October 2026 Precious-Metal Sulfosalts, Lengenbach Thallium & Långban Skarn Breakthroughs
Direct Ore Microscopy & Collector Mineralogy Summary: Opaque and semi-transparent metallic ore minerals cannot be studied in standard transmitted-light microscopes; instead, mineralogists examine polished sections under a Reflected-Light Polarizing Microscope to measure spectral reflectance (R1 and R2 from 400 to 700 nm), reflection pleochroism (bireflectance), crossed-nicols anisotropy, Vickers micro-indentation hardness (VHN), and semiconductor internal reflections. In the October 2026 issue of American Mineralogist (Vol. 111, Issue 10, pp. 1715–1721) and 2026 European journals, researchers unveiled an extraordinary suite of precious-metal, thallium, and skarn collector species: (1) Auropolybasite ([Ag6Sb2S7][Ag9AuS4]), Auropearceite ([Ag6As2S7][Ag9AuS4]), and Fengruiite ([Ag6Sb2S7][Ag9CuS2Te2]) (Mikuš et al., 2026; Tian et al., 2026), three new gold- and tellurium-essential members of the pearceite-polybasite group; (2) Spaltiite (Tl2Cu2As2S5) (Graeser, Topa, Effenberger, Makovicky et al., 2026, Eur. J. Mineral. 38:27–37), a new thallium sulfosalt from the world-famous Lengenbach quarry in Binn, Switzerland; (3) Argentotennantite-(Fe) (Ag6(Cu4Fe2)As4S13) from the San Genaro mine, Peru (Sejkora et al., 2026); and (4) Hugotibergite (Långban, Sweden), Zincocabrerite (Bou Azzer, Morocco), and Babunaite-(Nd) (NdAsO4, North Macedonia, the first rare-earth arsenate in the scheelite group).
🔬 Fresnel Specular Reflectance, Semiconductor Bandgap & Vickers VHN Equations
- Fresnel Normal-Incidence Specular Reflectance in Air (Refractive Index n & Extinction Coefficient k):
R (%) = [ (n − 1)² + k² ] / [ (n + 1)² + k² ] × 100|ΔR = |R₁(ω) − R₂(ε)| - Semiconductor Bandgap Absorption Edge & Crimson Internal Reflection Cutoff (λedge):
λ_edge (nm) = 1239.84 / E_g (eV)→ ForE_g ≈ 1.95 eV, photons withλ > 636 nm (Red)transmit & glow internally - Vickers Diamond Pyramid Micro-Indentation Hardness (VHN in kg/mm2) & Youngbowitz Mohs Relation:
VHN = 1.8544 · P_load (g) / d_diag² (µm²)|Mohs ≈ 0.675 · (VHN)^(1/3)
Complete Reference Chart: 8 October 2026 Precious-Metal, Thallium & Skarn Minerals
| Mineral Species & Formula | Symmetry & Group | Reflectance / VHN / Mohs | Type Locality & 2026 Discovery Significance |
|---|---|---|---|
Auropolybasite & Auropearceite[Ag₆(Sb,As)₂S₇][Ag₉AuS₄] | Trigonal P3̄m1 / P321Pearceite-Polybasite | R ≈ 31.4% · VHN ≈ 145 (Mohs ~2.5–3.0) | Mikuš et al. (Oct 2026 Am. Min.); linear two-coordinated Au+ ordered in B-module |
Fengruiite (Oct 2026 IMA)[Ag₆Sb₂S₇][Ag₉CuS₂Te₂] | Trigonal Pearceite-Polybasite | R ≈ 32.8% · VHN ≈ 158 (Mohs ~3.0) | Tian et al. (Oct 2026 Am. Min.); Haopinggou Ag–Pb–Zn–Au deposit, Qinling, China |
Spaltiite (2026 IMA)Tl₂Cu₂As₂S₅ | Orthorhombic / Monoclinic Thallium Sulfosalt | R ≈ 29.5% · VHN ≈ 112 Deep Crimson Internal Glow | Graeser, Topa et al. (2026 EJM 38:27); Lengenbach quarry, Binn Valley, Switzerland |
