Ruizhongite, (Ag₂☐)Pb₃Ge₂S₈, a thiogermanate mineral from the Wusihe Pb-Zn deposit, Sichuan Province, Southwest China

Germanium (Ge) is an important element that exhibits a low average crustal abundance of ~1.6 ppm (Taylor and McLennan 1985). Owing to its diverse applications, including the production of fiber-optic systems, infrared optics, polyethylene catalysts, and solar cells, this element has been designated as a critical metal (USGS 2018). Considering that approximately 30% of Ge consumed around the world is produced from recycled materials, primary sources, such as zinc ore residues, coal ash, and flue dust, are unable to satisfy the demand (Frenzel et al. 2016; USGS 2018). China is a major producer of Ge, primarily as a byproduct of Pb-Zn ores, especially sphalerite, and galena. The occurrence and enrichment mechanism of Ge have been extensively investigated because of its economic importance.

According to Höll et al. (2007) and mindat.org, 37 naturally occurring Ge-bearing minerals have been identified and approved by the Commission of New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA-CNMNC). Among them, 18 are Ge-sulfide minerals, with argyrodite, briartite, renierite, and germanite being relatively common. In laboratory, a series of thiogermanates, such as (A_1–2xM_x)₂M₃Ge₂Q₈ (0 ≤ x ≤ 0.5, A = Ag, Cu, Na; M = Pb, Eu, Ba; Q = S, Se), have been prepared, but their occurrence in nature has not been reported (Poduska et al. 2002; Iyer et al. 2004).

Ruizhongite, a thiogermanate mineral, was identified in the Wusihe Pb-Zn deposit in Sichuan Province, Southwest China, during an investigation of the mineralogy of this deposit. In the present study, the polarized optical microscopy, scanning electron microscopy, electron microprobe, μ-X-ray diffraction, and Raman spectroscopy analyses were utilized to characterize the occurrence, optical property, chemical composition, and crystal structure of ruizhongite. Both the mineral and its name have been approved by the IMA-CNMNC (2022-066). Type specimens are preserved in the Geological Museum of China, Beijing, China (Catalog number M16138).

Ruizhongite was named in honor of Ruizhong Hu (1958) of the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS). Prof. Hu obtained his Ph.D. from Chengdu College of Geology (currently known as Chengdu University of Technology) in 1988. Including two years of post-doctoral research, he has been working in the IGCAS since 1989 and earned a high reputation with over 200 publications and seven major awards on mineral resources and the geochemistry of ore deposits. These studies include the proposal of new methods and elucidation of intracontinental metallogenesis and mechanisms of the formation of the ores involving critical metals such as Ge. In 2021, he was elected a member of the Chinese Academy of Sciences. In the present study, the morphology, composition, physical properties, and crystallography of ruizhongite are described. A comparison with synthetic analogs, including AgPb_0.5Pb₃Ge₂S₈, PbPb₃Ge₂S₈, and (CuPb_0.5)Pb₃Ge₂S₈ is also provided.

Ruizhongite occurs in the Wusihe Pb-Zn deposit at E102°53′23.0″ and N29°16′27.0″, ~25 km southeast of the Hanyuan County, Sichuan Province. This deposit is estimated to contain 3.7 Mt of Pb and Zn with grades of 8.6 and 2.0%, respectively (Xiong et al. 2018). The Wusihe deposit is part of the Sichuan–Yunnan–Guizhou (SYG) Pb-Zn metallogenic province in the western Yangtze Block (Online Materials¹ Fig. OM1a; Wang 2005; Zheng 2012; Hu et al. 2017). Exposed rocks in the Wusihe deposit area are comprised mainly of the Ediacaran Dengying Formation and Cambrian to Permian marine sequences (Online Materials¹ Fig. OM1b). Mineralization in the Wusihe deposit was controlled primarily by the Wangmaoshan and Maotuo faults, and ore bodies are hosted predominantly in carbonaceous shales of the Lower Cambrian Qiongzusi Formation and siliceous dolomites of the Dengying Formation (Online Materials¹ Fig. OM1b). The contact zone between the Qiongzusi and Dengying formations also hosts some ore bodies. Despite the fact that the ores are either massive, disseminated, or vein-type, the mineral assemblages are similar.

The Wusihe deposit has been characterized as a Mississippi Valley-type (MVT) Pb-Zn deposit that formed at ~411 Ma (Xiong et al. 2018), and according to previous studies, the Pb and Zn originated from both the basement and host rocks (e.g., Xiong et al. 2018; Zhang et al. 2019; Wei et al. 2020). Sphalerite samples that were collected from the deposit have Ge concentrations that range from ~3 to 1934 ppm (mean = 563 ppm), attributed to the substitution of Zn (Luo et al. 2021). Ruizhongite occurs as tiny disseminated grains, 1 to 10 μm in size, in close association with galena, jordanite, and argutite in the sphalerite matrix (Fig. 1).

