Lead silicates represent a chemically and structurally diverse category of rare minerals (with about 50 species known so far) that crystallize under specific geological conditions, typically in Pb-rich skarn environments or oxidized zones of polymetallic ore deposits. These minerals, characterized by the incorporation of Pb²⁺ ions, with frequently stereo-chemically active lone-pair electrons, and (SiO₄)⁴⁻ units within their crystal structures, often exhibit complex compositions and unusual structural motifs, reflecting the variable coordination chemistry of lead and the polymerization behavior of silica tetrahedra. Notable species include exotica like alamosite, larsenite, ganomalite, barysilite, the kentrolite–melanotekite series, and wickenburgite. The study of lead silicates potentially provides valuable insights into mineral formation processes across a range of temperatures, particularly concerning the mobility of lead in nature and, by extension, the technogenic environment.

During an investigation of the rare mineral jagoite, Pb₁₁Fe₅Si₁₂O₄₁Cl₃, and its paragenesis (Holtstam et al., 2025), from the Långban mine in Värmland, Sweden, a new, structurally related lead silicate was discovered. It is named “friisite” (IMA2024-047) in honor of the Danish mineralogist Henrik Friis (b. 1977), professor at the Natural History Museum, University of Oslo, Norway, in recognition of his significant contributions to Nordic (including Greenlandic) mineralogy. It must not be confused with “frieseite”, an obsolete name and near-homophone, referring to a mixture of sulfide minerals (Ondruš et al., 2003). The mineral symbol is “Fii”. The holotype material is housed in the type mineral collection of the Department of Geosciences, Swedish Museum of Natural History, Box 50007, SE-10405 Stockholm, Sweden, under collection no. GEO-NRM #19610234.

The Långban Fe–Mn–(Ba–As–Pb–Sb–W–Be–B) deposit, located in the Filipstad district (lat 59°51′ N, long 14°15′ E), was mined continuously from 1711 to 1972 for iron (hematite–magnetite) and manganese (braunite–hausmannite) ores, as well as dolomitic marble. The syngenetic, carbonate-hosted Långban-type deposits of the Bergslagen ore region (Moore, 1970) are interpreted to have formed at ca. 1.9 Ga from hydrothermal–volcanogenic fluids in a shallow submarine setting (Boström et al., 1979; Holtstam and Mansfeld, 2001). These deposits occur within a supracrustal sequence dominated by rhyolitic volcanic and volcaniclastic rocks that, together with the ores and interbedded dolomitic carbonate horizons, experienced regional metamorphism under amphibolite-facies conditions. Subsequent thermal overprinting occurred during the emplacement of Svecokarelian magmatic intrusions between 1.87 and 1.75 Ga (Stephens and Jansson, 2020). Långban is internationally renowned for its exceptional mineral diversity (Holtstam and Langhof, 1999) and significance as a type locality, with 81 IMA-approved mineral species currently recognized (including friisite; see https://www.mindat.org/, last access: 24 February 2026).

In a comprehensive geological and mineralogical study of the Långban deposit, Magnusson (1930) delineated four major paragenetic stages, denominated A, B, C, and D. The earliest, A and B, are associated with ore and skarn formation, with stage B encompassing the regional metamorphic peak (at approximately 600 °C and 0.3 GPa; Skelton et al., 2018). The later periods, C and D, correspond to mineral formation in vugs and fissures, respectively, under progressively lower temperature and pressure conditions.

The new mineral (Figs. 1 and 2) was found in a museum specimen of jagoite with heterogeneous skarn associated with hematite ore, collected from the Canberra stope at 220 m depth in the mine and provided by John Gustaf Herman Weslien in 1961. This specimen also contains melanotekite, aegirine–augite, jagoite, albite, baryte, fluorapophyllite-K, margarosanite, alamosite, native lead, a serpentine group mineral, and a wickenburgite-like mineral. Mineral identification was mainly done using energy-dispersive X-ray spectroscopy under the scanning-electron microscope (FEI Quanta 650 field emission gun scanning-electron microscope fitted with a back-scattered electron detector and an 80 mm² X-Max^N Oxford Instruments energy dispersion micro-analyzer) on polished sections embedded in epoxy resin.

