I consider the conditions for defining a mineral by its dominant end-member formula. One can calculate the end-member proportions of the end-members of a mineral provided that the end-members are linearly independent (i.e. they are phase components of the mineral); the result includes the dominant end-member of the mineral. If the end-members used in this calculation are not linearly independent, the corresponding set of simultaneous equations is indeterminate. One may remove an end-member from the system, removing the linear dependence; however, any end-member formula may be removed, leaving various sets of end-members that function as phase components. Each set of end-members produces a different solution for the end-member proportions. Each set of positive end-member proportions may (or may not) result in a different dominant end-member; however, within the compositional limits of the species, the same end-member is dominant over all others calculated with different combinations of component end-members. Problems previously encountered in attempting to calculate the dominant end-member formula were due to (1) using mineral formulae that do not accord with the requirements of stoichiometry, and (2) using end-members that are not components of the system. Where the set of end-members chosen to relate mineral composition to end-member proportions contains an end-member that is a linear combination of the other end-members, one must calculate the end-member proportions for all distinct subsets of linearly independent end-members. The dominant end-member over all sets of end-member proportions with all proportions positive is the dominant end-member. Thus for any mineral formula, the dominant end-member formula may be identified and serves to uniquely characterize and identify the mineral. The arguments used here are illustrated by reference to the minerals of the garnet supergroup.

The triphylite–lithiophilite series, Li(Fe2+,Mn2+)PO4, of primary phosphate minerals are notable for their susceptibility to alteration and their extensive range of alteration products. This article considers the principal crystallographic motifs that relate the crystal structures of the alteration minerals and their relationships to the parent structures. For alteration under oxidizing conditions the common structural motif is a laueite-like heteropolyhedral layer comprising 7 Å corner-connected octahedral chains, cross-linked by M2(TO4)2Φ8 cyclic tetramers. With decreasing temperature of hydrothermal alteration, the structures change, from containing two sets of interpenetrating, quasi-orthogonal, laueite-type layers, to containing one orientation of the laueite-type layers, in two-layer-wide slabs. Alteration minerals formed under supergene conditions, such as the laueite-group minerals have single laueite-type layers interconnected via corner-sharing with hydrated octahedrally coordinated divalent cations. Under reducing conditions, the alteration phases are hydrated minerals, which have structures based on strings of edge-sharing octahedra. They have, in common with the oxidized alteration products, the linking of the octahedral chains by cyclic tetramers that polymerize as kröhnkite-related chains. Motifs present in the structures of the alteration phases can also be identified in the parent structures, suggesting that they may be nucleating sites for the formation/growth of the alteration phases.

Groutite, α-Mn3⁺O(OH), is a mineral that belongs to the diaspore group. We used X-ray synchrotron radiation in the pressure range from 0.65(6) GPa to 39.85(8) GPa to collect diffraction data for three single crystals of groutite placed in a diamond anvil cell. Groutite is orthorhombic, space group Pbnm. Analysis of the reciprocal lattice planes and the intensities of the measured reflections revealed that above 30 GPa, a phase transformation begins in the structure. This transition is gradual, with observable changes in the intensity of specific reflections, particularly those that are forbidden in the Cmcm space group. Although we see significant decreases in the intensities of these reflections, many of them still have intensities significantly greater than zero after the beginning of transformation. This is accompanied by a smooth, continuous shrinkage in the unit cell volume, symmetrization of hydrogen positions, and distortion of the MnO6 octahedra. Analysis of the reflection intensities confirms that the observed changes are due to structural transformations and not experimental or sample effects. This means that the starting Pbnm symmetry is preserved although the mineral transforms as a function of pressure close to the Cmcm space-group symmetry. We propose a mechanism for this phase transition in the Pbnm space group.

