Fe-dominant cordierite, the mineral sekaninaite, is relatively rare in calc-alkaline volcanic rocks, with only a handful of recorded localities. The compositionally zoned phenocrysts in the Late Devonian Rubicon Rhyolite in central Victoria, Australia are the most Fe-rich known, with core compositions having Mg# of 10–30. Similar cordierite–sekaninaite (Crd–Skn) phenocrysts occur in: rhyolites and rhyodacites from San Vincenzo and Roccastrada in Tuscany; on Lipari Island in Italy; in the Nefza province in northern Tunisia; and in a newly discovered Devonian rhyolite at Arthurs Seat, Victoria. All show well-developed sector twinning in response to structural ordering during cooling. The Victorian sekaninaite crystals all show strong zonation from Fe-rich cores to rims richer in Mg, i.e. reverse zoning. In contrast, the Italian and Tunisian examples have relatively weak normal zoning. In the Roccastrada and Lipari rocks, a second type of Crd–Skn occurs as turbid fragments and as groundmass crystals, with the latter showing reverse zoning. Reverse zoning in the Rubicon and Arthurs Seat rhyolite phenocrysts and the Lipari groundmass crystals is attributed to the reheating of their host magmas due to interaction, such as mixing, with a higher-temperature magmas prior to eruption. The ranges of Crd–Skn compositions in all volcanic rocks bear no systematic relationship to the bulk compositions of their host rocks. Assuming that the whole-rock compositions approximate the original magmatic liquids, and an initial H2O content of 5 wt.% throughout, enabled a comparison to be made between the relative P–T conditions of most occurrences, apart from Lipari. Results suggest that the Rubicon Rhyolite had the lowest P–T conditions with Roccastrada next, and San Vincenzo the highest. It appears that sekaninaite might be stable in silicic volcanic magmas over a wide range of melt compositions, pressures and temperatures but is favoured for low-Mg# bulk compositions at low P and low T.

Heavy metal being immobilised in the lattice of a mineral is beneficial for its removal, recovery and reuse from wastewater. It is therefore essential to determine how heavy metals can be transferred into minerals controllably. This work developed a potential way for transforming heavy metals (Cu2+, Pb2+, Ni2+, Zn2+ and Cd2+) in wastewater into solids with high efficiency by introducing crystal seeds. The results of this work demonstrate that the addition of hydrotalcite and paratacamite crystal seeds can enhance heavy metal removal, both in simulated and actual acid mine wastewater. The removal rate can be increased by 18–47% and 31.8% for each heavy metal and total heavy metals in the presence of each crystal seed, respectively. Additionally, the recovery products of heavy metals can be changed by crystal seeds. In the systems without crystal seeds, the recovery products are mixtures; but the pure phase can be achieved if crystal seeds are added. For instance, in the Cu2+–Al3+–Cl– system without crystal seeds, the products were mixtures of paratacamite and layered double hydroxides (LDHs). But the products could be altered easily by hydrotalcite or paratacamite seeds. Paratacamite seeds induced Cu2+ to form paratacamite at pH 5.0, but a mixture of LDHs and paratacamite at pH 7.0. In contrast, hydrotalcite seeds induced Cu2+ to form LDHs at both pH 5.0 and 7.0. From the perspective of enthalpies of formation, CuAl-LDH and paratacamite are potential products, but the former is generally more stable, and thus it becomes the dominant product of the reaction systems using crystal seeds. It is believed that the crystal seeds can accelerate the dynamic process of LDH formation. This work suggests a controllable way for heavy metals removal, recovery and reuse.

