The discovery site is the Haopinggou Ag-Pb-Zn-Au deposit, located roughly 60 kilometers southwest of Luoning County in the eastern Qinling mountains, one of China’s great metallogenic provinces. Haopinggou sits within the world-class East Qinling porphyry molybdenum belt, and the deposit itself is notable as the first documented example of an intermediate-sulfidation epithermal system in that belt. Epithermal deposits, formed from hot aqueous fluids circulating at shallow crustal levels, represent some of the planet’s most significant silver resources, and understanding precisely which minerals carry the silver in such systems is central both to exploration science and to the broader geochemistry of metal transport. The sample that yielded fengruiite came from the H5 polymetallic sulfide orebody, a vein structure between 0.3 and 2.8 meters wide that extends more than 1.5 kilometers, and was collected underground at an elevation of 380 meters from a galena-quartz vein.

The host rocks tell an ancient story. The H5 orebody cuts through amphibolite-facies metamorphic rocks of the Taihua Group, dominated by biotite plagiogneiss, amphibolite gneiss, and amphibolite, rocks likely formed in the Neoarchean and metamorphosed during the Paleoproterozoic. Into this ancient basement, Late Cretaceous magmatic-hydrothermal events injected the metal-bearing fluids that precipitated quartz, carbonates including siderite, ankerite, and calcite, pyrite, sphalerite, galena, chalcopyrite, and tetrahedrite in open-space fillings, veins, and hydrothermal breccias. Silver-bearing minerals are abundant throughout the galena-sphalerite-tetrahedrite assemblage, including argentiferous tetrahedrite, stromeyerite, pyrargyrite, polybasite, jalpaite, cervelleite, argentite, and native silver. Fluid inclusion studies indicate that fengruiite was deposited at intermediate temperatures of 163 to 213 degrees Celsius from fluids of moderate salinity, between 7.2 and 14.0 weight percent NaCl equivalent.

Physically, fengruiite is unassuming. It occurs as irregular grains smaller than 20 by 80 micrometers, intergrown with galena, cervelleite, and chalcopyrite. The mineral is opaque, gray with a bluish tint in reflected light, and shows a metallic luster with a steel-gray to iron-black streak. It is brittle, fractures unevenly, and has an estimated Mohs hardness of 3 to 4, judged from scratch tests with fine calcite and fluorite particles. Its calculated density is a hefty 6.62 grams per cubic centimeter, a direct consequence of its extraordinary silver content. Electron probe microanalysis revealed that silver makes up between 65.89 and 68.63 weight percent of the mineral, with a mean of 66.92 weight percent, accompanied by sulfur at roughly 13 weight percent, antimony near 10 percent, tellurium between 6.34 and 7.56 percent, and copper around 3.5 percent, plus traces of arsenic.

Extracting a crystal suitable for structural analysis from such a tiny intergrowth demanded considerable ingenuity. The team selected a single homogeneous domain under the reflected-light microscope, then used a dual-beam focused ion beam platform to carve out a crystal measuring just 0.02 by 0.02 by 0.01 millimeters. Single-crystal X-ray diffraction at the China University of Geosciences in Beijing, using a rotating anode microfocus source, revealed a trigonal structure in space group P-3m1 with unit-cell parameters a = 7.6087 angstroms and c = 11.970 angstroms. The architecture is that of the Tac polytype of the pearceite-polybasite group: two chemically distinct layers, an A module of composition [Ag6Sb2S7] carrying negative charge and a B module of composition [Ag9CuS2Te2] carrying positive charge, stacked in alternation along the crystallographic c axis. In the A layer, antimony atoms form well-defined SbS3 pyramids, while in the B layer copper sits in linear coordination between two sulfur atoms.

The decisive discovery lay in where the tellurium resides. Refinement of the diffraction data showed that the mixed Te1/S1 site near the center of the B layer is occupied by tellurium at 0.680 and sulfur at 0.320, meaning tellurium is the dominant constituent at that crystallographic position. Meanwhile, every anion site in the A layer remains sulfur-dominant; tests for tellurium substitution at the S2 site yielded only a negligible 0.05 occupancy, and the S3 and S4 sites refined to essentially full sulfur. This distribution is what separates fengruiite from its closest relatives. In the Te-rich polybasite-Tac described in earlier work, tellurium is present in significant amounts but never dominates any individual site, which is why that crystal was not classified as a new mineral. In benleonardite, by contrast, tellurium invades anion sites in both the A and B modules. Fengruiite alone combines a Te-dominant site in the B layer with sulfur-dominant sites throughout the A layer.