Silica enrichment in cratonic peridotites, expressed as high modal contents of orthopyroxene is a common yet enigmatic feature of Archean lithospheric mantle. Less widely recognised, silica depletion, expressed as high olivine content is also present in some locations. Although high-pressure melting alone cannot account for the full range of observed silica contents, open-system interaction between mantle melts and lithospheric peridotite offers a viable alternative. We present results from high-pressure (5 GPa), high-temperature (1600–1690°C) multi-anvil experiments designed to investigate the reaction between Al-depleted komatiitic melts and both fertile and moderately depleted peridotite. Using hybrid and reaction-couple experimental configurations, we track mineralogical and compositional changes across controlled thermal gradients within the experimental capsules. Modal and compositional data reveal a two-stage reaction process. Stage 1 involves high-temperature olivine precipitation from primitive Al-depleted komatiitic melts interacting with peridotite to produce olivine-rich residues with high MgO/SiO2 and low Al2O3/ SiO2. As a result of olivine removal, the coexisting melt becomes enriched in SiO2. In stage 2, the now silica-enriched komatiite melt reacts with residual olivine (±clinopyroxene and garnet) to form orthopyroxene at moderate temperature, increasing the bulk SiO2 content. Residue bulk compositions from olivine-rich zones resemble high-Mg# garnet dunites from Archean lithosphere, whereas orthopyroxene-rich zones are analogues to natural silica-rich cratonic peridotites. Coexisting melts evolve from komatiitic toward picritic compositions as the reaction progresses. We infer that reactive porous flow of komatiitic melts through depleted lithosphere can simultaneously generate silica-depleted and silica-enriched, refractory residues and diversify melt compositions, providing a process-based framework for the chemical and textural diversity of cratonic mantle.

A series of high-entropy sodalite-and cancrinite-type feldspathoids with mixed Cl–, Br–, I–, S4, S2O32–, Na+, K+ and Ca2+ extra-framework species were synthesized under soft hydrothermal conditions, in order to evaluate the influence of heteroatomic substitutions on luminescence properties and thermal stability of feldspathoids. It was found that relative stability of either cubic sodalite or the hexagonal cancrinite framework is strongly dependent on the type of anionic species. The partial substitution of (Na, K) for Ca, along with the mixed halogen occupancy, substantially improves the thermal stability of sodalites. Novel data on intrinsic luminescence attributed to recombination of electron and hole on halogen-metal complexes within cages is presented. The intrinsic luminescence bands span the entire visible region and depend on the geometry of the cages. The excitation of the luminescence occurs at ∼5 eV (250 nm).

Hydrated sodium and magnesium sulfates are fairly common minerals. This study is focused on löweite from the fumaroles of the Tolbachik volcano in Kamchatka. The crystal structure of löweite was refined using single-crystal X-ray diffraction, and the hydrogen atoms were localized for the first time. In situ single-crystal (temperature range –173 to 227°C) and powder (temperature range –173 to 900°C) X-ray studies were performed. Both techniques show that löweite remains stable up to ∼220°C without showing any signs of potential phase transitions. The mineral is also stable under a vacuum of ∼600 Pa. The following transformation sequence for löweite was observed upon heating: löweite → metathénardite + vanthoffite + ‘x-phase’ → metathénardite + periclase.

The thermal expansion of löweite demonstrates two distinct patterns in two temperature ranges. There is virtually no expansion in the structure until –93°C. Following this, the structure rapidly expands, exhibiting a highly anisotropic behaviour. Shear deformations of the soft S–O–Mg and S–O–Na hinges explain the structural flexibility and adaptability of löweite to changes in physicochemical environments.

