Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride ${\mathrm{FeB}}_{4}$ were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in ${\mathrm{FeB}}_{4}$ is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth’s lower mantle. However, their range of stability, crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Here we investigate the behaviour of pure iron carbonate at pressures over 100 GPa and temperatures over 2,500 K using single-crystal X-ray diffraction and Mössbauer spectroscopy in laser-heated diamond anvil cells. On heating to temperatures of the Earth’s geotherm at pressures to ∼50 GPa FeCO₃ partially dissociates to form various iron oxides. At higher pressures FeCO₃ forms two new structures—tetrairon(III) orthocarbonate Fe₄³⁺C₃O₁₂, and diiron(II) diiron(III) tetracarbonate Fe₂²⁺Fe₂³⁺C₄O₁₃, both phases containing CO₄ tetrahedra. Fe₄C₄O₁₃ is stable at conditions along the entire geotherm to depths of at least 2,500 km, thus demonstrating that self-oxidation-reduction reactions can preserve carbonates in the Earth’s lower mantle.

The new mineral chukanovite, Fe₂(CO₃)(OH)₂, occurs in cavities of weathered fragments of the Dronino ataxite iron meteorite found near the Dronino village, Kasimov district, Ryazan' Oblast, Russia. It is a product of terrestrial alteration of meteorite iron. Associated minerals are goethite, akaganéite, hematite, hibbingite, reevesite, honessite, etc. Chukanovite forms acicular to fibrous individuals (up to 0.5 mm long and up to 2–3 μm thick) combined in spherulites up to 1 mm in diameter, botryoidal spherulitic clusters and parallel- or radial-columnar aggregates which form crusts up to 1 mm thick. Unaltered chukanovite is transparent, pale-green or colourless. The surface of aggregates is brownish-green. Streak is white. Lustre is vitreous. Cleavage is perfect, probably on {0–21}, fracture is uneven. The mineral is brittle, the Mohs' hardness is 3.5–4, the calculated density is 3.60 g/cm³. It is optically biaxial (–) with α 1.673(3), β 1.770(5), γ 1.780(5), 2V _meas. 10(5)°. Average chemical composition (wt. %; electron probe, H₂O by modified Penfield method, CO₂ by selective sorption) is: MgO 0.1, FeO 68.8, NiO 0.6, CO₂ 19.8, H₂O 10.9, total 100.2. The empirical formula calculated on the basis of two metal atoms is (Fe²⁺ _1.97Ni_0.02Mg_0.01) _Σ2.00(CO₃)_0.93(OH)_2.14·0.18H₂O, ideally Fe₂(CO₃)(OH)₂. Chukanovite is monoclinic P2₁/a, with a = 12.396(1) Å, b = 9.407(1) Å, c = 3.2152(3) Å, β = 97.78°. The strongest lines of the X-ray powder pattern [d(Å), I, (hkl)] are: 6.14, 40, (200); 5.15, 60, (231); 3.73, 80, (310); 2.645, 100, (230); 2.361, 40, (510); 2.171, 40, (520). The structure of chukanovite was refined on synchrotron data by the Rietveld method up to Rp = 3.43 %, wRp = 4.51 %, R_Bragg = 2.48 %. Chukanovite is closely related to the minerals of the malachite-rosasite group. It was named in honour of Nikita V. Chukanov (b. 1953), Russian physicist and mineralogist. The holotype specimen is deposited in the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.