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

The research, led by George Yandem, Katarzyna Grygoyć and Magdalena Jabłońska-Czapla of the Polish Academy of Sciences together with Joanna Willner and Tomasz Matuła of the Silesian University of Technology, tackles a question that has received surprisingly little systematic attention: how the way you pre-treat a dead panel interacts with the chemistry of the leaching solution to determine which metals dissolve, and how much. Instead of testing one process in isolation, the team ran a full factorial comparison of three pre-treatment routes against four chemically distinct leaching agents, all under identical ultrasonic conditions, using a decommissioned polycrystalline silicon module originally manufactured by ALGATEC Solar AG in Germany.

The three pre-treatments represented the main strategies competing in the recycling world today. Mechanical milling simply ground the module into particles sieved below 0.5 millimetres. Pyrolysis heated the material to 500 degrees Celsius for two hours, burning away the plastic encapsulants that glue the sandwich of glass, silicon and metal together. Acetone washing softened the ethylene–vinyl acetate encapsulant chemically, allowing the glass to be scraped away and the dissolved polymer to be filtered off under vacuum. Each route was then paired with citric acid, oxalic acid, the chelating agent EDTA, or concentrated nitric acid, and leached in an ultrasonic bath at 40 kilohertz and ambient temperature, with a solid-to-liquid ratio of one gram to twenty millilitres over treatment times from ten to sixty minutes.

Before any leaching began, the team characterised exactly what the waste contained and where those elements ended up after grinding. Using microwave digestion and inductively coupled plasma mass spectrometry, they found the material dominated by copper, tin, lead, antimony, chromium, silver and nickel, with lower but environmentally significant concentrations of cobalt, gallium, germanium, indium, molybdenum and tellurium. Crucially, sieving revealed that most of these elements concentrate in the finest dust. Particles smaller than 0.05 millimetres made up only 13.4 to 14.4 percent of the total mass, yet carried the highest concentrations of most elements. That means a recycler could process just the fine fraction, cutting reagent demand, treatment cost and pollutant load dramatically—an insight with immediate practical value, since industrial-scale crushing can push residues toward concentrations comparable to metal ores.

The headline results concern which combination extracts which element. Pyrolysis followed by nitric acid delivered the strongest overall performance, dissolving up to 300 milligrams per litre of silver, 888 milligrams per litre of copper, 256 milligrams per litre of lead and 109 milligrams per litre of tin. Thermodynamic analysis explains why: tin dissolution in nitric acid carries an equilibrium constant of roughly 10 to the power 37, and lead is barely less favourable, so once the polymer barriers are burned away the oxidation reactions proceed almost irreversibly. Silver, by contrast, is the most noble metal in the set, with a small cell potential of just 0.156 volts, yet the ultrasonically assisted nitric acid route still outperformed earlier studies by a factor of nearly six in leached silver concentration—a difference the authors attribute to the sonication and the fine pre-milling.

The greener acids proved unexpectedly powerful for the rarer, geopolitically sensitive elements. Oxalic acid, especially after pyrolysis or acetone treatment, was the best route for antimony and indium, reaching 5.07 and 8.11 milligrams per kilogram of indium respectively—close to the total indium content of the finest digestible fraction. Citric acid excelled at tellurium and germanium, while EDTA with simple milling pulled out cobalt, and acetone treatment followed by oxalic or citric acid preferentially recovered indium, tin and tellurium. The chemistry behind this selectivity is ligand-driven: citrate ions bind metals through tridentate carboxylate and hydroxyl groups, oxalate forms highly stable bis- and tris-oxalate complexes with indium(III) and gallium(III), and antimony(III) oxalate complexes carry a stability constant that makes dissolution spontaneous. In effect, the organic acids act as molecular tweezers, plucking specific ions from the particle matrix while leaving others behind.

