# X-ray Photoelectron Spectroscopy in Mineral Processing Studies
## 2 Speciation of Surface Products
### Oxidized Surfaces of Metal Sulfides
#### 2.1.2 Metal-Deficient and Polysulfide Surfaces  
[PERSON] and [PERSON] have also found [44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55] that oxidation of metal sulfides in atmosphere and aqueous media generally produces surfaces depleted in metal and so enriched in sulfur. The initial oxidation typically yields metal oxides and hydroxide with no oxidized sulfur species, suggesting the formation of \"defective metal-deficient\" sulfide surfaces. If the deficit of a metal becomes large, the S 2p lines at higher binding energies arise, suggesting S-S bonding and \"polysulfide\" surfaces. These effects were found for a majority of sulfide minerals, including pyrite [49; 57; 58] containing disulfide groups, which is the most resistive to oxidation so quantities of polysulfide are the smallest in comparison with other sulfides. The surface non-stoichiometries induced by the preferential release of metals from the lattice are important phenomena for mineral processing and adjacent areas because they affect hydrophobic/hydrophilic characteristics (polysulfide surface is commonly assumed to be hydrophobic [10; 17; 18]), reactivity and mechanisms of various reactions of metal sulfide phases, including passivation in the hydrometallurgical leaching. Even small, within the error of XPS analysis, stoichiometry deviations can dramatically change the semiconducting properties of metal chalcogenides, surface charge distribution, etc., as shown, for example, for galena [59; 60; 61].  
The studies [62; 63; 64; 65; 66; 67; 68] performed with the aim to understand the surface S-S species have elucidated that the end atoms in polysulfide anions bear negative charges similar to that in disulfide groups with the S 2p3/2 BE about 162.5 eV. A small charge may be localized at S atoms nearest to the terminal ones [66], while intermediate atoms are close to elemental sulfur with almost no charge and BE between 163 and 164 eV. The occurrence of polysulfides on reacted sulfide mineral surfaces was confirmed with Raman spectroscopy (not quite surely, though) [57; 69; 70] and TOF-SIMS [71] and other techniques. Polysulfides are thought to be stable in the spectrometer vacuum, in contrast to elemental sulfur [44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 63; 64; 65; 66; 67; 68].  
At the same time, [PERSON] et al. [72; 73; 74] have reported that the intensity of S 2p bands attributable to polysulfides at chalcopyrite decreased with time as the sample was kept in UHV at room temperature. The \"polysulfide\" lines have been suggested to belong to elemental sulfur adsorbed at metal-deficient surface and the sulfur gradually evaporated at room temperature, whereas polysulfides do not actually exist [72]. This, however, contradicts numerous findings, including the S-S species detected in much deeper sulfide layers and transformations of S-S bonding observed in (electro)chemical reactions [75; 76; 77; 78]. The inconsistency seems to be resolved by using hard XPS depth profiling [89; 90] which elucidated that the metal-depleted surfaces are composed of several layers with only the outer one containing polysulfide anions. [PERSON] et al. [68] found with DFT+\(U\) simulation and cryo-XPS of fast-frozen chalcopyrite (Figure 1) that the polysulfides as intermediates to elemental sulfur are surprisingly stable and hinder the release of elemental sulfur (see Section 3.3 for details).