# X-ray Photoelectron Spectroscopy in Mineral Processing Studies
## 3 Spatial Resolution of Photoelectron Spectroscopy
### Hard X-ray Photoelectron Spectroscopy  
The probing depth can be increased with increasing energies of excitation photons far beyond those applied in the common laboratory spectrometers. The high-energy photoemission spectroscopy, or hard XPS (HAXPES) [128; 153], most often utilizes the photon energies tunable from 2 keV to 10 keV at many synchrotron facilities and characterizes the layers extending up to 20-30 nm in depth. Commercial instruments employing hard X-ray sources, particularly with Cr (5.417 keV) and liquid Ga (9.243 keV) anodes [154; 155], are developed now too. The HAXPES is an effective tool for destruction-free examination of multilayered structures, and surfaces and interfaces buried under reaction products, which provides information about both the concentration as a function of the excitation photon energy (depth profiles) and the depth-resolved chemical form of elements [130]. Certain shortcomings of the compositional analysis arise for samples with complex morphology of near-surface regions due to multiple elastic scattering, and because the photoionization cross-sections of core shells essentially depend on the excitation energy [128; 153; 154; 155]. HAXPES looks as a very promising technique for characterization of real heavily reacted and contaminated surfaces in mineral processing but, to the best of our knowledge, only few HAXPES investigations of sulfide minerals were reported in the literature.  
As an example, [PERSON] et al. [88; 89] have applied HAXPES in conjunction with Fe K- and S K-XANES, and DFT+\(U\) simulation to study polycrystalline chalcopyrite, pyrite, and pyrrhotite plates oxidized in air and aqueous Fe\({}^{3+}\) solutions and developed Fe-depleted surfaces. It has been known that the metal deficiency induced by chemical reactions is not just a surface phenomenon and extends in depth up to hundreds nanometers as established with AES depth profiling [11; 76; 77; 78; 79], X-ray absorption and emission, and Mossbauer spectroscopy [80; 81; 87; 88; 89; 148; 87; 89] but the nature of such heavily disordered structures has remained puzzling. HAXPES study discovered that the near-surfaces of mineral samples beneath some surface Fe oxides are stratified (Figure 3) and composed of an outer layer with high S excess and polysulfide species, and then a metal-deficient zone with mono- and disulfide anions, whose composition and thickness varied from a few nm to several tens of nm, depending on the nature of mineral and reaction conditions. The underlayers up to a hundred nm in depth had no variations in the composition discernible in HAXPES but the changes in XANES suggested their high defectivity. The polysulfide centers located in the 1-2 outmost atomic layers were found to be energetically favorable under the oxidation conditions [68], so arresting the yield of elemental sulfur and promoting the formation of thick metal-depleted layers. These findings shed new light on the origin of metal-deficient surfaces and highly debatable mineral \"passivation\" [11; 12; 13; 14; 156; 16; 17; 157; 158; 159; 160].  
Figure 3: Atomic ratios and the results of fitting of the S 2p and Fe 2p spectra as a function of excitation photon energy for pyrrhotite leached in acidic 0.5 M FeCl\({}_{3}\) solution (top panels), and scheme illustrating layered structures of pyrite and pyrrhotite oxidized in air and the aqueous solution as derived from HAXPES and XANES. Adapted from [88] with permission from Elsevier.