The viscosity (η) of a geofluid dictates its roles within the Earth, such as migration in subduction zones and volcanism. High pressures and temperatures at depth influence visocity (η). The falling sphere method is effective to measure viscosity at high-pressures for geofluids that form solids at ambient temperature. A typically metallic sphere is placed atop a solid sample, which is then compressed to high-pressure and later melted by heating. This method is more challenging for geofluids that do not form solids at ambient temperature. A diamond-anvil-cell (DAC) is often used to contain such geofluids, but requires a small sample chamber and that the sphere either falls parallel to the diamond culet faces or rolls along one face. This geometry produces complicated drag effects on a sphere fall and additional frictional forces for a roll. The sphere may also adhere to chamber surfaces, preventing its fall/roll. To circumvent these issues, in this study, we quantify the viscosity of a geofluid (H2O) at pressures <2.5 GPa using the Brownian motions of suspended particles in DAC. Previous high-pressure efforts used particles of polystyrene, which are unstable at ≥300°C, or silica, but only at ambient temperatures. Such temperatures are relatively low for hydrothermal to supercritical geofluids. We tested quartz particles (∼1–2 µm diameter) with heating, as quartz does not significantly dissolve/melt until ≥600°C. Although three times denser than water, the particles remained suspended and displayed Brownian motions for long timescales at temperatures ≤200°C. The measured viscosities are relatively high due to drag from the culets and particle–particle interactions. Regardless, our measured pressure-effect on viscosity shows excellent agreement with the standard reference for water. After correcting for the drag, the η are very low (< 2 mPa s) highlighting that H2O-rich geofluids should be highly mobile at depth.

The new mineral wiperamingaite, NaCaFe3+Al(PO4)F5(OH)·H2O, was found at the Wiperaminga Hill West Quarry, Boolcoomatta Reserve, Olary Province, South Australia, Australia where it has formed by hydrothermal alteration of triplite–zwieselite. Wiperamingaite occurs in a matrix of quartz, minor triplite and pyrite in association with fluorite, bermanite, leucophosphite and phosphosiderite. Crystals are transparent to translucent, brownish-orange to brownish-pink tablets, up to 0.25 mm across. The mineral has a white streak and vitreous lustre. It is brittle with a splintery fracture. The calculated density is 3.11 g/cm3. Optically, the mineral is biaxial (–) with α = 1.538(2), β = 1.599(2), γ = 1.614(2) (white light); 2V = 52(2)°; distinct r > v dispersion; orientation: X = a, Y = b, Z = c; pleochroism: X colourless, Y brown yellow, Z yellow; Y > Z > X.

Electron microprobe analysis provided the empirical formula Na0.97Ca1.01Fe3+0.92Al1.11(PO4)0.97F4.85(OH)1.32·0.95H2O. Wiperamingaite is orthorhombic, P212121, a = 5.3537(11), b = 5.5911(11), c = 26.279(5) Å, V = 786.6(3) Å3 and Z = 4. The structure of wiperamingaite contains chains of cis-corner connected Feφ6 octahedra (φ = O, OH and H2O) running parallel to [010] decorated with corner-connected PO4 tetrahedra. Adjacent chains link by corner-connection between the octahedra and tetrahedra to form sheets parallel to the (001) plane. Alφ6 octahedra (φ = O and F) attach to both sides of the sheets via corner-sharing with PO4 tetrahedra. Naφ11 polyhedra share edges and faces to form a layer between the sheets that links to the sheets via Alφ6 octahedra and Caφ8 polyhedra.