Two uranyl carbonate silicate minerals have been studied in detail, lepersonnite-(Gd) and the new mineral lepersonnite-(Nd). Both minerals originate from Katanga province in the Democratic Republic of Congo (Africa): lepersonnite-(Gd) from the Shinkolobwe mine (type locality) and lepersonnite-(Nd) from the Swambo mine (type locality) and Shinkolobwe mine. Each occurs as radial to acicular aggregates composed of long prismatic, thin crystals, as the products of hydration–oxidation weathering of uraninite. A detailed description of the chemical and physical properties of lepersonnite-(Nd) is provided, including the thermal analysis of the -(Gd) member. Moreover, Raman and infrared spectroscopy data are provided for both minerals. The crystal structure of lepersonnite-(Gd) has been solved based on 3D electron diffraction data. According to the best dataset at 95 K, lepersonnite-(Gd) is orthorhombic, with a = 11.838(134) Å, b = 15.822(333) Å, c = 39.147(190) Å and V = 7353(282) Å3 (Z = 4). The dynamic refinement of the crystal structure (Robs/wRobs = 0.1204/0.1088 and Rall/wRall = 0.1204/0.1088 for 3090/50507 observed/all reflections and 247 refined parameters) revealed a large complex sheet structure with structural units defining a new lepersonnite topology. It contains infinite sheets of uranyl polyhedra, planar CO3 groups, protonated Si-tetrahedra, and Gd3+ polyhedra, and a thick interlayer hosted with H2O and partially occupied Ca2+ sites. Based on the structure refinement, bond-valence considerations, and new electron microprobe data, we infer that the ideal formula of lepersonnite-(Gd) is [Ca0.5Gd0.5(H2O)18(OH)1.5] [Gd(UO2)12(SiO3OH)2(CO3)4(OH)10O2(H2O)5]. On the basis of their closely related powder X-ray diffraction patterns and the similar behaviour of Nd and Gd, we expect the two minerals to adopt the same structures, apart from Ca, which was absent in lepersonnite-(Nd). The ideal formula for lepersonnite-(Nd) is [Nd(H2O)18(OH)2][Nd(UO2)12(SiO3OH)2(CO3)4(OH)10O2(H2O)5].

Diamonds are found occasionally in the United States of America. Diamonds from the Prairie Creek lamproite in Arkansas, USA occur within a north to south corridor of Neoproterozoic-to-mid-Cretaceous magmatism that extends across North America. These diamond-bearing lamproites are unusual because they intrude adjacent to sutured and strongly thinned lithosphere rather than stable within-plate settings and the diamonds themselves provide physical evidence of processes related to diamond formation at the cratonic margin. Indeed, A review of previously published geophysical data, isotopic compositions, inclusion suites and inclusion geochemistry suggest most diamonds were formed in subducted and eclogitic rocks within a highly localised diamondiferous lithosphere beneath the cratonic margin.

The morphology and spectroscopic character of 155 diamonds from the Prairie Creek lamproite suggest typical diamond formation conditions in an otherwise thinned continental lithosphere. Most diamonds examined during this investigation have spectroscopic features indicating strong nitrogen aggregation, a history of thermal perturbation and plastic deformation. Nitrogen contents range up to 1882 ppm and the diamonds preserve ∼70% aggregated nitrogen in the B aggregation state. Furthermore, inclusion elastic barometry and time-averaged mantle residence temperatures suggest most Arkansas diamonds formed at 5.2±0.2 GPa and 1205±63°C (1σ). However, a subpopulation of ∼4% of relatively large and inclusion free, colourless, flattened-to-irregular habit Arkansas diamonds are Type IIa with <5 at.ppm nitrogen. Those stones size, morphology, colour and N content might warrant their inclusion in the class of Cullinan-like, Large, Inclusion-Poor, Pure, Irregular and Resorbed or ‘CLIPPIR’ diamonds. Other diamonds examined commonly exhibit physical evidence of plastic deformation, including brown body colour and deformation lamellae.

