The basic principle of XPS analysis is that when a beam of X-rays with specific energy irradiates a sample and the photoelectric effect occurs on the surface of the sample, photoelectrons with characteristic energy related to the electron energy level in the inner layer of the measured element will be generated. By analyzing the energy distribution of these photoelectrons, the photoelectron spectrum can be obtained. Each element has a series of photoelectron spectrum peaks with different binding energies, and its intensity is related to the quantum number of related electrons. Generally, the strongest peak of an element is selected as the characteristic peak of the element. Because of the different physical and chemical conditions of the element, the characteristic peak shifts, which leads to the separation of peaks in the internal energy level spectrum of the same atom in different chemical environments. This is called chemical shift effect, which can be used to analyze the chemical valence state of the element.
Yang (2006) selected {100}, {2 10} arsenic-bearing pyrite and {230} arsenopyrite in the second hydrothermal period of Yangshan Gold Mine, studied the valence characteristics of elements such As Fe, As and S on their surfaces by XPS, discussed the possible occurrence forms of As in pyrite, and then discussed Yangshan Gold Mine.
6.3. 1 XPS experiment and results
The experiment was completed in the surface analysis (electron spectroscopy) laboratory of the analysis and testing center of Peking University Institute of Chemistry. AXIS Ultra XPS spectrometer of British Kratos company was used, and aluminum target X-ray source with monochromator (Al K, h= 1486.7 1 eV) was used, with power of about 225 W (working voltage 15 k V, emission current 15m A) and minimum energy resolution of 0. Vision(PR2. 1.3) and Casa XPS(2.3. 12Dev7) were used for data processing and peak fitting.
Before 10min analysis and injection, all samples were crushed into powder particles with a particle size of 0.0 1 ~ 0. 1 mm with agate mortar, which were pasted on double-sided adhesive tape and sent to the analysis room for detection.
Table 6.2 XPS Analysis Results of Crushed Surface Elements of Pyrite and Arsenopyrite
Table 6.2 summarizes the analysis results of XPS peak position, full width at half maximum, integrated area and relative content of elements of fresh sections of pyrite and arsenopyrite in Yangshan Gold Mine. It can be seen that the atomic ratios of Fe:S:As on the surfaces of two pyrite samples are 1: 1.327:0.037 and 1: 1.409:0.073, respectively, and the arsenopyrite is1:0.955. In addition, there are a lot of pollution O and pollution C on the surface of pyrite and arsenopyrite, and their atomic percentages are 26.88% ~ 2 1.26% and 37. 12% ~ 22.34% respectively.
Table 6.3 summarizes the atomic percentages of ions with different valence states of Fe, S, As and As on the fresh sections of pyrite and arsenopyrite. It can be seen from the table that on the surface of pyrite and arsenopyrite, Fe mainly exists in the valence state of +2, and a small amount exists in the valence state of +3; S mainly exists in the form of-1, with a small amount of elemental sulfur and sulfate. On the surface of pyrite, more than 50% As exists in sulfide in the form of-1, in addition, 18% ~ 20% As exists in the form of +3 oxide, and about 30% As exists in the form of +5 oxide. On the surface of arsenopyrite, more than 60% of As exists in the form of-1 valence in sulfide, nearly 30% As exists in the form of +3 valence oxide, and the rest As is mainly +5 valence oxide. Figs. 6.9 to 6. 14 are XPS spectra of Fe 2p, S 2p and As 3d on the surface of pyrite and arsenopyrite particles, respectively.
Table 6.3 Substance types and (relative) atomic percentages of As 3d, S 2p, Fe 2p and Fe 2p on pyrite and arsenopyrite surfaces
sequential
Fig. 6.9 Energy spectrum of Fe 2p XPS on pyrite particle surface
(According to Yang, 2006)
A-PD 1 12-2 pyrite sample; B-45 pyrite sample
Fig. 6. Fe 2p XPS spectrum of10 arsenopyrite particle surface.
(According to Yang, 2006)
A-45 arsenopyrite sample; B-27 arsenopyrite sample
Fig. 6. XPS spectrum of11pyrite particle surface.
(According to Yang, 2006)
A-PD 1 12-2 pyrite sample; B-45 pyrite sample
Fig. 6. XPS spectrum of12s2p arsenopyrite particle surface.