The new mineral boevskite Pb4(TeO3)2(SO4)(S2O3) was found at the Boevskoe W-Be deposit, Chelyabinsk Oblast, Southern Urals, Russia. It occurs as very rare euhedral grains up to 0.25 mm at the contact of galena and pyrite, as inclusions in galena up to 0.2 mm, and as thin veinlets up to 0.2 × 0.03 mm filling cracks in sphalerite. Other associated minerals include anglesite, cerussite, chalcopyrite, empressite, hessite, ingodite, joséite-B, matildite and pyrrhotite. Boevskite is colourless and transparent with an adamantine lustre. It is brittle, with uneven fracture. No cleavage or parting are observed. The Vickers’ micro-indentation hardness (VHN, 10 g load) is 110 kg/mm2, corresponding to a Mohs’ hardness of 2.5–3. Dcalc. = 6.599 g/cm3. Boevskite is optically biaxial, colourless and nonpleochroic. The mean refractive index calculated using the Gladstone–Dale equation is 2.08. The empirical formula calculated on the basis of 13 O apfu is Pb4.01Te4+2.03S6+1.97S2–0.98O13. Boevskite is orthorhombic, space group Pnma, with a = 9.7764(7), b = 13.3622(10), c = 10.7213(9) Å, V = 1400.57(19) Å3 and Z = 4. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (h k l)] are: 6.683 (65) (0 2 0), 3.355 (44) (1 0 3), 3.344 (52) (0 4 0), 3.289 (60) (2 3 0), 3.230 (100) (1 3 2), 3.144 (92) (2 3 1), 3.120 (51) (3 0 1), 2.787 (50) (0 3 3). The crystal structure of boevskite was refined from single-crystal X-ray diffraction data to R1 = 0.0491 for 1538 reflections with I > 2σ(I). Boevskite has a unique structure formed by Te–Pb–O layers coplanar to the ac plane with thiosulfate groups and SO4 tetrahedra located between them. This is the first mineral having both sulfate and thiosulfate groups as species-defining constituents. It is named after the type locality.

Ferroåkermanite, Ca2FeSi2O7 – a new member of the melilite group, has been found in coarse-grained kirschsteinite-bearing paralava in the Hatrurim Basin outcrop between the Zohar and Halamish Wadies of the Hatrurim Complex in Israel. Ferroåkermanite rarely forms single subhedral light-yellow crystals up to 30–50 μm in size with a prismatic habit. The most common are irregular grains, aggregates and intergrowths with gehlenite or ferroåkermanite crystals with perovskite inclusions. The mineral is transparent, exhibits vitreous lustre and has a distinct cleavage on (001). It is non-fluorescent, brittle and has a conchoidal fracture, a Mohs hardness of ∼4.5–5 and a calculated density of 3.20 g/cm3. Ferroåkermanite is uniaxial (–), ω = 1.652(3) and ε = 1.643(3) (λ = 589 nm), and exhibits a visible pleochroism from light-yellow (ω) to intense yellow (ε). The empirical formula of ferroåkermanite calculated on the basis of 7 O is (Ca1.77Na0.18Sr0.02Ba0.02K0.02)Σ2.01(Fe2+0.68Al0.28Mg0.04)Σ1.00(Si1.93Al0.07)Σ2.00O7. The chemical data obtained confirm the presence of ferroåkermanite–gehlenite solid solution (Fe2+ + Si4+ ↔ 2Al3+) in the studied rock, which was verified by Raman spectroscopy investigation. The crystal structure of the new mineral was refined to R = 0.0617 in the space group P$\overline4$21m with the following unit-cell parameters a = 7.7813(7) Å, c = 5.0114(5) Å, V = 303.43(6) Å3, Z = 2. Ferroåkermanite has a melilite-type structure with layers consisting of (Si2O7)6– disilicate units and (Fe2+O4)6– tetrahedra intercalated by layers formed of eightfold-coordinated Ca atoms. Moreover, the T1 site in the holotype specimen shows a mixed occupancy refined to 0.63(3) Fe2+ and 0.37(3) Al. The presence of rock-forming minerals such as gehlenite or rankinite suggests that the paralava analysed formed under high-temperature conditions, confirming that the new mineral ferroåkermanite is indeed a high-temperature phase. Furthermore, the presence of Fe2+-bearing phases, such as kirschsteinite, ferroåkermanite, chromite, ulvöspinel and bennesherite indicates the reduced conditions.