Friisite occurs as subhedral, flaky grains up to 150 µm in size, forming aggregates within a medium-grained skarn matrix and contiguous to jagoite. It is white to colorless with a white streak and exhibits a sub-adamantine luster. The mineral is transparent and does not fluoresce under UV light. It is brittle, with an uneven fracture and perfect cleavage on {001}; no parting was observed. Based on analogy with jagoite, the Mohs hardness is estimated to be 4–5. The density could not be measured due to the small grain size and an expected value exceeding the limits of available heavy liquids. The calculated value is 5.54(1) g cm⁻³, derived from the empirical formula and single-crystal X-ray diffraction data (see below). Optically, friisite is non-pleochroic and uniaxial negative, with a calculated average refractive index of n=1.88 using Gladstone–Dale constants (Mandarino, 2007).

The composition of friisite was determined with electron microprobe analyses (EMPAs) collected with an AZtecWave wavelength–dispersion (WDS) system attached to an FEI Quanta 650 field emission scanning-electron microscope at 20 kV and 14 nA, with a working distance of 10 mm. The polished sample surface was carbon-coated to 20 nm thickness prior to analysis. The mineral reference materials (SPI 53) used and the analytical results are shown in Table 1. The number of spot analyses was six. Mg, Sb, and Br concentrations in the sample were close to or below the detection limit with this method. Neither H₂O nor CO₂ was indicated in structural or spectroscopic data for the mineral vide infra, and these are thus not considered in the formulae.

The empirical formula calculated from the EMPA data, normalized on a basis of 30 anions (O + Cl), is (Pb7.89Na0.11Ca0.08)∑=8.08 (Al2.19Si0.31Fe0.203+Zn0.13Mn0.112+)∑=2.94Si8O27.02Cl2.98. The analyzed crystal is heterogeneous with respect to some metal components, where the Al₂O₃ content is negatively correlated to Fe₂O3+MnO+ZnO (linear R2=0.95).

The simplified formula is (Pb,Na,Ca)₈(Al,Si,Fe,Zn,Mn)₃Si₈O₂₇Cl₃, whereas the ideal formula is Pb₈Al₃Si₈O₂₇Cl₃, which would require PbO 71.38 %, Al₂O₃ 6.11 %, SiO₂ 19.22 %, Cl 4.25 %, Cl ≡ O-0.96 %, and a sum of 100.00 wt %.

Micro-Raman spectra were obtained at Laboratorio di Mineralogia Sperimentale “Fiorenzo Mazzi”, Università di Pavia, with a Horiba LabRAM HR Evolution spectrometer equipped with an Olympus BX41 confocal microscope and a Symphony BIDD detector and a solid-state (YAG) 532 nm laser focused to a 1 µm diameter spot (power of 2.06 and 5.02 mW). Measurements were done with a 100×VIS objective, 600 groves mm⁻¹ grating, and spectra calibrated against pure silicon. The acquisition time was 15 s in 10 separate accumulations. Spectra were processed using LabSpec 6 software suite by Horiba for background subtraction and peak fitting. The spectra (Fig. 3) have distinct common features with that of jagoite (Holtstam et al., 2025): very intense bands at values <250 cm⁻¹ and weaker bands up to 1100 cm⁻¹. No Raman bands are observed at >1100 cm⁻¹. The spectra exhibit polarization effects that vary with crystal orientation, with one component perpendicular to and the other parallel to the (001) cleavage plane. The most intense bands are observed at 29, 42, 52, 63, 72, 96 123, 179, and ∼213 cm⁻¹; the strongest polarization effects are observed for the bands at 52, 73, and 123 cm⁻¹. Jagoite and friisite have common bands at 52, 64, 72, 96 (100 for jagoite), and 179 (183 cm⁻¹ for jagoite; see Holtstam et al., 2025 and Fig. 4). This is not surprising since those modes are related to lattice vibrations and considering the fact that both minerals belong to a polysomatic series with similar types of layers in the crystal structure (see below). The two latter bands can possibly be assigned to O–Pb–O and/or Cl–Pb–Cl bending modes. In the intermediate-range (300–700 cm⁻¹) bands at ∼306, ∼340, ∼406, 541, and 562 are much weaker but present in both orientations, with some other peaks showing polarization at 436 and 482 cm⁻¹ (laser beam parallel to (001)) and 611 and 659 cm⁻¹ (perpendicular to (001)); all of these bands are related to bending modes of SiO4/AlO4 groups. In the higher range (700–1200 cm⁻¹), weak bands are observed at ∼ 869, ∼ 885, 905, 964, ∼ 970, and ∼ 1060 cm⁻¹, with minor shoulders poorly defined; these are believed to be related to stretching modes of SiO4/AlO4 groups. Very similar band positions are observed for jagoite (Fig. 4). The only differences are bands at 390, 459, and 524 cm⁻¹ for jagoite (with numbers in red in Fig. 4).