Subcalcic Cr-rich pyrope is a typical inclusion in natural diamond and considered as the main indicator mineral in diamond exploration. This article presents the results of experiments using a model garnet–spinel–harzburgite with the objective of determining the maximum Cr content of the garnet. This study supplements the existing CaO vs Cr2O3 diagram with new experimentally obtained data on Cr-rich garnets. A high-pressure apparatus (BARS) was used to conduct experiments at a pressure of 5.5 GPa and temperature of 1300°С, which corresponds to the stability field of both garnet and diamond. A model harzburgite was obtained using natural Mg-serpentine, which decomposes at the pressures and temperatures of this experiment into olivine and orthopyroxene. Natural chromite and carbonatite were used as the sources of Cr and Ca, respectively. The samples formed are composed of forsterite, enstatite, Cr-pyrope and Cr-spinel. The maximum Cr2O3 content, 16.23 wt.%, was detected in grains which grew in contact with chromite. The addition of 1–2 wt.% carbonatite resulted in the crystallisation of garnets with varying Ca contents (2.83–7.49 wt.% CaO). The experiments confirmed the boundary at 16 wt.% Cr2O3 for subcalcic pyrope associated with diamond. It is concluded that the origin of natural samples of Ca-rich lherzolitic/wherlitic and Ca-poor harzburgitic garnets with >16 wt.% Cr2O3 can be attributed to the specific Ca/Cr/Al ratios of the host medium.

The solubility of Ag, As, Bi, Cu, Pb, Sb and Sn in the synthetic analogue of kotulskite (PdTe) was studied experimentally at 400°C (for Bi at 500°C). The extent of the solid solution was studied on the line PdTe–PdA with a compositional range Pd(Te1–xAx) with x = 0.1, 0.2, 0.4 and 0.6 and A = Ag, As, Bi, Cu, Pb, Sb and Sn. For the purpose of this study the silica-glass tube method was used. The experimental products were evaluated by means of powder X-ray diffraction analysis, reflected light and electron microscopy. Kotulskite does not dissolve Ag. It may dissolve up 3 at.% As, up to 5.6 at.% Cu, up to 20 at.% Sn and up to 30 at.% Pb. A complete solid solution between kotulskite (PdTe) – sudburyite (PdSb) and kotulskite (PdTe) – sobolevskite (PdBi) as suggest from natural occurrences has not been confirmed within this study. There is a composition gap between Pd0.99(Te0.21Bi0.80)Σ1.01 (PdTe dissolving 40.3 at.% Bi) and Pd1.00(Bi0.97Te0.03)Σ1.00 (PdBi dissolving 1.6 at.% Te). Also, there seems to be a gap between Pd1.00(Te0.18Sb0.82)Σ1.00 (PdTe dissolving 41.1 at.% Sb) and PdSb. The Cu substitutes for Pd whereas As, Bi, Pb, Sb and Sn substitute for Te in the crystal structure of kotulskite. Assessed solid solution series should be sought in assemblages with other platinum-group minerals and known Pd tellurides, likely in magmatic Cu–Ni–PGE mineral deposits, associated with mafic and ultramafic igneous rocks.