Ruizhongite is gray with a black streak, and the Mohs hardness is estimated to be 3 to 3.5 (comparable to that of jordanite). It is brittle without cleavage. The calculated density is 5.706 g/cm³ based on the empirical formula and unit-cell volume refined from single-crystal XRD data. According to tests using a magnetic needle, the mineral is nonmagnetic.

In reflected light, ruizhongite shows a greenish-gray color (Figs. 1a–1d) without bireflectance, pleochroism, anisotropy, or internal reflection. Reflectance values measured in air by Leitz MSP-UV-VIS 2000 microphotospectrometer using SiC as the reference material are presented in Table 1.

Raman spectra for ruizhongite were obtained using a Horiba ARAMIS micro-Raman system at the School of Geosciences and Info-physics, Central South University. A solid-state laser instrument with a wavelength of 532 nm and a thermoelectric-cooled CCD detector with a resolution of 2 cm⁻¹ and a spot size of 1 µm was used for excitation. Raman spectra were acquired from 100 to 1000 cm⁻¹, and the measurement time for each spectrum was 180 s. Polished thin sections of randomly oriented ruizhongite crystals were utilized for the Raman spectroscopy measurements. In a typical spectrum, sharp peaks are observed at 81, 217, and 348 cm⁻¹, whereas intermediate peaks are visible at 111 and 406 cm⁻¹, and weak peaks are present at 261 and 361 cm⁻¹ (Fig. 2a). The Raman spectrum of ruizhongite is comparable to that of synthetic Ag_0.5Pb_1.75GeS₄ (Fig. 2b) (Iyer et al. 2004). The peaks between 300 and 500 cm⁻¹ can be assigned to Ge-S stretching vibrations in GeS₄ tetrahedra, whereas peaks lower than 300 cm⁻¹ are attributed to S-Ge-S bending vibrations in GeS₄ tetrahedra, as well as Pb-S and (Ag,Pb,Zn)-S stretching vibrations.

The chemical composition of ruizhongite was characterized using a Shimadzu-1720 electron microprobe via wavelength-dispersive spectrometry. Measurements were conducted at the School of Geosciences and Info-physics, Central South University, and the acceleration voltage, beam current, and beam size were 15 kV, 10 nA, and 1 μm, respectively. A qualitative scan of elements reveals the presence of S, Pb, Fe, Ag, Zn, and Ge, and the quantitative analyses were made using pure Ge, PbS, Ag, ZnS, and FeS₂ as standards, and the ZAF4 program of the instrument was used for correction. Data for the composition of ruizhongite are presented in Table 2. Average data from the 18 analyses yield Pb (57.37 wt%), S (21.39 wt%), Ge (11.53 wt%), Ag (7.34 wt%), Zn (1.57 wt%), and Fe (0.27 wt%), with a total of 99.46 wt%. The empirical formula calculated on the basis of 8 S apfu was (Ag_0.82Pb_0.32Zn_0.28Fe_0.06)_Σ1.48Pb₃Ge_1.90S₈, and the simplified formula is (Ag,Pb,Zn,Fe,☐)₃Pb₃Ge₂S₈. According to the crystal structure and IMA rule of end-member (Hatert and Burke 2008), the ideal formula is (Ag₂☐)Pb₃Ge₂S₈.

A single crystal of ruizhongite about 6 × 5 × 4 μm³ (Fig. 1d) was extracted for X-ray diffraction by focused ion (Ga+) beam (FIB) workstation equipped in the FEI Helios Nanolab 600i systems at the National Key Lab of Powder Metallurgy, Central South University. Powder XRD measurements were performed using a Rigaku XtaLAB Synergy diffractometer. Measurements were conducted using CuKα radiation at 50 kV and 1 mA in the Gandolphi powder mode. However, only incomplete lines are observed due to the extremely small size of sample. Thus, calculated X-ray powder diffraction data according to the crystal structure are presented (Table 3). The strongest lines [d in Å (I) (hkl)] are 3.755 (100) (123), 3.511 (76) (004), 2.992 (73) (233), 2.574 (21) (125), 2.482 (79) (044), 2.276 (46) (235), 1.784 (39) (237), and 2.075 (24) (136). The refined unit-cell parameters from powder X-ray diffraction data are a = 14.0443(5) Å, V = 2770.14(28) ų, and Z = 8.

