The new mineral leishmanite (IMA2026-002), Ba(H2O)4[(UO2)3O2(OH)3]2·3H2O, was found in the small uranium occurrence near Nendaz, Valais, Switzerland, where it occurs as elongated tabular crystals (up to 0.1 mm) in a peculiar assemblage with parauranophane and ‘biotite’ in a thin quartz veinlet, concordant with the schistosity of a very finely bedded paragneiss; the origin of the mineral is most probably from the retrograde metamorphic fluids. The streak is pale yellow. Crystals are translucent with vitreous lustre. The tenacity is brittle, the Mohs hardness is ∼2½, and the fracture is curved. The mineral has Dcalc. of 5.217 g·cm–3 for the ideal formula. The mineral is optically biaxial (–) with α = 1.755(3), β = 1.796(3) and γ = 1.799(3); 2V is 28(2)°; orientation is X = b, Y and Z lie in {010}; pleochroism: X colourless, Y yellow, Z yellow; X < < Y < Z. Electron microprobe analyses provided (Ba0.91Pb0.07K0.06)Σ1.04(H2O)4[(UO2)6O4.02(OH)5.98]·3H2O. Leishmanite is orthorhombic, Pmn21, a = 12.079(2), b = 15.096(3), c = 7.155(2) Å, V = 1304.7(5) Å3 and Z = 2. The crystal structure (R1 = 0.0392 for 6324 I > 3σI reflections) contains uranyl-oxo-hydroxo sheets of both the α-U3O8 (protasite) topology with the PD stacking sequence and β-U3O8 topology with the DRU stacking sequence. Between the sheets, Ba2+ is coordinated by three H2O molecules, and an additional four H2O molecules are not bonded to any metal cation. The structure and chemical composition of leishmanite are discussed with respect to the paragenetic relationships within alteration associations of uraninite, based on the bond-valence approach.

Feldspars are among the most common phases in the Earth’s crust and are characterized by wide variations in chemical composition and crystal structure motifs. In this paper, we synthesized SrZn2P2O8 and studied it by means of scanning electron microscopy, electron microprobe analysis and powder X-ray diffraction at ambient and high-temperature (up to 900°C) conditions. We have structurally characterized for the first time layered triclinic β-modification of SrZn2P2O8 (space group P$\bar 1$, a = 5.0109(8), b = 8.6202(13), c = 9.7527(15) Å, α = 118.088(3), β = 74.622(6), γ = 87.525(6)° and V = 351.29(10) Å3), which is structurally related to hexacelsian. The high-temperature study demonstrated that β-SrZn2P2O8 transformed irreversibly into the framework monoclinic paracelsian-like α-SrZn2P2O8 (space group P21/c, a = 8.3163(2), b = 9.5023(2), c = 9.0248(2) Å, β = 92.285(1)°, V = 712.60(1) Å3) above 600°C, which, in turn, is stable up to 900°C, when it melts. Though the crystal structures of α- and β-modification of SrZn2P2O8 are different (framework and layered, respectively), their thermal expansion is similar: αV ≈ 26 ± 1 × 10–6 °C–1, αmax/αmin ≈ 5. The possible occurrence of the corresponding minerals in nature is discussed.

The bulk modulus of a crystal is often closely related to the compressibility of its constituent elements, such as cation-centred coordination polyhedra. The compressive behaviour of regular polyhedra with a fixed coordination number was studied thoroughly some decades ago and was found to follow a simple universal rule. However, the compressive behaviour of large cations with irregular and poorly defined coordination polyhedra has not been studied systematically because of methodological challenges and biases. Here, an original approach to defining and analysing polyhedral compressibility is proposed, based on the concepts of effective coordination number (ECoN) and weighted mean bond length. This approach is demonstrated using K+ coordination polyhedra as an example.

Arzamastsevite (IMA2024-085), ideally K6Al5Si6O20(OH)4Cl, is a new zeolite with a unique Si(OH)4 group. The mineral was found in natrolitized sodalite syenite from Parguaiv Mt. (67°47′22.4″ N, 34°35′10″ E), Lovozero alkaline massif, Kola Peninsula, Russia. It occurs as short-prismatic crystals (average 60 µm across) and anhedral grains (average 50 µm across) and is associated with alkali feldspar, sodalite, natrolite, aegirine, magnesio-arfvedsonite, loparite, rhabdophane-(Ce), lamprophyllite, sphalerite, galena, pyrrhotite and pyrite. The largest observed arzamastsevite crystal is 180 µm across. Aggregates of 2–5 crystals are typically observed. Arzamastsevite is colourless, with a vitreous lustre and white streak. The mineral is brittle. Cleavage was not observed; the fracture is uneven. The measured density is 2.50(5) g⋅cm–3, the density calculated using the empirical formula and single-crystal unit-cell parameters is 2.51 g⋅cm–3. Arzamastsevite is optically uniaxial (+), ω = 1.530(2), ε = 1.532(2) (589 nm). Chemical data (electron microprobe, wt.%) are: SiO2 37.17, Al2O3 25.98, Fe2O3 0.05, K2O 28.88, Na2O 0.27, Cl 3.54, H2O (calculated from the crystal structure information, i.e. H = 4) 3.71, O=Cl2 –0.80, total 98.80. The empirical formula calculated on the basis of [20 O + 4 (OH)] = –44 apfu is (K5.96Na0.08)Σ6.04[Al4.95Fe3+0.01Si5.01O20]Cl0.97·[Si(OH)4]. Arzamastsevite is tetragonal, space group: I$\overline 4 $2m, with a = 9.8280(5) Å, c = 13.1342(13) Å, V = 1268.63(18) Å3 and Z = 2. The crystal structure was determined from single-crystal data and refined to R1 = 0.08. The crystal structure of arzamastsevite is characterized by the EDI type zeolite tetrahedral framework. The crystal structure of arzamastsevite is generally similar to the crystal structure of kalborsite, but differs in the substitution of the negatively charged [B(OH)4]–-tetrahedra by the neutral [Si(OH)4]0-tetrahedra, which is controlled by the Si:Al ratio of 1:1 within the framework. The strongest lines of the powder X-ray diffraction pattern [d in Å (I) (hkl)] are: 3.4782 (35) (220); 3.2857 (26) (004); 3.1103 (44) (310); 3.0750 (100) (222); 2.9708 (84) (114); 2.8132 (73) (312); and 2.7321 (28) (204). The mineral is named in honour of Dr. Andrey A. Arzamastsev (b. 1950), a prominent Russian geologist and an expert in the petrology of alkaline rocks.