The abundance and distribution of rare-earth elements (REE) within fish debris from the Japanese Exclusive Economic Zone around Minami-Torishima Island have been investigated to determine the host materials of REE in the REE-rich deep-sea mud. The study was performed using REE-extraction experiments, field emission electron probe microanalysis (FE-EPMA), transmission electron microscopy with an energy-dispersive X-ray spectrometer (TEM-EDX), laser-induced breakdown spectroscopy (LIBS), Fourier-transform infrared (FTIR) spectroscopy and X-ray absorption near edge structure (XANES) analysis.

The EPMA data indicate that the amount of REE in the apatite in the teeth is less than that in the bone debris. According to TEM-EDX analysis, yttrium (Y) is contained in the rod-like apatite crystals in the bone debris, but not in the long rod-like apatite crystals of the teeth. Analyses by LIBS and EPMA indicate that Y-rich bone debris is also rich in C. The FTIR spectroscopic analysis indicates the presence of carbonate groups within the Y-rich apatite structure. Moreover, the results of the XANES analysis and REE-extraction experiments indicate that Y occurs within the crystal structure of C-bearing apatite in the bone debris. These results suggest that the REE within C-bearing apatite in the fish debris from the Minami-Torishima REE-rich deep-sea mud substitute for Ca in apatite via an ionic exchange reaction, probably Ca2⁺ + P⁵⁺ ↔ REE3⁺ + C⁴⁺.

Terrestrial vascular plants affect Earth’s long-term geological processes, contributing to carbon cycling, chemical weathering and soil formation. Plants transport elements from the soil to their above-ground structures, accumulating a range of macroelements including Na, K, Mg, Ca, Si, S, P and Cl. Wildfire combustion concentrates these macroelements into inorganic ash. This ash is dominated by oxides, carbonates, halides, sulfates and phosphates of Na, K, Mg and Ca. This work describes K₂Ca₂(CO₃)₃, which occurs abundantly in the ash of the desert spoon (Dasylirion wheeleri), a plant native to the Sonoran Desert. Electron microprobe analysis, powder X-ray diffraction Rietveld refinement and Raman spectroscopy confirm that this phase matches synthetic rhombohedral (R3) K₂Ca₂(CO₃)₃. This phase forms during the smouldering combustion of D. wheeleri trunks, producing friable, decimetre-sized, porous, ash lumps that pseudomorphically preserve the plant’s fibrous structure. This ash occurs as glassy, sintered, porous aggregates, dominated by K₂Ca₂(CO₃)₃, with sylvite, calcite, fairchildite, arcanite and minor hydroxyapatite and periclase. Several double K–Ca carbonates form under surficial pressures and temperatures below ~800°C, including K₂Ca₂(CO₃)₃, and bütschliite (K₂Ca(CO₃)₂) and its dimorph, fairchildite. The occurrence of rhombohedral K₂Ca₂(CO₃)₃ and fairchildite are consistent with smouldering between 518 and 780°C. Upon exposure to water, K₂Ca₂(CO₃)₃ rapidly decomposes, leaving calcite. The occurrence of K₂Ca₂(CO₃)₃ as a major phase in the plant ash expands our understanding of Earth’s mineral diversity, provides new insights into the widespread geological process of wildfire ash formation and highlights the role that these fires play in forming mineral phases that are rare in other geological settings. Though K₂Ca₂(CO₃)₃ was first identified in Dasylirion wheeleri, this phase probably forms in other fire-adapted plant species. The occurrence of K₂Ca₂(CO₃)₃ in plant ash is an example of an inorganic phase that bridges the gap between biomineralisation and geological mineral formation.

Mineralogical crystallography has evolved from the geometric and observational studies of the eighteenth century to a dynamic, predictive science capable of probing matter at atomic and nano-scales. Contemporary advances, including ultrafast X-ray free-electron lasers, high-pressure diamond anvil cells, cryo- and environmental electron microscopy, and multimodal in situ techniques, now permit real-time observation of mineral transformations under extreme conditions. Coupled with computational modelling and predictive simulations, these methods are transforming crystallography into an integrative, interdisciplinary discipline with applications ranging from Earth and planetary sciences to materials engineering. This essay explores technological innovations and emerging frontiers of mineralogical crystallography, highlighting its enduring role in revealing the hidden architectures of matter and guiding the exploration of both natural and synthetic materials.