Argentotennantite-(Fe) (2026)Ag₆(Cu₄Fe₂)As₄S₁₃ | Cubic I4̄3mTetrahedrite Supergroup | R ≈ 30.2% (Isotropic) VHN ≈ 315 (Mohs ~4.0) | Sejkora et al. (Oct 2026 Am. Min.); San Genaro Ag–Pb–Zn mine, Huancavelica, Peru |
Hugotibergite (IMA 2026-014)Ca₆CaPb₂(Si₃O₉)₂(SO₄)₂(OH)₂(H₂O)₄ | Monoclinic C2/mRoeblingite Group | d100 = 3.229 Å · High Pb RI Mohs ~3.5 | Yang, Kampf, Downs et al. (2026); Långban mine, Sweden; Ca-analog of roeblingite |
Zincocabrerite (IMA 2026-013)ZnMg₂(AsO₄)₂(H₂O)₈ | Monoclinic C2/mVivianite Group | d100 = 6.684 Å Mohs ~2.5 (Sectile Blades) | Kampf, Favreau, Olds et al. (2026); Bou Azzer East mine, Morocco; Zn-analog of cabrerite |
Babunaite-(Nd) (Oct 2026)Nd(AsO₄) | Tetragonal I4₁/aScheelite Group | VHN25 = 578 kg/mm2 Mohs 5.0 · Adamantine | Galuskina et al. (Oct 2026 Am. Min.); Nežilovo, North Macedonia; 1st REE scheelite arsenate |
Bayanoboite-(Y) (Oct 2026)Ba₂Y(CO₃)₂F₃ | Orthorhombic / Monoclinic Ba-HREE Fluorocarbonate | Vitreous · High Δn Mohs ~4.5 | Xue et al. (Oct 2026 Am. Min.); Bayan Obo deposit; 1st Ba–heavy-REE fluorocarbonate |
Frequently Asked Questions
Why is the Lengenbach Quarry in Switzerland (home of 2026 Spaltiite) famous among mineral collectors?
Located in the Binntal (Binn Valley) of Valais, Switzerland, the Lengenbach quarry is a metamorphosed Triassic dolomitic marble where hydrothermal solutions enriched in Arsenic (As), Thallium (Tl), Silver (Ag), and Lead (Pb) crystallized in small cavities. It is the type locality for over 45 ultra-rare sulfosalt minerals—including the 2026 discovery Spaltiite (Tl2Cu2As2S5) and iconic species like Hutchinsonite, Hatchite, and Imhofite.
How do geologists use a Vickers Diamond Indenter (VHN) to measure the hardness of microscopic new minerals?
Many newly discovered minerals—such as Babunaite-(Nd) (crystals up to 70 μm) or Auropolybasite grains inside polished ore mounts—are far too small to scratch with a traditional Mohs hardness kit. Instead, mineralogists press a microscopic square-based diamond pyramid (with a 136° apex angle) into the polished crystal under a precise load (e.g., 25 grams for 15 seconds) and measure the two diagonal lengths (d1 and d2 in microns) of the resulting diamond-shaped impression. For Babunaite-(Nd), a VHN25 of 578 kg/mm2 converts directly to Mohs 5.0.
What makes Långban (Sweden) and Bou Azzer (Morocco) two of Earth's greatest mineral hotspots?
Långban in Värmland, Sweden (type locality of 2026 Hugotibergite and Friisite) is a metamorphosed manganese-iron-lead skarn hosting over 300 mineral species (and 70+ type species) formed where oxidizing Mn3+/Mn4+ horizons met reducing sulfide pods. Bou Azzer in Morocco (type locality of 2026 Zincocabrerite) is a Precambrian ophiolite cobalt-nickel-arsenide district famous for vivid crimson Roselite, Wendwilsonite, and Skutterudite crystals.