Neotype material of chukhrovite-(Ce), ideally (Ca3Ce)[AlF6]2(SO4)F·12H2O, was established on the basis of a new occurrence from the Tripi mine, Alì, Peloritani Mountains, Sicily, Italy. In fact the Russian specimens originally considered as type material of this species were later found to correspond to Ce-rich chukhrovite-(Y). At the Tripi mine, chukhrovite-(Ce) occurs as white cube-octahedral crystals, up to 0.05 mm in size, associated with creedite, quartz and fluorite. Electron microprobe analysis gave (in wt.%, normalised to sum = 100.00): SO3 8.40, P2O5 0.16, ZrO2 0.12, Al2O3 11.95, Y2O3 0.48, La2O3 3.64, Ce2O3 7.63, Pr2O3 1.00, Nd2O3 3.23, Sm2O3 0.97, Eu2O3 0.13, Gd2O3 0.97, Dy2O3 0.21, CaO 20.47, F 22.09, Cl 0.07, O = (F, Cl) –9.31, H2Ocalc 27.80. On the basis of 29 anions per formula unit, assuming the presence of 13 (F+Cl+OH) atoms and 12 H2O groups, the empirical formula of chukhrovite-(Ce) from the Tripi mine is Ca3.17(Ce0.40La0.19Nd0.17Pr0.05Sm0.05Gd0.05Y0.04Eu0.01Dy0.01)Σ0.97Zr0.01Al2.04(S0.91P0.02)Σ0.93O4[F10.10Cl0.02(OH)2.88]Σ13.00·12H2O. The occurrence of H2O groups was supported by micro-Raman spectroscopy. Chukhrovite-(Ce) is cubic, Fd$\bar 3$, with a = 16.7608(5) Å, V = 4708.5(4) Å3 and Z = 8. Its crystal structure was refined to R1 = 0.0378 for 604 unique reflections with Fo > 4σ(Fo) and 36 least-square parameters. Chukhrovite-(Ce) belongs to the chukhrovite group and it is isotypic with chukhrovite-(Y), whereas it is homeotypic with chukhrovite-(Ca).

Fulgurites are natural glasses that form when lightning strikes sand, soil, or rock and fuses the individual grains together to generate what is usually a tubular structure that follows the path of the strike. During this process, localised reducing conditions are conducive to forming rare minerals including iron silicides. This paper examines a fulgurite formed in Southwick, Massachusetts, USA, which displays an iron silicide that has a clearly defined reaction rim. The reaction rim demonstrates the production of a more silicon-rich rind consisting of Fe5Si3 on a core of Fe2Si, and the most likely route to forming this material is by reaction of silicon gas with Fe2Si at high temperature (>1000°C), with a reaction timescale of about one second. This reaction suggests the high temperature, reducing conditions of a lightning strike favour reactions of condensed matter (e.g. liquid or solid iron minerals) with gas that occurs rapidly during the lightning strike. The conditions necessary to form these minerals suggest that the fulgurite became more reducing over time, as more Si entered the solid phase, perhaps as oxygen left the system, either as CO2 or from the breakdown of SiO2 gas.

Five typical metal cations (i.e. Na+, K+, Ca2+, Mg2+ and Al3+) were selected as representatives to study the influence of metal cations on the dissolution and transformation of biotite. This work focussed on the mineralogical features of transformation products and phase transformation mechanisms by utilising modern spectroscopic methods and micro-beam characterisation techniques. In comparison with a control system, K+ inhibited the dissolution and transformation of biotite, leading to the generation of amorphous iron hydroxides on the biotite surface. Na+, Mg2+ and Ca2+ promoted the dissolution of biotite but inhibited its transformation into kaolinite, with the Na system producing sodium-bearing biotite, vermiculite, hematite and a small amount of kaolinite, and the Mg and Ca systems producing mainly vermiculite, chlorite and hematite. Al3+ notably accelerated the dissolution and transformation of biotite, resulting in well-crystallised kaolinite and hematite. Furthermore, metal cations changed the formation mechanism of kaolinite by altering the dissolution rate of biotite. Within the blank system, biotite dissolved slowly, with elements (i.e. Al and Si) accumulating on the biotite surface and growing epitaxially into kaolinite; whereas in the Al system, the rapid dissolution of biotite provided a large amount of Si, which combined with Al in the solution, forming kaolinite via a dissolution–recrystallisation process. In addition, the exchange reactions of metal-cation–K+ and the competitive adsorption of metal-cation–proton simultaneously constrained the dissolution process of biotite. This work offers a theoretical basis for an in-depth comprehension of the factors influencing biotite weathering and new insights into the evolution of clay minerals in terrestrial surface environments.