Jatobá is a magnetite-hosted Cu-Ni deposit in the Carajás Domain, Brazil. The deposit is located along E–W structures belonging to the Canaã dos Carajás shear zone and hosted within the Neoarchean Grão Pará Group. Micron to nanoscale investigation of magnetite from mafic host lithologies and Cu-Ni-mineralisation facilitate constraints on ore genesis. Two texturally and geochemically distinct types of magnetite are defined: ‘trellis’ (country rocks) and ‘silicate-mottled’ (ore). An overlap between the types is recognised as ilmenite changes from lamellar trellis to blebby and patchy textures in the silicate-mottled magnetite. The blebby type comprises Al-hydroxides (gibbsite) and talc, replacing hercynite and Mg-amphibole, respectively. The mottled magnetite contains Mg-(Fe)- and Ce-bearing calcic amphiboles, both associated with non-classical pyriboles (NCP). Geochemical signatures change from Ti-Cr-Co-Mn in trellis magnetite to a pronounced REE enrichment in the mottled type. Nano-inclusions of allanite occur as epitactic intergrowths with actinolite within magnetite. Amphiboles in the host rocks mirror those found in magnetite, with ferro-tschermakite present in both cases.

Ilmenite-magnetite nano-thermobarometry yields a range of temperature and logfO2 values (temperature from 728°C at logfO2= –12 to 414°C at logfO2= –31) for re-equilibration between magnetite and ilmenite from initial trellis to the trellis + blebby and to patchy ilmenite in the densely mottled magnetite. Ferro-tschermakite geobarometry enables an estimate of 6.4–7.4 kbar, compatible with amphibolite-facies metamorphism at ∼20 km depth. Syn-metamorphic deformation textures include magnetite + apatite as pods, banding and folds, as well as sigmoidal scapolite and pressure shadows surrounding magnetite. Collectively, these data support a genetic model implying deep shear-zone metamorphism at the base of the Canaã dos Carajás strike-slip structure. This area is a reservoir for metal sources as fluids can tap into granitoids and ultramafic lithologies in the basement. The telescoped transition from trellis to silicate-mottled magnetite records fluid fluxes of variable overprinting effects during protracted fluid–rock interaction.

Allanite is a REE-rich accessory mineral commonly enriched in Th and U and is therefore well suited to U-Th-Pb geochronology. However, widely used international allanite reference materials typically contain common Pb and are not readily accessible. Zircon reference materials are robust, available, and have been applied to allanite age determination via non-matrix-matched LA-ICP-MS calibration, but their performance requires further evaluation. In this study, we provide radiometric ages for magmatic allanite from two Cretaceous granite plutons (Wanganzhen, sample 10581-1 and Heigoubei, sample 10426-1) in the northern Taihang Orogen, China, using zircon reference materials 91500 and Plešovice as primary standards and for quality control. Petrography and trace-element signatures indicate negligible hydrothermal alteration. Tera-Wasserburg lower-intercept ages for allanite sample 10581-1 range from 128.0 ± 4.9 Ma (24 μm spot size) to 135.0 ± 3.0 Ma (60 μm), and for sample 10426-1 from 110.0 ± 22.6 Ma (24 μm) to 108.9 ± 9.9 Ma (60 μm). Plešovice-calibrated results are comparable (123.9 ± 5.4 to 131.9 ± 4.0 Ma; 100.0 ± 28 to 113.0 ± 15 Ma). All allanite ages overlap within uncertainty with coeval zircon U-Pb ages (132.0 ± 0.8 Ma and 110.3 ± 0.8 Ma). Although Th-Pb ages decouple from U-Pb ages at 24 and 44 μm, probably due to different fractionation behaviours between zircon and allanite, the overall results demonstrate that zircon 91500 and Plešovice are suitable reference materials for allanite U-Th-Pb age determination. A spot size of 32–44 μm is recommended. This study provides validation for the non-matrix-matched U-Th-Pb age determination method of allanite using zircon standards, thereby expanding the applicability of allanite geochronology.