Kinetic analysis added a further layer of control. The elements fell neatly into three time groups: silver, arsenic, cobalt, gallium, germanium, molybdenum and nickel peaked within ten minutes; copper, indium, manganese, lead and tin needed thirty; and chromium, antimony, tellurium, thallium and zinc required the full hour. This pattern reflects where each element sits in the material—surface-accessible phases dissolve quickly, while elements buried deeper in the matrix or held by slower ligand-exchange kinetics take longer. Shrinking-core models, the classical framework for hydrometallurgical dissolution, only fit gallium and germanium, indicating that those two elements leach from deep within the particles under chemical reaction control, with germanium showing R-squared values above 83 percent for EDTA and nitric acid. For everything else, multiple mechanisms operate simultaneously, a reminder that ultrasonic cavitation scrambles the tidy assumptions of conventional leaching theory.

Ultrasound itself deserves attention. Acoustic cavitation—the collapse of microscopic bubbles generated by sound waves—strips blockages from mineral surfaces and shatters particles, expanding the reactive area. Previous work has shown ultrasound can shrink waste particles from 30 micrometres to 1 micrometre and cut chemical activation energy from 34.68 to 6.21 kilojoules per mole, producing dramatic jumps in recovery. Here, the ultrasonic bath allowed leaching at ambient temperature in as little as ten minutes, avoiding the heated, hours-long acid treatments typical of the literature. The method’s principal component analysis further confirmed that pre-treatment dominates the overall variance, with pyrolysis samples clustering apart from milled and acetone-washed material, and low-melting-point elements such as cadmium, gallium, vanadium, zinc and thallium responding distinctly to thermal processing.

The industrial context is moving fast. Recent reporting from the International Energy Agency’s Photovoltaic Power Systems Programme shows commercial recyclers such as SOLARCYCLE, Reiling and SPR already running hybrid lines that combine automated deframing, shredding, mechanical sorting and chemical leaching, with capacities between 20,000 and 63,000 tonnes per year. Downstream, dissolved metals are typically converted into sellable products through clarification, selective separation, concentration and final recovery by precipitation, solvent extraction, ion exchange, cementation or electrolysis. The challenge this study addresses is that these hydrometallurgical stages traditionally depend on nitric, sulfuric and hydrofluoric acids—highly corrosive and toxic reagents whose handling and disposal carry their own environmental burden, threatening the very sustainability credentials that solar recycling is meant to deliver.

The authors are candid about the limits of scaling sound waves. Cavitation intensity becomes spatially uneven in large reactors, potentially producing inconsistent leaching, and the energy cost of prolonged ultrasonication must be weighed against the savings from milder reagents. They suggest that combining ultrasound with conventional agitation, rather than relying on it exclusively, may be the pragmatic path, and they call for pilot-scale trials with techno-economic analysis and life-cycle assessment. Still, the core message is striking: end-of-life solar panels are not merely a disposal problem but a concentrated, pre-sorted ore body, and with the right marriage of mechanical, thermal or solvent pre-treatment and green complexing acids—activated by nothing more exotic than sound—a recycler can dial in exactly which critical element to extract. As millions of tonnes of panels head toward retirement, that tunability may prove as valuable as the metals themselves.

Subject of Research: Green sonochemical recovery of technology-critical elements from end-of-life photovoltaic modules using aqueous organic-acid leaching solutions

Article Title: Green sonochemical recovery of technology-critical elements from the end-of-life photovoltaic module using aqueous organic-acid solutions

Article References: Green sonochemical recovery of technology-critical elements from the end-of-life photovoltaic module using aqueous organic-acid solutions. (n.d.). https://doi.org/10.1007/s10098-026-03606-5

Image Credits: AI Generated

DOI: 10.1007/s10098-026-03606-5

Keywords: photovoltaic recycling, sonochemical leaching, technology-critical elements, organic acids, urban mining, silver recovery, indium, tellurium, pyrolysis pretreatment, green chemistry, e-waste, circular economy