Unveiling the pressure‒temperature path of low-grade metamorphic rocks is challenging because of the occurrence of detrital minerals and high-variance mineral assemblages (i.e. chlorite–white mica–quartz). This paper is an attempt to reconstruct the pressure–temperature history on metapelites from a low-grade metamorphic unit, i.e. the Cabanaira Unit, located in the Marguareis Massif (Western Ligurian Alps, Italy). In order to obtain the most robust result possible, multi-equilibrium thermobarometry, forward modelling and crystallochemical index measurements are used together to reconstruct a pressure–temperature path, with consideration of the strengths and weaknesses of these methods.

This multidisciplinary approach allowed us to reconstruct the metamorphic evolution of the unit of interest, characterised by a pressure peak reached under low-temperature conditions (0.85–0.68 GPa and 250–285°C) followed by decompressional warming (low pressure–high temperature, 0.4-0.6 GPa and 300–335°C).

This pressure‒temperature path is consistent with the tectonic evolution of the investigated area proposed by previous studies, where a geological scenario in which the Cabanaira Unit experienced subduction-related processes was postulated, even if the reasons for warming remain unclear.

Multi-equilibrium thermobarometry is considered to be the most suitable method to unravel the metamorphic history of low-grade rocks, whereas forward thermodynamic modelling and the calculation of crystallochemical indexes seem to resolve only some segments of the pressure‒temperature path.

Rubinite, a garnet with the ideal formula Ca3Ti3+2Si3O12, is an indicator of super-reduced conditions and has been found recently in refractory inclusions in a few CV3 chondrites. We discovered rubinite in phosphide-bearing breccia from the pyrometamorphic Hatrurim Complex, Negev Desert, Israel. Aggregates of phosphide and native iron are concentrated at the boundary of thermally altered sedimentary xenoliths encased in flamite–gehlenite paralava. Rubinite, with the average empirical formula (Ca2.97Mg0.02Fe2+0.01)Σ3.00(Ti3+1.10Al0.44Ti4+0.37Mg0.08Cr0.01)Σ2(Si2.71Al0.29)Σ3.00O12, was found in a small xenolith composed of hydrogrossular, tacharanite and calcite, and containing relics of high-temperature minerals such as pseudowollastonite, cuspidine, gehlenite, baghdadite, barringerite, murashkoite, osbornite, paqueite and oldhamite. For the first time, the structure of rubinite, with the composition (Ca2.99Mg0.01)Σ3(Ti3+0.78Al0.62Ti4+0.43Mg0.17)Σ2(Si2.74Al0.26)Σ3O12, has been refined. Its unit-cell parameter a = 12.0193(4) Å, is significantly smaller than that of the synthetic analogue of Ca3Ti2Si3O12, 12.1875 Å. In the rubinite Raman spectrum weak bands corresponding to the vibrations of Ti4+–O in the (TiO6)8– octahedra: 610 cm–1 ν1(TiO6)8– and 438 cm–1 ν4(TiO6)8– are present in addition to the bands related to Si–O and Al–O vibrations in the TO4 tetrahedra. Rubinite forms a thin reactive rim (<10 μm) on pseudowollastonite grains. It probably formed during a sharp increase in rock porosity in the course of natural clinkerisation of sedimentary xenoliths caused by the thermal impact of the paralava. The high porosity increased the effect of reductive gases on the rocks, which were by-products of pyrometamorphism. The brief appearance of super-reduced conditions defined the formation of the Ti3+-bearing minerals osbornite and rubinite. Paqueite, Ca3Ti4+(Ti4+Al2)Si2O14, which crystallised in a thin melting zone of xenolith at the boundary with the paralava, does not contain Ti3+.

