used in the framework of the mineralogical research: Polarizing Olimpus BX51 and electron (SEM) FEI Quanta 200FEG equipped with an X-ray spectrometer (EDX Genesis) and backscattered electron detector (BSE). In addition, observations were carried out using a cathodoluminescence apparatus, the Cambridge Image Technolgy Ltd CCL 8200 mk3 model, and a polarizing microscope, type Nikon Optiphot 2. Determination of the phase composition (qualitative and quantitative) was made using X-ray diffraction and utilizing the powder method of Debye-Sherrer. An X-ray diffractometer, a Philips PW 3020 X'PERT, was also used in the study. Analyses by Fourier transmission infrared spectroscopy were performed using the production apparatus BIO-RAD, model FTS 165, equipped with a package of programs for the digital processing of results. The chemical composition was determined by atomic absorption spectroscopy (AAS) using a spectrophotometer Philips PU 9100Xi Camera SX-100 and atomic emission spectroscopy with inductively coupled plasma (ICP AES) using spectrometer 40 PLASMA. The geomechanical properties were determined in accordance with the following standards: open and total porosity (PN-EN 1936:2001) compressive strength in air – dried state (PN-EN 1926:2001), bulk density and density (PN-EN 1936:2001), relative humidity (PN-EN 1925:2001); absorbability of stone material (PN-EN