Single-crystal XRD measurements were conducted using a Rigaku XtaLAB Synergy-DS diffractometer at the School of Geosciences and Info-physics, Central South University, China. The instrument, which involves a microfocus-sealed Cu anode tube, was operated at 50 kV and 1 mA. The unit-cell parameters are: a = 14.0559(2) Å, V = 2777.00(7) ų, and Z = 8. Based on reflection data, the candidate space group is I43d (no. 220). The Rigaku CrysAlisPro software package was used to process diffraction data, including the Lorentz and polarization corrections. An empirical absorption correction was employed via a multi-scan method using ABSCOR (Higashi 2001).

The crystal structure of ruizhongite was determined and refined using the SHELX (Sheldrick 2015a, 2015b) and Olex2 (Dolomanov et al. 2009) packages. Crystallographic and refinement statistics data are presented in Table 4. The structure based on the I43d space group involves two anion (S1 and S2) and three cation (Ge1, Pb1, and Ag1) sites (Fig. 3a). Refinement shows that the Ge1 and Pb1 sites are fully occupied, whereas Ag1 is partially occupied. Therefore, based on compositional data, the occupancies of Ag, Pb, Zn, and Fe at Ag1 site were manually adjusted. The atomic coordinates and displacement parameters are presented in Table 5, while selected bond lengths and angles are listed in Table 6. The final anisotropic full-matrix least-squares refinement of F² was converged to an R₁ of 2.98% and wR₂ of 6.66% for the 447 independent reflections (F_o>4σ) and an R₁ of 3.23% and wR₂ of 6.76% for all 2594 (474 unique) reflections (Table 4).

Ruizhongite is isostructural with synthetic Ag_0.5Pb_1.75GeS₄ (Iyer et al. 2004) and Pb₂GeS₄ (Poduska et al. 2002). The structure involves a non-centrosymmetric arrangement of [GeS₄]⁴⁻ tetrahedra, with interstices occupied by Pb1 and Ag1 aligned along the a-, b-, and c-axes (Fig. 3b). The Ge-S bond lengths in the slightly distorted [GeS₄]⁴⁻ tetrahedra vary from 2.208 to 2.242 Å (mean = 2.217 Å) (Table 6). The Pb1 is coordinated to four sulfur atoms, forming pyramid-like geometry at distances ranging from 2.839 to 2.868 Å (mean = 2.854 Å) (Figs. 3a and 3c), and to four additional sulfur atoms at distances that vary from 3.421 to 3.468 Å. In contrast, the Ag1 site is partially occupied by Ag (0.28), Pb (0.12), Zn (0.10), and Fe (0.02), and coordinated to four sulfur atoms, thereby in a shape of quadrangles at distances from 2.708 to 2.898 Å (Figs. 3a and 3c). The Ag1 sites are clustered and aligned along the a-, b-, and c-axes, and the Ag1-Ag1 distance is 0.717 Å (Table 6; Fig. 3b).

Ruizhongite is the first natural occurrence of non-centro-symmetric cubic thio- and seleno-germanates with the following general formula: (A_1–2xM_x)₂M₃Ge₂Q₈ (0 ≤ x ≤0.5, A = Ag, Cu, Na; M = Pb, Eu, Ba; Q = S, Se), that have previously been synthesized (e.g., Poduska et al. 2002; Iyer et al. 2004; Choudhury et al. 2007; Reshak et al. 2013). This group can be extended to include thiostannates with similar cubic structure and space group, such as Ag₂CdBa₆Sn₄S₁₆ and Ba₃CdSn₂S₈, if A = Ag, Cd, M = Ba, and Q = Sn (e.g., Teske 1985; Zhen et al. 2016). The Ba₃CdSn₂S₈ structure is highly tolerant of vacancies and disorder, and thus many elements can be substituted into it (Iyer et al. 2004). Moreover, the Ba₃CdSn₂S₈ structure can be formed with different tetrahedral units such as [PSe₄]^3–, [GeS₄]^4–, or [SnS₄]^4- when cation sites were substituted by +1 or +2 metals with or without introducing disorder and/or vacancies (Aitken et al. 2000). Therefore, it can be predicted that more minerals with similar structure can be present in nature or synthesized in laboratory. Iyer et al. (2004) inferred that a series of compounds such as Sr_0.5Pb_1.5GeQ₄, Sr_0.25Pb_1.75GeQ₄, Na_0.5Sn_1.75GeQ₄, Cu_0.5Eu_1.75SnQ₄, and Cu_1.5Pb_0.75AsQ₄ (Q = S, Se) can be present. Therefore, Sr- or Na-bearing Ge minerals will be an important target of new minerals in the future. Considering that Na and Sr are incompatible in sulfides such as sphalerite, galena and pyrite, the discovery of Sr- or Na-bearing Ge minerals should focus on gangue minerals such as calcite, dolomite, and feldspar.