Feldspars are the most common minerals in the Earth’s crust, and commonly contain trace concentrations of gallium and germanium. However, their gallium/germanium synthetic analogues are widely used, both in industrial applications and geological reconstructions. Here KMeGe3O8 (Me = Al, Ga) compounds were synthesized using solid-state reactions and hydrothermal synthesis and studied by microprobe analysis and powder X-ray diffraction (under both ambient and high-temperature conditions). In both (Al and Ga) cases, paracelsian-like phases were the first to form, even though feldspar-like phases are predicted to be thermodynamically stable. Paracelsian-like phases are transformed irreversibly upon heating (930 ± 30 and 960 ± 30°C for KGaGe3O8 and KAlGe3O8, respectively) into feldspar-like modifications. According to the obtained data, the feldspar-like modifications exhibit a higher degree of anisotropy, whereas the volume thermal expansion coefficients are higher for paracelsian-like modifications. The influence of chemical composition and topology on the crystallization process, thermal transformation and thermal expansion of feldspar- and paracelsian-like compounds is discussed.

Hydrocalumite, a natural Ca-Al Layered Double Hydroxide (LDH), also known as a hydrated calcium aluminate and an aluminate ferrite monosubstituted (AFm) phase in cement chemistry, has been studied by single-crystal and powder X-ray diffraction, electron probe microanalysis and Raman spectroscopy on a sample from Boisséjour, Puy-de-Dôme, Auvergne-Rhône-Alpes, France. The mineral is monoclinic, space group P2/c, a = 10.0234(3), b = 11.5131(3), c = 16.2989(5) Å, β = 104.205(3)°, V = 1823.39(9) Å3 and Z = 4. The crystal structure has been refined to R1 = 0.0505 based on 6296 unique reflections. The empirical chemical formula of the mineral (Ca, Al, Cl according to electron probe microanalysis; CO2, OH and H2O on the basis of crystal-structure refinement and Raman spectroscopy) is Ca3.96Al2.04(OH)12.04Cl0.96(CO3)0.5˙5H2O that can be idealized as Ca4Al2(OH)12Cl(CO3)0.5·5H2O. The presence of (CO3)2– and (OH)– anionic groups and (H2O)0 molecules is confirmed by Raman spectroscopy. The strongest reflections in the powder X-ray diffraction pattern are (d in Å, Irel in %): 7.89, 100; 3.951, 34; 3.860, 59; 3.753, 26; 2.884, 73; 2.527, 27; 2.501, 38; 2.453, 64; 2.433, 30; 2.327, 25. The crystal structure study reveals the ordering of interlayer species: Cl– and (CO3)2– anions and H2O molecules, which means that both anions play a species-defining role in contrast to the previous suggestions on mono-anionic end-members (with either Cl or OH or CO3 anions). The ordering of two different anions is usually not observed for other LDHs and appears to be a crystal-chemical feature of the Ca-Al LDH members. This work is dedicated to the 150th anniversary of the Mineralogical Society of the UK and Ireland. Hydrocalumite is a mineral with a 90-year history starting from North Ireland, UK, and is a good example of joint efforts of different mineralogical scientific societies in deciphering scientific puzzles.

Metaheimite (IMA 2023-020a), PbCu2(AsO4)(OH)3, has been identified as a new secondary mineral at the Grosses Chalttal deposit, Mürtschenalp district, Glarus, Switzerland. It forms radial aggregates of blade-like crystals of light blue to turquoise blue colour. Metaheimite has a light blue streak and vitreous to silky lustre. Its calculated density is 5.47 g cm–3. The empirical chemical formula based on seven anions per formula unit is (Pb0.96Ca0.03)Cu1.98(As1.01O4)(OH)3. Metaheimite is pseudo-orthorhombic, with monoclinic symmetry, space group P21/n and unit cell parameters a = 5.8347(4), b = 7.7528(6), c = 13.8899(9) Å, β = 90.018(3)°, V = 628.31(8) Å3 and Z = 4. The five strongest lines in the calculated powder diffraction pattern are (d in Å(I)hkl) as follows: 6.945(100)002, 3.870(75)112, 3.169(78)014, 3.145(99)121 and 2.615(81)015. The crystal structure, refined to Robs = 6.13% for 1393 reflections with I > 3σ(I), consists of layers similar to those occurring in heimite, PbCu2(AsO4)(OH)3·2H2O, but with 6+2 coordinated Pb2+. The lack of water molecules in metaheimite causes different interlayer hydrogen bonding and consequently, different layer stacking order as compared to that in heimite. In metaheimite, Cu2+ therefore occurs in a square-pyramidal coordination by five oxygen atoms. Hydrous species in metaheimite have been examined by Raman and by infrared spectroscopy. Metaheimite is structurally related to duftite, PbCu(AsO4)(OH) and may be considered a transitional state between heimite and duftite.