The new mineral tarutinoite, ideally Ag3Pb7Bi7S19, was found in a fragment of a drill core extracted at the 178.5 m level of borehole #4604 at the Tarutinskoe (Tarutino) copper-skarn deposit, Chelyabinsk Oblast, Southern Urals, Russia. It occurs as anhedral grains up to 0.10 × 0.05 mm intergrown with hessite and galena in magnetite and calcite. Tarutinoite is grey, opaque with metallic lustre, brittle tenacity and uneven fracture. No cleavage and parting are observed. The Vickers’ micro-indentation hardness (VHN, 25 g load) is 178 kg/mm2 (range 165–194, n = 4), corresponding to a Mohs’ hardness of 3.5–4, and calculated density is 7.180 g/cm3. In reflected light, tarutinoite is greyish-white, very weakly bireflectant and non-pleochroic. Under crossed polarisers the new mineral exhibits moderate anisotropy, in grey and dark grey tones with bluish tints. The reflectance values for wavelengths recommended by the Commission on Ore Mineralogy of the International Mineralogical Association are (Rmin/Rmax, %): 45.5/47.9 (470 nm), 43.5/45.0 (546 nm), 43.3/44.1 (589 nm) and 41.8/42.5 (650 nm). The chemical composition (wt.%, electron microprobe data, mean of 7 spot analyses) is Cu 0.30, Ag 8.33, Cd 0.04, Pb 37.12, Bi 37.52, S 15.15, Se 0.40, Te 0.66, total 99.52. The empirical formula calculated on the basis of 36 atoms per formula unit is (Ag3.01Cu0.18)Σ3.19(Pb6.98Cd0.01)Σ6.99Bi7.00(S18.42Se0.20Te0.20)Σ18.82. Tarutinoite is monoclinic, space group C2/m, with a = 13.5447(12), b = 4.1027(3), c = 32.481(4) Å, β = 96.433(9)°, V = 1793.6(3) Å3 and Z = 2. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 16.15 (48) (0 0 2), 3.407 (69) (1 1 –5), 3.328 (95) (2 0 –9), 3.042 (65) (2 0 –10), 2.941 (100) (3 1 2), 2.910 (55) (3 1 –4), 2.053 (44) (0 2 0). The crystal structure of tarutinoite was refined to R1 = 0.1349 for 2024 reflections with Fo > 4σ(Fo) and 84 refined parameters. The new mineral is the first 7,8L member of the lillianite homologous series. It is named after its type locality.

Members of norsethite-type carbonate solid solutions with the compositions Ba(Mg1–xMnx)(CO3)2, (x = 0, 0.25, 0.50 and 0.75) have been synthesised under high-pressure and -temperature conditions (3GPa, 800°C) for the first time. The synthetic transparent crystals gradually changed their appearance from colourless to blue lustre with the increasing Mn2+ content (XMn). The results of the crystal structure analyses reveal that the lattice parameters (a, c, unit-cell volume, Mg/Mn–O bond lengths and Ba–O bond lengths) complied with a linear increase with XMn. In contrast, the C–O bond lengths and O–C–O bond angles decreased, because the CO32– group was squeezed by the expansion of the (Mg/Mn)O6 octahedra. Moreover, the Raman and infrared vibrations, except for the lattice mode T, shift to low frequency with the increasing XMn, and the slight corresponding variations of the atomic positions were also determined. These new results demonstrate the impact of Mg2+–Mn2+ substitution on the crystal chemistry of norsethite-type solid solutions, with further implications for the natural occurrence and environmental of norsethite-type and dolomite/ankerite-type carbonates.

Nano-silicon has been regarded as the most promising anode material for next-generation lithium-ion batteries (LIBs). However, the preparation of nano-silicon suffers from high cost, complex procedures, and low yield, which hinders its commercial application. In this study, porous nano-silicon with particle sizes in the range of 50–100 nm was prepared through molten salt-assisted magnesiothermic reduction using porous nano-silica derived from clay minerals as the precursor. Through combining ball milling and acid activation, the synthesised nano-silica derived from montmorillonite exhibited smaller particle sizes (below 50 nm), higher specific surface area (647 m2 g–1), and total pore volume (0.71 cm3 g–1). This unique structure greatly facilitated the conversion efficiency of silica into nano-silicon by maximising the contact area between silica and magnesium powder and optimising the diffusion kinetics of magnesium atoms. When used as anodes in LIBs, the synthesised nano-silicon materials demonstrated a high specific capacity of up to 1222 mAh g–1 and an excellent capacity retention rate of 79% after 150 cycles at a current density of 0.5 A g–1. This method provides a novel approach for the cost-effective and large-scale production of nano-silicon materials for high-performance anodes.