(According to Yang, 2006)
A-45 arsenopyrite sample; B-27 arsenopyrite sample
Sulfides such as pyrite and arsenopyrite will be oxidized when exposed to air. A lot of work has been done on the oxidation of sulfide (Hyland et al., 1989,1990; Naisbitt et al.,1995,2000; Schaufuss et al., 2000; Lan et al., 2000; Jia Jianye et al., 2000; Yu et al., 2000) found that various oxidation products can be formed during sulfide oxidation, such as FeSO4, Fe2 (SO4) 3, FeO, FeOOH, Fe2O3, Fe3O4, SO, polysulfate (etc. ) and polysulfides. By removing the characteristic information of surface oxidation products of pyrite, arsenopyrite and other sulfides, the crystal characteristic information before oxidation can be obtained.
Fig. 6. Three-dimensional XPS energy spectrum of13 pyrite particle surface.
A-PD 1 12-2 pyrite sample; B-45 pyrite sample
Fig. 6. Three-dimensional XPS energy spectrum of14 arsenopyrite particle surface
A-45 arsenopyrite sample; B-27 arsenopyrite sample
The oxidation of sulfide minerals in O2, steam or atmosphere is equivalent to neutral medium conditions. The reaction mechanism is that Fe2 ++ in pyrite, pyrrhotite and chalcopyrite and As- in arsenopyrite diffuse from lattice to crystal face to react with oxygen (Buckley et al.,1984; Mycroft et al.,1995; Naisbitt et al.,1995; Yin et al.,1995; Knipe et al., 1995).
6.3.2 Occurrence state of arsenic, iron and sulfur on arsenic-bearing pyrite surface
The experimental results show that besides As 3+, As 5+, Fe 3+, SO and plasma, as-, Fe2+ and s- are still the main existing forms of these three elements on the surface of pyrite oxide (Tables 6.2 and 6.3; Figure 6.9, Figure 6. 1 1, Figure 6. 13). It can be seen that the surface of pyrite oxide still retains its original internal structural characteristics.
After removing the information of these oxidation products, it is found that more than 50% of As exists in arsenic-bearing pyrite in the form of As-, which indicates that some As elements in arsenic-bearing pyrite replace one s- in [S2]2- to form [As] 2-, and combine with Fe2 ++ to form Fe As SPy with pyrite structure.
There may be two reasons for the 0/8% ~ 20% as3+ on pyrite surface: First, As- only exists in pyrite (Table 6.3; Fig. 6. 13), when pyrite is exposed to air, As- is oxidized to intermediate product As3+, and As3+ is further oxidized to final product As5+; Secondly, not only As- exists in pyrite, but also As 3+ exists in its lattice cation position. As mainly exists in the oxidation state of as3+ and as5+ in natural fluid system (Ballantyne et al.,1988; Arehart et al., 1993), but under the reduction condition of pyrite precipitation, As3 ++ is the main one, and when H2S in the solution is completely oxidized, As5 ++ will be formed (Stauffer et al., 1980). When pyrite is formed, part of as3+ in natural fluid is reduced to As- instead of S- in p- sulfur ions to form [As S] 2-. Bostick et al. (2003) studied the adsorption reaction of +3 valence As on the surface of meteorite iron sulfide (Fe S) and pyrite (Fe S2) in anoxic environment. It is found that in the weak acidic fluid with low sulfide content, As3+ is adsorbed on Fe and S at the same time, forming amorphous Fe As S structure. The bond length of arsenic sulfur and arsenic iron is about 2.4 A, which is close to that of arsenic sulfur and arsenic iron in arsenopyrite (2.37 A and 2.35? )。 Cook et al. (1990) think that when As enters the pyrite structure, some [Ass]4- anion pairs will be generated, which will lead to the unbalanced electricity price of pyrite structure. At this time, some trivalent positive ions such as As 3+, au3+ and Sb3+ will be located in the Fe position to balance the electricity price. It can be seen that there are many factors that cause As ~ (2+) to exist on the surface of pyrite, and which factor is dominant still needs further study.
The Fe ~ (3+) on the surface of pyrite after crushing is caused by the oxidation-reduction reaction in which Fe ~ (2+) is oxidized to Fe ~ (3+) and S- is reduced to S ~ (2-) (Nesbitt et al.,1998,2000; Uhlig et al, 2001; Harmer et al., 2004), partly because Fe2 ++ ions are oxidized to Fe3 ++ ions in air (Figure 6. 15).