Marinaite, ideally Cu2Fe3+O2(BO3), a new mineral, was found in a high-temperature (400–600°C) crack on the cooling lava field of the 2012–2013 fissure eruption at Tolbachik volcano, Kamchatka, Russia. The associated minerals are belomarinaite, tenorite, hematite, anglesite and gold. Marinaite forms prismatic, tabular, lamellar or acicular crystals up to 0.06 × 0.06 × 0.3 mm or aggregates of these forms up to 3 mm across. The mineral has a brown to black colour and submetallic lustre. Under the microscope in reflected light marinaite is grey with reddish-brown internal reflections. D(calc.)= 5.026 g/cm3. Electron microprobe analyses yielded (wt.%) 62.15 CuO, 16.71 Fe2O3, 5.11 Al2O3, 3.14 TiO2, 0.16 SiO2, 12.84 B2O3, total 100.11 wt.%. The empirical formula calculated based on 5 O apfu is Cu2.07Fe0.56Al0.27Ti0.10Si0.01B0.98O5. Marinaite is monoclinic, space group P21/c, unit-cell parameters are: a = 3.12750(18), b = 11.9690(8), c = 9.4657(5) Å, β = 97.568(6)°, V = 351.24(4) Å3 and Z = 4. The strongest lines of the powder XRD pattern [d in Å(I)(hkl)] are: 5.051(95)(021); 4.700(69)(002); 3.675(26)(031); 2.764(100)(023, $\bar 1$02, 111, 120); 2.524(94)(042); 2.444(45)(130, 102, $\bar 1$31); 2.320(31)(051, $\bar 1$13, 131); 2.037(22) (113, $\bar 1$33); 1.840(22)(044, 133, 062). Marinaite is isostructural with synthetic Cu2Fe3+O2(BO3) and has a monoclinically distorted ludwigite-type structure. The mineral is named in honour of the Russian mineralogist Marina Vsevolodovna Kuznetsova (1948–2015).

Iron oxide-apatite (IOA) deposits and the related iron oxide-copper-gold (IOCG) deposits, are major repositories of base metals (Fe, Cu). However, the genesis of IOA deposits remains a topic of debate, with both magmatic and hydrothermal models. Close parallels exist between IOA deposits and some skarns, which exhibit sodic alteration in silicic host rocks, but are unequivocally metasomatic in origin. In this study we compared the trace element composition of magnetite from IOA deposits in the Kiruna District, Sweden, with magnetite skarns from the Turgai district, Kazakhstan. Comparison with published discrimination diagrams for deposit types shows poor correspondence with defined fields. The two districts correspond closely in terms of Sn and Ga contents, with close correspondence to previous analyses of porphyry and skarn deposits. When estimates of temperature (T) from Mg in magnetite are considered Sn and Ga show little correlation with T, whereas Ni increases and Mn decreases with decreasing T. Rare earth element distribution patterns correspond to local igneous rocks, albeit at lower absolute concentration. Tin and Ga, as high valence ions in tetrahedral sites in magnetite are potentially more resistant to re-equilibration and preserve a high temperature magmatic-hydrothermal signature comparable to Fe skarns and the early magmatic stages of some IOA deposits in the Kiruna district. Overall, these data are consistent with an early high-temperature mineralisation stage, potentially resulting from hypersaline brines or salt melts interacting with volcanic rocks (Kiruna district) or limestone and volcanic rocks (Turgai district), followed by subsequent hydrothermal magnetite mineralisation to relative low T. The high-temperature stage is better represented in the Turgai skarns compared to the Kiruna district IOA deposits. Overprint of sulfide mineralisation on magnetite results in an increase in Ni content which may be an effective tracer for IOCG mineralisation related to IOA deposits, or sulfide mineralisation in skarns, whilst metamorphism may homogenise and reduce trace element concentrations.

