Zipserite is a new mineral species discovered in a sample collected from the old mine dumps of the abandoned epithermal deposit Nagybörzsöny in Hungary. Zipserite occurs as anhedral to subhedral, lath-like grains, up to 500 μm in size, in hydrothermally strongly altered rocks. It is found at a contact between bismuth and bismuthinite, also associated with rare ikunolite and joséite-A. Zipserite is silvery white with a metallic lustre. Mohs hardness is ca. 2–3 and the calculated density is 7.815 g.cm–3. In reflected light, zipserite is grey–white, with colour and reflectance essentially matching those of bismuthinite. Bireflectance is weak, internal reflections not present. Anisotropy is moderately strong, with dark blue and grey colours of anisotropy. Reflectance values for the four Commission on Ore Mineralogy wavelengths of zipserite in air [Rmax, Rmin (%) (λ in nm)] are: 48.4, 46.4 (470); 47.8, 45.9 (546); 47.8, 45.8 (589); and 47.5, 45.6 (650). The empirical formula, based on electron-microprobe analyses, is (Bi4.74Pb0.31)Σ5.05(S3.38Se0.56Te0.02)Σ3.96, that can be simplified as Bi5(S,Se)4. The ideal end-member formula of zipserite is Bi5S4, which requires Bi 89.07 and S 10.93, total 100 wt.%. Zipserite possesses a fascinating crystal structure. The average structure is trigonal, with space group P$\bar{3}$m, a = 4.162(1) Å, c = 16.397(1) Å, V = 245.94(4) Å3 and Z = 2. The structure is built by the alternation of a double bismuth layer Bi2 and the Bi3S4 block which is a three octahedra thick layer. Its general formula can be expressed as Bi2 + Bi3S4, which corresponds directly to the observed stacking. At 98 K, the structure can be described using the superspace formalism with an R-centred trigonal unit cell a = 4.209(2) Å, c0 = 5.616(6) Å, a modulation vector q ≈ 4/3 c* and the superspace group R$\bar{3}$m(00γ)00. Zipserite is not only a new mineral but also the first named member of a new sub-group of compounds within the broader family of bismuth chalcogenides, characterised by complex stacking of structural units (Bi2 layers and Bi3S4 blocks). Some of these phases are being investigated as promising thermoelectric materials and synthetic analogues of zipserite could also be inspected for similar physical properties.

The thermal behaviour of fluorcarletonite, KNa4Ca4Si8O18(CO3)4(F,OH)⋅H2O, from the charoitites of the Severny district at the Malyy Murun massif, Murun complex, NW Aldan Shield, Siberia, Russia, has been investigated in order to understand the temperature-induced changes in the crystal structure of this rare silicate. The study has been carried out combining in situ high-temperature single-crystal X-ray diffraction (T range 25–550°C), ex situ high-temperature Fourier-transform infrared spectroscopy (25–700°C) and ab initio calculations. An increasing trend of lattice parameters and cell volume was observed in the 150–550°C temperature range, when the mineral underwent a progressive dehydration process. At 550°C ~40% water loss was detected. If compared with the fluorcarletonite structure at room temperature, the partially dehydrated fluorcarletonite shows: the same space group (P4/mbm); increased distances between the oxygens of the H2O molecules (O11w and O12w) and their Na-centred octahedral cations (Na1 and Na2, respectively); distortion of the four- and six-member tetrahedral rings of the double silicate layer. The dehydration process mainly involves the oxygen at the O11w site which has a different local environment with respect to the oxygen at the O12w site. At T > 600°C, the complete dehydration is accompanied by deprotonation of the OH groups substituting for the F atoms and by the collapse of the structure when the CO2 is released. The adopted approach allowed definition of the temperature thresholds at which modifications occur in the fluorcarletonite crystal structure when subjected to controlled heating conditions. Our findings contribute to assessment of stability, reactivity and, more generally, the thermal behaviour of sheet silicates with fluorcarletonite-like topology.

Markwelchite, ideally TlPbSbS3, is a new mineral from the hydrothermal deposit of Jas Roux, Hautes-Alpes, France. It occurs as a black anhedral crystal associated closely with protochabournéite. Microhardness measurements (VHN15) gave a mean value of 197 kg/mm2 corresponding to a Mohs hardness of ~3–4. In plane-polarised incident light, markwelchite is grey in colour. Under crossed polars, it is distinctly anisotropic with greyish white to bluish rotation tints, with bright red internal reflections. Reflectance percentages (Rmin and Rmax) are: 28.5, 31.5 (471.1 nm); 28.3, 30.7 (548.3 nm); 27.9, 30.3 (586.6 nm); and 27.6, 29.8 (652.3 nm). The mean of 5 electron microprobe spot analyses gave Tl 34.67(45), Pb 31.86(25), Sb 15.06(15), As 2.37(5), S 15.35(20), total 99.31 wt.%, corresponding, on the basis of a total of 6 atoms per formula unit and structural results, to Tl1.063Pb0.964(Sb0.775As0.198)Σ0.973S3.000. Single-crystal X-ray diffraction studies revealed that markwelchite is isotypic with richardsollyite, TlPbAsS3. It is monoclinic, space group P21/c, with the following unit-cell parameters: a = 8.9144(3), b = 8.4513(3), c = 8.6511(3) Å, β = 108.723(4)°, V = 617.27(4) Å3 and Z = 4. The five strongest observed powder-diffraction lines [d in Å (Irel)(hkl)] are: 3.88 (100)($\bar{2}$11); 3.78 (90)(210); 3.29 (90)(102); 2.73 (85)($\bar{1}$13); and 2.93 (75)(022). The crystal structure can be described as formed by (100) [Me2(SbS3)]– layers sandwiching the Me1+ cations. The Me1 site has a seven-fold coordination, whereas the Me2 site has an 6+2 coordination corresponding to a distorted, bicapped trigonal prismatic coordination, and the Sb site displays a trigonal pyramidal coordination with three S atoms and Sb at the apex.

The name markwelchite honours Dr Mark D. Welch of the Natural History Museum, London, UK.

The new mineral has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2024–001). A discussion on the relationships between markwelchite and synthetic TlPbSbS3 is also provided.