The structural refinement also exposed something stranger: the silver atoms refuse to sit still. In the A layer, the electron density assigned to silver is locally droplet-like and was modeled by two mutually exclusive split positions whose occupancies sum to about one, representing alternative statistical positions of a disordered population rather than two simultaneously occupied sites. In the B layer, the silver electron density extends along pseudohexagonal two-dimensional pathways parallel to the crystallographic ab plane, sampled by three partially occupied positions. An auxiliary anharmonic refinement using third-order Gram-Charlier coefficients confirmed the strongly non-harmonic character of this density and improved the agreement factors. Comparable extended silver distributions in other pearceite-polybasite minerals have been interpreted as potential pathways for silver-ion migration, consistent with the ionic-conducting behavior documented for this group, although the authors caution that no conductivity or temperature-dependent diffraction experiments were performed, so dynamic diffusion remains unproven in fengruiite itself.

The researchers were equally careful about the chemistry. Because silver-rich sulfosalts can suffer electron-beam-induced silver migration during microanalysis, they compared results from a standard 3 to 5 micrometer beam with a defocused 8 micrometer beam and found no systematic silver depletion, indicating the reported compositions are robust. Reflectance measurements across the visible spectrum showed that fengruiite and coexisting cervelleite overlap substantially in optical properties, meaning reflectance alone cannot distinguish them and identification rests on composition and structure. The team also emphasizes that bulk tellurium content measured by electron microprobe cannot by itself assign a species among Te-rich polybasite-Tac, fengruiite, and benleonardite, since the probe cannot resolve where tellurium sits within the layered structure; provisional total-tellurium guidelines exist, but definitive identification requires site-occupancy data from single-crystal diffraction.

Beyond taxonomy, the find carries a provocative geochemical message. Low-melting chalcophile elements such as arsenic, antimony, bismuth, selenium, and tellurium are well known as agents of gold enrichment in hydrothermal systems, yet their role in silver concentration has remained underexplored. Fengruiite, the first structurally characterized Ag-Sb-Te sulfosalt from the East Qinling metallogenic belt, suggests that tellurium helped trap and concentrate silver as hot fluids cooled. The Te-dominant site indicates that tellurization, coupled with antimony-rich sulfidation, created a favorable thermodynamic pathway for stabilizing silver in a complex sulfosalt framework rather than in minor substitutions within simple sulfides. In effect, antimony and tellurium acted as anchors, selectively partitioning silver into intricate crystalline structures during fluid cooling. If similar low-melting chalcophile element associations with silver turn up in other epithermal systems worldwide, the humble gray grain from Haopinggou may prove to be a template for understanding, and perhaps finding, the silver deposits of the future. The mineral is named in honor of Rui Feng, the exploration geologist whose guidance added more than 5,000 tonnes of silver metal to the Xiayu orefield, and the type material is preserved at the Geological Museum of China in Beijing.

Subject of Research: Crystal structure and ore-forming significance of the new tellurium-rich silver sulfosalt mineral fengruiite from an epithermal deposit in eastern Qinling, China

Article Title: Fengruiite, [Ag6Sb2S7][Ag9CuS2Te2], a new Ag–Sb–Te sulfosalt mineral from the Haopinggou Ag–Pb–Zn–Au deposit, eastern Qinling, China

Article References: Tian, Y., Li, G., Sun, N., Liu, M., Mao, J., Dong, Y., Liu, P., Jian, W., Yao, W., Wang, X., & Ye, H. (2026). Fengruiite, [Ag 6 Sb 2 S 7 ][Ag 9 CuS 2 Te 2 ], a new Ag–Sb–Te sulfosalt mineral from the Haopinggou Ag–Pb–Zn–Au deposit, eastern Qinling, China. European Journal of Mineralogy, 38(4), 519-530. https://doi.org/10.5194/ejm-38-519-2026

Image Credits: AI Generated

DOI: 10.5194/ejm-38-519-2026

Keywords: fengruiite, sulfosalt minerals, silver deposits, tellurium, epithermal systems, crystal structure, East Qinling, mineralogy, IMA new mineral, hydrothermal fluids, pearceite-polybasite group, ore deposit geochemistry