The Khaderpet carbonatite (15°58′N, 77°33′E) occurs as a small plug-like intrusion (45 m × 60 m) within altered ultramafic volcaniclastic breccia of unknown parentage. Both the carbonatite and its host rock contain crustal xenoliths of granite and quartz monzonite. Although the absence of primary silicate and oxide phases obscures its direct genetic link with the host rocks, the carbonatite preserves clear evidence of magmatic crystallization subsequently overprinted by hydrothermal alteration, crustal assimilation and supergene oxidation. The rock is dominated by calcite, which occurs in three distinct generations. Early Sr-Ba-rich calcite-1 (0.8–2.1 wt.% SrO and 0.4–2.2 wt.% BaO) phenocrysts co-crystallized with rounded fluorite at temperatures above ∼600°C, and are hosted within a Ba-Sr-poor calcite-2 matrix. Mantle-like bulk-rock δ13C values (–4.21 to –4.62 ‰, VPDB), together with (La/Yb)Cn (>1–100) and Y/Ho (24–34) ratios in calcite-1 and calcite-2, support a primary magmatic origin. Evidence for crustal assimilation includes REE-Si enrichment in apatite (up to 1.3 wt.% SiO2) by a britholite-type substitution, increased allanite abundance near xenolith contacts, Si-rich pyrochlore and interstitial quartz. Coarse calcite-3 veins crosscut the calcite-2 matrix and comprise Mn-Fe-Mg-rich bright calcite-3a cores and nearly pure, dark calcite-3b peripheries. Elevated Mn-Fe-Mg contents and high Y/Ho ratios (up to 64) in calcite-3a reflect rapid crystallization during waning hydrothermal stages. High δ18O values (+9.17 to +11.54 ‰ VSMOW) indicate low-temperature H2O-rich, CO2-poor meteoric fluid alteration. Negative Ce anomalies in apatite (Ce/Ce*: 0.8–0.3) and calcite (Ce/Ce*: 0.8–0.4), most pronounced in calcite-3b (Ce/Ce*: 0.2–0.6), together with apatite trace element compositions, indicate supergene alteration. Textural evidence of supergene alteration includes replacement of pyrochlore-1 by pyrochlore-2, pyrochlore-1 and pyrite by goethite, allanite-(La) by ferriallanite-(Ce), and late precipitation of baryte, REE-fluorocarbonates and vanadinite, indicating involvement of F–, SO42–, Pb and V in oxidizing hydrothermal fluids.

Iron silicides, including hapkeite (Fe2Si), are rare minerals found in both terrestrial and extraterrestrial environments. In this study, we present the discovery of trigonal Fe2Si in glass from the Blackville site, South Carolina, USA, in a discrete, deeply buried layer where it is associated with suessite and other impact-related materials. The site has been previously studied for its rich assemblage of Fe–Si spherules, platinum, iridium and nanodiamonds. Using advanced micro-computed tomography, scanning electron microscopy, electron microprobe analysis, and single-crystal X-ray diffraction, we have characterised the crystal structure of trigonal Fe2Si, identifying it as a distinct phase from the cubic hapkeite. The high concentrations of V, Ti and P in trigonal Fe2Si and its co-occurrence with suessite suggest an impact-related origin or a lightning strike. Anthropogenic processes, although unlikely, cannot yet be completely ruled out. While this compound represents a new polymorph of Fe2Si, we refrain at this time from proposing it as a new mineral species to the International Mineralogical Association (IMA) because of remaining uncertainties about its formation. Further research is needed to determine whether this trigonal Fe2Si was produced by natural processes or by anthropogenesis.