The new mineral popugaevaite Ca3[B5O6(OH)6]FCl2·8H2O was found at the Internatsional’nyi diamond mine, Internatsional’naya kimberlite pipe, Sakha (Yakutia) Republic, Russia. It belongs to the low-temperature hydrothermal mineral assemblage formed in the contact zone between kimberlite and a boron-bearing halite rock. Popugaevaite occurs as veinlets in massive aggregates of ekaterinite and crusts (up to 0.7 mm thick and up to 1 cm × 4 cm in area) on ekaterinite nodules embedded in halite. Other associated minerals are Fe-rich szaibélyite, serpentine, dolomite, pyrrhotite and chalcopyrite. Crude prismatic crystals of popugaevaite are up to 0.3 × 1 mm. The mineral is transparent, colourless, with vitreous lustre and perfect {010} cleavage. It is optically biaxial (–), α 1.502(2), β 1.523(2), γ 1.530(2) and 2Vmeas = 50(10)°. The chemical composition (wt.%, electron-microprobe, boron by ICP-MS, H2O calculated by stoichiometry) is: CaO 28.54, B2O3 28.62, F 3.19, Cl 11.50, H2O 32.83, O = (F,Cl) –3.94, total 100.74. The empirical formula, calculated based on 23 O+F+Cl and 22 H atoms per formula unit, is Ca3.07B4.96O6.03(OH)6F1.01Cl1.96·8H2O. Popugaevaite is monoclinic, space group Pn, a = 8.7055(11), b = 8.1025(11), c = 14.812(2) Å, β = 91.367(7)°, V = 1044.5(2) Å3 and Z = 2. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I,%)(hkl)] are: 8.12(100)(010), 4.058(27)(020), 3.577(15)($\bar 1$21), 2.936(10)(123), 2.834(16)(301, $\bar 1$05) and 2.283(10)(133). The crystal structure was solved based on single-crystal XRD data and refined on powder data by the Rietveld method, Rwp = 0.0058, Rp = 0.0043 and Robs = 0.0241. Popugaevaite is an isostructural analogue of brianroulstonite Ca3[B5O6(OH)6](OH)Cl2·8H2O with F– instead of the OH– group non-bound with boron. The structure is based upon the layers of twelve-membered rings of alternating BO3 triangles and BO2(OH)2 tetrahedra. The mineral is named in honour of the Russian geologist Larisa Anatol’evna Popugaeva (1923–1977), one of the principal discoverers of diamondiferous kimberlite pipes in Yakutia.

