Combined Raman and EDXS analysis on drill core samples

This presentation deals with the results of combined spectroscopic analyses performed on a sample extracted from a core drill obtained from a geological survey for raw materials mining purposes. The aim of the work was to detect the major mineral component phases following an experimental protocol setting up the bases that will be developed further within the EU-H2020 SOLSA project http://www.solsa-mining.eu), funded by the EU-H2020 Raw Material program. In this framework, an expert system, coupling sonic drilling and combined analytical tools for automatic recognition of core minerals will be developed and tested. In the present study, both micro-Raman as well as Energy dispersive X-Ray spectroscopy tests were carried out in a complementary way. The sample, in form of a solid bar with a very dark/black colour, was labelled as BC6 (18/84, 60 m), and, according to the provider (the French mining and metallurgical group ERAMET), it consists of sulfur minerals, mainly pyrite. Micro-Raman spectra were carried out in backscattering geometry by means of a Horiba-Jobin Yvon apparatus (model LabRam HR), consisting of a single spectrograph an objective 80X, a notch filter and an holographic grating (600 lines/mm). The spectra here reported were excited by the 633 nm of a He-Ne laser and recorded by a CCD (256x1024 pixels) detector, cooled at -134oC by liquid N2 . The laser spot size at sample surface was about 3 μm, the laser power was 1.5 or 3 mW, and the integration time of the order of 100 s. The energy dispersive X-ray spectra (EDXS) were acquired with a system (mod. Falcon Zaphire, EDAX) attached to a scanning electron microscope (SEM- mod. L30, FEI)) operated in a low-vacuum mode and using an accelerating voltage for the primary electrons of 20 kV. When the low-vacuum mode is used, the specimens can be observed in the as delivered conditions, with no need of surface coating with a conductive film, to prevent electrostatic charging, in case the observed sample was partially or fully non conductive. Film coating would have been unacceptable in the present case for possible interference of the X-ray lines from the film (C or Au are typically used) and those coming from the underlying sample material. SEM images of several millimeter or sub-millimeter regions of the BC6 sample clearly show its poly-crystalline and multi-phase structure, with the presence of grains of about 100 micrometers in size. EDXS spectra of the same sample regions allow for the determination of the local relevant chemical composition. These results still do not provide precise indications on the mineral phases that are there in the analyzed sample. Phase composition is an extremely critical information, in view of the drilling conditions to be adopted and, most importantly, the extraction methodology to be used to obtain from the mined ores the metal(s) of interest. Therefore, the analytical data were cross-checked and implemented using Raman spectroscopy. Raman spectra observed from two different micro-regions in the central region of this sample indicates in both cases a significant occurrence of microcrystalline graphite, revealed by the very strong and quite narrow D and G bands in the region of 1300-1650 cm-1. In addition, a noticeable pyrite FeS2 component is observed, in the first case, while, in the other case, dolomite (CaMgCO3 ) with a relatively minor occurrence of pyrite FeS2 is observed. Several additional Raman measurements on morphologically unlike micro-regions, especially on microcrystalline grains, allowed to detect, besides the always present microcrystalline graphite, also dolomite (MgCO3 ), and, in turn, quartz SiO2 alone or, even, accompanied by a minor amount of pyrite. Finally, in some cases microcrystalline graphite alone was observed. These results provide now a clear picture concerning of the chemical and phase composition of the analyzed sample. For the experimental tools that we have used in this feasibility study, the sample regions are extremely small as compared to the interesting size for real mineralogical core drills. Indeed, within the SOLSA project the acquisition of analytical and crystallographic data will rely also on “larger scale” tools, e.g., X-ray diffraction and X-Ray fluorescence, carried out in a hierarchical way so to optimize the acquisition times and relevant combined data processing. As a further step forward, the whole knowledge acquired from the experimental and test strategy will be used for designing a multi-analytical instrumentation, that will render the whole data acquisition procedure and analysis protocol comparatively fast and user friendly.