# X-ray Photoelectron Spectroscopy in Mineral Processing Studies
## 3 Spatial Resolution of Photoelectron Spectroscopy
### Lateral Resolution of Photoelectron Spectroscopy  
Natural mineral systems are composed of plentiful phases with the particle size less than 100 \(\upmu\)m after the comminution [1,2], with the surfaces of single mineral particles being further inhomogeneous in terms of crystal faces exposed, roughness, impurity distributions, and so on. Conventional laboratory and synchrotron photoelectron instruments have resolutions on the order of a hundred \(\upmu\)m and several \(\upmu\)m, respectively, at best. The high spatial resolution methods available at present are scanning X-ray photoelectron microscopy (SPEM, or XPEM) and photoemission electron microscopy (PEEM) [161, 162, 163, 164]. In synchrotron SPEM, the photon beam focused to a spot of about 100 nm scans over the surface. Emitted photoelectrons are collected with an analyzer, providing local photoelectron spectra similar to conventional XPS or/and spectrom microscopy images with submicrometer resolution at selected excitation photon energies [161, 162].  
PEEM can be realized both as laboratory and synchrotron set-ups, with the latter often coupled with XANES technique [163, 164]. PEEM provides surface chemical imaging by parallel acquisition of the photoelectrons emitted from the illuminated area. This allows reaching the lateral resolution as good as 10 nm at short measurement times, but information on chemical bonding is limited and should be extracted from a coupled technique, e.g. XANES. The methods have been applied to image distributions of various phases in natural mineral samples, including surface oxidation products across natural samples [164, 165, 166].  
Interesting SPEM studies on reacted sulfide minerals have been performed by [PERSON] and co-workers [168, 169, 170]. The experiments were carried out using polished mineral slabs frozen after the treatment and measured using about 100 nm photon beam at LN temperatures at the ESCA Microscopy Beamline (ELETTRA). It has been found, for example, that Fe-O bonds form prior to Fe-OH bonds in initial stages of pyrite oxidation, S oxidation proceeds through interactions of OH radicals formed at the Fe sites, and sulfate forms, via thiosulfate/sulfite intermediates, in the patches, where adsorption of oxygen has been localized and Fe oxidation occurred. Chalcopyrite dissolution in 4 mM Fe\({}^{3+}\) solution has been observed to be spatially heterogeneous at the submicrometer scale, producing di- and polysulfide anions, elemental sulfur, sulfite, and sulfate (Figure 4). The surface heterogeneity has been concluded to indicate that the minerals are not passivated by these products.  
Figure 4: S 2p SPEM speciation maps (**a**) \(S^{2-}\), (**b**) \(S_{2}{}^{2-}\), (**c**) \(S_{n}{}^{2-}\), (**d**) \(S^{0}\) and (**e**) \(SO_{3}{}^{2-}\) collected from a region (100 \(\times\) 100 \(\upmu\)m) on the chalcopyrite surface partially dissolved with pyrite for 10 days containing spots 4A-4D (S 2p XPS spectra are shown in top left corner). Reproduced from [170] with permission from Elsevier.  
In general, PEEM and SPEM are not broadly used in mineral processing studies until now, in particular, because these methods require rather flat surfaces and well-conducting materials, in addition to access to SR facilities. XPS measurements spatially resolved in lateral dimension remain a challenge.