Beschreibung:
<jats:p>The new mineral radvaniceite, GeS2, was found on the burning coal mine dump of the abandoned Kateřina coal mine at Radvanice, near Trutnov, northern Bohemia, Czech Republic. It occurs as aggregates resembling cotton tufts up to 5 mm in size; they are composed of acicular crystals up to fibres about 1–5 μm thick and up to 3 mm in length. Individual fibres are distorted and partly resemble bent wires nucleated on rock fragments or on black, crumbly ash, in association with minerals of solid solutions of Bi-Sb and stangersite, herzenbergite, and greenockite. Radvaniceite was also observed as irregular grains in a range of 10–50 μm in size, forming part of earlier multicomponent aggregates upon which the above-described crystals grow. These aggregates are formed, in addition to radvaniceite, by minerals of Bi-Sb, Bi2S3-Sb2S3 and Bi2S3-Bi2Se3 solid solutions, Bi3S2, Bi-sulpho/seleno/tellurides, tellurium, unnamed PbGeS3, Cd4GeS6, GeAsS, Sn5Sb3S7, stangersite, greenockite, cadmoindite, herzenbergite, teallite, and Sn- and/or Se-bearing galena. Radvaniceite is formed under reducing conditions by direct crystallization from hot gasses (250–350 °C) containing Cl and F at a depth of 30–60 cm under the surface of a burning coal mine dump; the mine dump fire started spontaneously, and no anthropogenic material was deposited there. Acicular crystals up to fibres of radvaniceite are elastic to flexible; are white to yellowish grey in colour, with white streaks; are translucent in transmitted light; and have vitreous to adamantine lustre. Cleavage and fracture were not observed. The calculated density is 3.05 and 2.99 g·cm−3 for the empirical and ideal formulae, respectively. Radvaniceite is transparent under the microscope, with a very weak pleochroism (from colourless to pale greenish yellow), and has a refraction index > 1.8. Under reflected light, radvaniceite is light grey; bireflectance and pleochroism were not observed due to abundant, white to grey, internal reflections. Anisotropy in crossed polars is distinct with grey rotation tints. Reflectance values of radvaniceite in air (Rmin–Rmax, %) are: 15.4–18.8 at 470 nm, 16.1–20.4 at 546 nm, 16.4–20.8 at 589 nm, and 16.9–20.9 at 650 nm. The empirical formula, based on electron-microprobe analyses, is (Ge0.99Bi0.01)Σ1.00(S1.97Se0.03)Σ2.00. The ideal formula is GeS2, which requires Ge 53.10, S 46.90, total 100 wt. %. Radvaniceite is monoclinic, Pc, a = 6.8831(12), b = 22.501(3), c = 6.8081(11) Å, β = 120.365(9)°, with V = 909.8(4) Å3 and Z = 12. The strongest reflections of the powder X-ray diffraction pattern [d, Å (I) (hkl)] are: 5.7395 (100) (11-1, 110), 5.2067 (16) (021), 3.3650 (33) (111, 11-2), 2.8417 (33) (022), 2.8236 (16) (170, 17-1), 2.8134 (20) (080) and 2.6257 (19) (240, 24-2). According to X-ray powder diffraction data and Raman spectroscopy, radvaniceite is a natural analogue of synthetic monoclinic low-temperature β-GeS2 with distorted GeS4 tetrahedra forming four corner-sharing tetrahedral chains, which are connected by corner-sharing tetrahedra in a three-dimensional structure. We named the mineral after its type locality, Radvanice, one of the past centres of coal mining in the Czech limb of the Intra-Sudetic Basin. This mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (number 2021-052).</jats:p>