Features of determining the composition of micro-sized palladium minerals using electron probe X-ray microanalysis

Electron probe X-ray microanalysis (EPMA) is the main method for determining the composition of micron-sized mineral phases containing palladium in ores and ore occurrences. This work is devoted to the study of factors influencing the correct determination of the composition of inclusions of mineral palladium phases (michenerite (PdTeBi), frudite (PdBi2) and some others) in chalcopyrite and pentlandite matrices. The measurements were made with the use of a MIRA 3 LMH (Tescan) scanning electron microscope equipped with an energy-dispersive spectrometer and a JXA-8200 (JEOL) microanalyzer equipped with wavelength-dispersive spectrometers. Estimates were made of the possibility of reducing the size of the local analysis region by reducing the accelerating voltage to 15 and 10 kV, compared to the manufacturer's recommended MIRA 3 (TESCAN) of 20 kV. It has been shown that mineral particles of palladium phases can be considered as a “massive” if their sizes are more than 5.2 μm at an accelerating voltage of 20 kV; 3.2 μm at 15 kV; 2.6 μm at 10 kV. Assessments have been obtained for the generation region of characteristic X-ray Pd, the dimensions of which allow the use of proven analytical signal correction methods developed for “massive” objects. An assessment of the fluorescence effect of the elements Fe, Ni, Cu, also contained in the sulfide matrix surrounding the particle, showed that the presence of an analytical signal of these elements does not mean their presence as an impurity in the particles, even if their sizes are larger than the mentioned sizes of “massive” particles. Comparison of the results of determining the elements Fe, Ni, Cu using X-ray lines of the K- and L-series allows us to conclude about the content of these elements in the form of impurities in the palladium phase. To assess the uncertainty in the composition of micron inclusions of palladium phases, the results of determination with a MIRA 3 SEM and with a JXA 8200 microanalyzer were compared. These instruments use different methods for measuring X-ray radiation and methods correction of the analytical signal. The uncertainty of contents is 1.2-1.8%, wt. at a content level of Pd ≈ 20%, Bi ≈ 80%, Te ≈ 30%, wt. The uncertainty estimates were almost independent of the accelerating voltage for the MIRA 3 SEM (10, 15 or 20 kV).