Characterization of synthetic and natural gold chalcogenides by electron microprobe analysis, X‐ray powder diffraction, and Raman spectroscopic methods

Abstract

Electron microprobe analysis (EMPA), X-ray powder diffraction (XRD), and Raman spectroscopy (RS) were applied to characterize the synthetic gold chalcogenides of the Au-Te-Se-S system and natural analogs from the Gaching deposit (Central Kamchatka, Russia). The EPMA results showed that the synthetic chalcogenides have different Te/Se/S and Au/X (X = Te + Se + S) ratios: AuX 2 , Au 3 X 10 , and AuX. They are similar in composition to natural compoundscalaverite (AuTe 2 ), maletoyvayamite (Au 3 Te 6 Se 4 ), and unnamed minerals AuTe 0.7 Se 0.3 and AuSe 0.7 S 0.3 . It was established that chalcogenides AuX, Au 3 X 10 , and AuX 2 have a specific Raman spectra with characteristic peaks. The position of the peaks and the character of the spectra of the synthetic phases and their natural analogs from the Gaching deposit coincide within the limits of accuracy. Different ratios of chalcogenes Te/Se/S in compounds influence the Raman peak positions. The positions of the peaks for natural compounds AuX differ depending on the predominance of Te (AuTe 0.7 Se 0.3 ) or Se (AuSe 0.7 S 0.3 ). AuSe synthetic phases consist of a mixture of αand β-polymorphs. Raman spectroscopy can be used for the identification of natural gold chalcogenides worldwide, which are difficult to diagnose by other methods due to the microscopic grain sizes and close intergrowths with other ore minerals. The similarity of the Raman spectra upon changing the concentrations of Se and S suggests identical structures and possible isomorphism in the composition range of AuTe 0.

Tolstykhite, Au3S4Te6, a new mineral from Maletoyvayam deposit, Kamchatka peninsula, Russia

“…The main bands observed in the spectrum are (in wavenumbers): 297, 203, 181, 151 and 127 cm –1 that are close to Raman spectra provided for natural ‘S-maletoyvayamite’ with the composition Au 3 Te 6 (S 3.4 Se 0.6 ) Σ4.0 and synthetic Au 2.92 Te 6.13 (S 2.15 Se 1.80 ) Σ3.95 (Palyanova et al , 2022) as well as for maletoyvayamite Au 3 Se 4 Te 6 (the Se-analogue of tolstykhite) and its synthetic analogue (Tolstykh et al , 2020; Palyanova et al , 2022). The strong bands at 127 and 151 cm –1 in tolstykhite could be tentatively assigned to Au–Te bonds whereas the ones at 181, 297 and the shoulder at 203 cm –1 – to Au–S bonds.…”

“…The strong bands at 127 and 151 cm -1 in tolstykhite could be tentatively assigned to Au-Te bonds whereas the ones at 181, 297 and the shoulder at 203 cm -1to Au-S bonds. According to Palyanova et al (2022), the band in the 280-290 cm -1 range appears when chemical analyses show the presence of S substituting for Se, and the more S is present in the sample, the stronger the corresponding band is. This is in line with the sharp peak of 297 cm -1 in tolstykhite, which is absent in maletoyvayamite and synthetic Au 3 Se 4 Te 6 .…”

Auroselenide, AuSe, a new mineral from Maletoyvayam deposit, Kamchatka peninsula, Russia

“…It is quite unusual that in all the synthesis runs, where α-AuSe and β-AuSe have been produced, these two polymorphs always coexist (see, e.g. Cranton and Heyding, 1968;Rabenau et al, 1971;Machogo et al, 2019;Palyanova et al, 2022), whereas auroselenide does not show any trace of its α polymorph (see Fig. 5, Table 4) in natural conditions.…”

“…The crystal structure of auroselenide shows two Au sites, Au1 with Au + and Au2 with Au 3+ , and one Se site (Figure 6). It is quite unusual that all synthesis runs, where α-AuSe and β-AuSe have been produced, these two polymorphs were always coexisting (see, e.g., Cranton and Heyding, 1968;Rabenau et al, 1971;Machogo et al, 2019;Palyanova et al, 2022), whereas auroselenide does not show any trace of its α polymorph (see Figure 5, Table 4) at natural conditions. This could be related to S and Te which substitute Se in not negligible amounts and likely stabilize the  polymorph.…”

“…Two synthetic polymorphs of gold selenide (α-AuSe and β-AuSe) have been known for a long time and are well studied in solid-state chemistry and experimental mineralogy (Cranton and Heyding, 1968;Rabenau et al, 1971;Cretier and Wiegers, 1973;Rabenau and Schulz, 1976;Ettema et al, 1994;Lee and Jung, 1999;Wagner et al, 2010;Feng and Taskinen, 2014;Palyanova et al, 2016aPalyanova et al, , 2016bPalyanova et al, , 2019Palyanova et al, , 2020Palyanova et al, , 2022Machogo et al, 2019 etc.). However, natural AuSe has now been found in the Maletoyvayam deposit and, as shown below, it corresponds to the β-AuSe polymorph.…”