The new mineral kantorite was found in the Arsenatnaya fumarole, Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. The associated minerals are aphthitalite, langbeinite, arcanite, krasheninnikovite, vanthoffite, kononovite, wulffite, halite, sylvite, flinteite, fluoborite, chubarovite, johillerite, urusovite, zincite, tenorite, pseudobrookite, hematite, sanidine and fluorophlogopite. Kantorite occurs as long-prismatic to acicular crystals up to 0.1 × 0.01 mm which form near-parallel and bush-like open-work clusters up to 0.2 mm across. It is transparent, colourless, with vitreous lustre. Dcalc is 2.498 g cm–3. Kantorite is optically biaxial (+), α = 1.447(2), β = 1.449(2), γ = 1.452(2) and 2Vcalc = 79°. The chemical composition (wt.%, electron microprobe data) is: Na2O 9.80, K2O 27.17, Rb2O 0.12, MgO 12.02, CaO 0.09, SO3 47.46, F 5.68, Cl 0.11, –O=(F,Cl) 2.42, total 100.03. The empirical formula calculated based on (O+F+Cl) = 9 apfu is (K1.94Na0.06)Σ2.00(Na1.00Ca0.01)Σ1.01Mg1.00(SO4.01)1.99(F1.01Cl0.01)Σ1.02. The idealised formula is K2NaMg(SO4)2F. Kantorite is orthorhombic, Pna21, a = 6.9894(7), b = 7.1378(7), c = 17.925(2) Å, V = 894.25(16) Å3 and Z = 4. Strong reflections of the powder XRD pattern [d,Å(I)(hkl)] are: 8.99(91)(002), 4.83(57)(111), 4.498(43)(004), 3.588(47)(020), 3.511(74)(200), 3.270(43)(202), 2.926(100)(115), 2.805(44)(024), 2.768(53)(204) and 2.510(55)(220). The crystal structure was solved from single-crystal XRD data, R1 = 0.031. The structure of kantorite is unique. It represents a quasi-framework in which the basic units are octahedra [MgO4F2]. They are linked via bridging F atoms to form infinite chains. These chains are encrusted by [SO4] tetrahedra and assembled into the quasi-framework by alkali cations. The sulfate-encrusted chains of octahedra [MgO4F2] are topologically identical in the structures of kantorite and krasheninnikovite KNa2CaMg(SO4)3F. The mineral is named in honour of the Russian mineralogist Boris Zinovievich Kantor (1930–2022).

‘Print stone’ is an iron-banded siltstone from the Pilbara Province of Western Australia that bears partial resemblance to iconic East Kimberley ‘zebra rock’ in both pattern morphology and mineralogical composition. Using a combination of mineralogy and elemental geochemistry, this investigation examines the mechanisms underlying the formation of periodic iron-oxide banding in print stone. We demonstrate that print stone patterns probably arose from the periodic deposition of hydrothermal pyrite during the early Palaeoproterozoic, as evidenced by the distinctive cuboid morphology of the hematite pigment, the deposition of iron oxides along fluid-transport pathways, the presence of extensive hydrothermal pyrite elsewhere in the formation, and the presence of a positive europium anomaly. Through spatial analysis of the iron-oxide banding, we further show that print stone adheres to the Liesegang spacing law with a spacing coefficient of 0.018. This suggests that the periodic deposition of pyrite in print stone arose due to the Liesegang phenomenon, which was probably triggered by the infiltration of near-neutral, sulfidic hydrothermal fluids into a ferruginous, feldspathic shale. Altogether, the present findings demonstrate the opportunity for iron-oxide Liesegang bands to develop in hydrothermal systems, providing additional insight into the mechanisms underlying the formation of the East Kimberley zebra rock and other banded geological material.

The minerals of the kröhnkite, talmessite and fairfieldite groups plus dondoellite, general formula X2M2+(TO4)2(H2O)2 where X = Na, Ca; M2+ = Mg, Fe, Mn, Co, Ni, Zn, Cu; and T = S, P, As; have been consolidated into the IMA-approved kröhnkite supergroup. The cell dimensions and space-group symmetries of the minerals in these three groups and dondoellite are distinct but the structures of these minerals are strongly related as they have topologically identical structural units. Differences in structure between the different groups involve the disposition of adjacent [M2+(TO4)2(H2O)2] chains and the stereochemical details of the hydrogen bonds that link adjacent [M2+(TO4)2(H2O)2] chains. The ions that form the interstitial complex of a mineral and the structural unit must satisfy the principle of correspondence of Lewis acidity–basicity whereby stable structures will form where the Lewis acidity of the structural unit closely matches the Lewis basicity of the interstitial complex. For pentavalent T cations (P5+or As5+), the Lewis acidity of Ca2+ shows the closest match of all cations to the Lewis basicity of the structural unit. For hexavalent T cations (S6+), the Lewis acidity of Na+ shows the closest match of all cations to the Lewis basicity of the structural unit.