Fig. 6. Schematic diagram of15 pyrite structure
(According to Harmer, 2004)
On the surface of crushed pyrite, the formation of Fe-O ion clusters is very slow (Schaufuss et al.,1998; Nesbitt et al., 2000). If formed, the peak of Fe-O will appear at 7 1 1 e V or 710 ~ 712 ev (Mcin Tyre et al., 1977).
Sulfur on the surface of pyrite is the most active. When exposed to air 1min, about 80% of S2- will be destroyed, and at the same time, S-O bonds with binding energy of 165 ~ 170 eV will be formed, and the strongest peak will be formed at the position of 169 e V, resulting in a large number of sulfate radicals (Schaufuss et al., 655
These are consistent with the experimental results (Table 6.3). Fe-O ion groups and a large number of iron ions are formed on the surface of pyrite.
6.3.3 Occurrence state of arsenic, iron and sulfur on arsenopyrite surface
As is mainly As- on the surface of unoxidized arsenopyrite, and there is also 15% As0(Nesbitt et al., 1995). In air, arsenic will be oxidized to ions with valence of-1 to +5 on the surface of arsenopyrite, among which As0, As2 ++ and As3 ++ are intermediate products. The oxidation of arsenic on arsenopyrite surface is realized by single electron migration. In negatively charged As-S ion clusters, As-S ions will migrate to the surface in the form of As0, leaving S2- on the lower horizon (Schaufuss et al., 2000).
The experimental results show that As mainly exists in the form of As-(more than 60%), and the rest as exists in the form of As 3+(about 30%) and As 5+. Obviously, arsenic is mainly combined with S- in arsenopyrite structure to form [As S]2-. As 5+ is the final product of arsenopyrite oxidation, while As3 ++ is not only an intermediate product, because about 65,438+05% of arsenopyrite will be generated after being exposed to air for 25 h, and As- will still be As. 1995), but the content of As3d(As3+) on arsenopyrite surface with binding energy of 43.9 e V is much higher than 15%, so the source of As3+ on arsenopyrite surface may be complicated.
Johan et al. (1989) pointed out that Au actually exists in arsenopyrite, not in excess As in Fe position, and the lack of Fe content is attributed to the existence of complex solid solutions (Fe, Au, As, Sb), such as 1 xs 1 x and 2As=Au (or Sb)+Fe. Cook et al. (1990) pointed out that invisible gold is preferentially enriched in arsenopyrite structure, and Au 3+mainly replaces Fe 3+in arsenopyrite and combines with [as s] 4-. Zhang Fuxin et al. (2000) studied Carlin-type gold deposits such As Jinlongshan and Qiu Ling in Qinling Mountains, and found that As can not only replace S2- in complex anions, but also replace some cations with As 3+ in pyrite and arsenopyrite, resulting in excessive As in pyrite and arsenopyrite.
Accordingly, arsenic in arsenopyrite of Yangshan gold mine may exist in two ways, one is only as-; Secondly, arsenic and arsenic coexist in arsenopyrite structure.
Iron will be enriched on the surface of arsenopyrite during oxidation, just as iron is enriched on the surface of pyrrhotite (Pratt et al., 1994). The different oxide thin layers formed on the surface of arsenopyrite are iron 1-X-ZAS 1-YS, Fexasyo and Z FeOOH (Schaufuss et al., 2000). It can be explained that in the atomic ratios of Fe:S:As on the surface of arsenopyrite (1:0.955:0.70 1 and 1:0.844:0.604 respectively), the content of Fe is obviously higher than that of S and As.
The content of Fe3+ on arsenopyrite surface is 4.45% and 4.04% respectively, which is higher than that on pyrite surface (1.06% and 2.70% respectively), indicating that it is easier to enrich high-valent iron on arsenopyrite surface (Table 6.3; Figure 6.9, Figure 6. 10).
Through the above analysis, it can be considered that As element in arsenic-bearing pyrite and arsenopyrite in Yangshan Gold Mine mainly exists in the form of-1, and As element mainly replaces S in the S-S anion group of pyrite and presents negative valence, and As is oxidized to +3 and+5 at the same time on the mineral surface; In addition, As may be located in the cation position of pyrite and arsenopyrite lattice in positive valence state, which needs further experimental confirmation. Iron in arsenopyrite of Yangshan Gold Mine mainly exists in valence state of +2, and Fe2 ++ in arsenopyrite is more easily oxidized than pyrite. The surface S of pyrite arsenopyrite in Yangshan Gold Mine is mainly-1, and a small amount of S exists in the form of elemental sulfur and sulfate.