Crystallographic data for ruizhongite, synthetic (AgPb_0.5) Pb₃Ge₂S₈, PbPb₃Ge₂S₈, and (CuPb_0.5)Pb₃Ge₂S₈ are presented in Table 7. Ruizhongite and synthetic Ag_0.5Pb_1.5GeS₄ have the same Ge-S distance but slightly different M-S and A-S distances. These differences indicate that significant variations in bond lengths at A and M sites may be caused by the replacement of Ag by Cu and more incorporation of Pb into the A sites, whereas GeS₄ tetrahedra are just slightly altered. Compared to synthetic Ag_0.5Pb_1.5GeS₄, the Raman spectra of ruizhongite shows characteristic peaks at 81 and 111 cm⁻¹ (Fig. 2a). This difference in Raman spectra is possibly caused by minor amounts of Zn and Fe in ruizhongite.

The synthetic compounds are mainly produced by high-temperature (510–650 °C) heating and annealing processes (Poduska et al. 2002; Iyer et al. 2004), which may provide constraints on the formation condition of ruizhongite from the Wusihe deposit. The pure crystal of Ag_0.5Pb_1.75GeS₄ can be obtained by direct combination of the elements at 650 °C in a sealed silica tube, cooling to 250 °C at a rate of 5 °C/h and then rapidly cooling to room temperature (Iyer et al. 2004). It is thus inferred that ruizhongite may have been formed at a rapid cooling rate from initial Ge-bearing fluids.

Ruizhongite appears chemically related to morozeviczite (Pb₃Ge_1–xS₄) (Harańczyk 1975; Anthony et al. 2016). However, the significant differences in the unit-cell parameters (a = 10.61 Å) and powder XRD data [3.08(10), 2.15(9), 2.80(6), 2.047(6), and 1.791(5)] exclude the possibility of morozeviczite being Pb-dominated at the disordered Ag1 site in ruizhongite.

Thiogermanate minerals are the principal carriers of Ge in nature. There are 17 thiogermanate minerals identified according to the data of http://www.mindat.org. Ruizhongite is the Ag end-member with the Ag1 site dominated by Ag in a group of noncentrosymmetric cubic thio- and seleno-germanate isostructural analogs, and the variation of chemical composition also indicates possible end-members of (Pb☐₂)Pb₃Ge₂S₈, (Zn☐₂)Pb₃Ge₂S₈, and (Fe☐₂)Pb₃Ge₂S₈.

Sphalerite may be an important bearer of Ge, because the ionic radii of Ge²⁺ is similar to Zn²⁺ (0.73–0.74 Å) but significantly different from Ge⁴⁺ (0.39–0.53 Å) (Shannon 1976). In previous studies, mechanisms for the substitution of Zn with Ge, including the simple and coupled substitution, have been proposed (e.g., Höll et al. 2007; Cook et al. 2009; Belissont et al. 2014). The discovery of ruizhongite in sphalerite further supports the existence of nano-metric to micrometric Ge-independent minerals in sphalerite under a local and relatively oxidized environment, in addition to those associated with isomorphic substitution. Therefore, correlations between the concentrations of Ge and those of Ag, Pb, and Fe for sphalerite samples determined using laser ablation ICP-MS must be treated with caution in regard to Ge substitution mechanisms in sphalerite. High-resolution scanning and transmission electron microscopy are also needed to characterize Ge-containing minerals in sphalerite and other mineral phases.

The presence of ruizhongite in sphalerite indicates that fluids responsible for sphalerite were periodically supersaturated with Ge. However, based on available data, the formation mechanism of ruizhongite remains unclear. Therefore, Ge-rich fluids and physicochemical conditions associated with the formation of ruizhongite in sphalerite require further investigation. The composition of ruizhongite suggests a close relationship between Ge and Pb, and thus, factors controlling the affinity of Ge for Pb relative to Zn also require attention. Therefore, an improved understanding of geochemical characteristics during the crystallization of sphalerite and galena can highlight the mechanism of Ge enrichment in Pb-Zn deposits.

This research was financially supported by the National Key Research and Development Program of China (Grant No. 2021YFC2900300), National Natural Science Foundation of China (42073043, 42072054), CAS Hundred Talents Program to XWH, Field Frontier Key Project of State Key Laboratory of the Ore Deposit Geochemistry (202101), and Guizhou Provincial 2020 Science and Technology Subsidies (No. GZ2020SIG).

We thank Shaohua Dong for her assistance with the SEM analysis, as well as Ming Wang at the Wusihe deposit of Sichuan Qiansheng Mining Co., Ltd., and Chuan Lv and Yunhe Zhou of the IGCAS for their help with the field investigation. The manuscript was improved based on critical comments of Luca Bindi and two anonymous reviewers.