Earth’s lithospheric mantle is dominated (usually >90% vol.%) by a nominally volatile-free mineral paragenesis with peridotitic composition. The remaining lithologies are typically hydrous and/or carbonate-bearing assemblages (the latter mostly stable at P >2 GPa), representing the products of mantle metasomatism with, or without, involvement of subduction. Despite their importance, the modal compositions of these metasomes are poorly constrained, probably containing hydrous phases (amphibole and/or phlogopite), coupled with accessory minerals, such as apatite, ilmenite, rutile and/or carbonates. These assemblages are usually considered to be the source of unusual magmas, in particular ultrapotassic compositions, especially kamafugites (K2O-rich, commonly ultracalcic, basic/ultrabasic lithologies). To evaluate if partial melting of such metasomes could effectively produce kamafugites, we performed partial melting experiments at 2.7 and 5 GPa, and 1200°C to 1550°C on clinopyroxenites variably enriched in phlogopite, olivine and accessory phases (apatite, oxides, titanite), and on a clinopyroxene glimmerite (with apatite and magnetite).

At low degrees of melting, the glasses show extremely high TiO2 (<15 wt.%), CaO (<18 wt.%) and P2O5 (<7.4 wt.%), coupled with low SiO2 (>21.6 wt.%), as accessory minerals are the principal contributors in the melting reactions. Although there are no known natural counterparts for these low-degree melt compositions, they might play a key role in re-fertilisation events in the upper mantle. At increased degrees of partial melting (∼50 to ∼90%), the experimental melts approach the compositions of silica-poor, potassic/ultrapotassic and ultracalcic rocks. Indeed, experimental-produced glasses share several geochemical similarities with natural kamafugites, partially overlapping for most of the major oxides. Clinopyroxene- and phlogopite-rich lithologies, variably enriched in olivine and accessory phases (apatite, oxides, titanite) probably occur as veins pervading the lithospheric peridotitic matrix, and their partial melting, especially at high degrees, may be a plausible explanation for the genesis of SiO2-poor, K2O- and CaO-rich compositions.

I present a new set of new rules for Crystal Chemistry that greatly increases our understanding of the factors affecting the stereochemistry of mineral and inorganic crystal structures.

The electric field in a crystal is a vector field; bond strengths from cations to anions are positive and bond strengths from anions to cations are negative. The incident bond strengths at all ion sites must equal the formal charges at those sites. Bond strengths along non-degenerate paths between symmetrically equivalent ions in the structure must sum to zero. This leads to rule 1: the a priori bond-strength rule: “A priori bond-strengths may be calculated for all bonds in a structure by constructing a bond-strength table that includes all bond-strengths as unknown variables. The corresponding charge-conservation matrix can be solved for all the unknown bond-strengths”. The resultant bond strengths depend only on the formal charges of the constituent ions and the bond topology of the structure. However, they correlate strongly with bond lengths.

Ion radii derived from experimental bond lengths do not represent the radii of ions in crystals as we cannot objectively divide bond lengths into the radii of the constituent ions. This leads to rule 2, the ion-radius rule: “Ratios of ion radii have no physical meaning whereas sums of ion radii can be used in crystal chemistry (e.g. correlating site occupancies with observed mean bond lengths).”

The characteristic Lewis acidity of a cation is defined as its characteristic bond strength, which is equal to its charge/characteristic-coordination-number. The characteristic Lewis basicity of an anion is defined as the characteristic strength of the bonds formed by the anion. This leads to rule 3, the bond-strength-matching rule: “Stable structures will form where the Lewis acidity of the cation closely matches the Lewis basicity of the anion.” Cation and anion coordination numbers adjust to optimize matching of Lewis acidities and Lewis basicities.

The hydrogen isotope composition of the mantle provides insight into the advection, melting and metasomatism of the mantle, and the Earth’s water cycle. Because of sampling challenges, most estimates for the hydrogen composition of the mantle have been derived from basaltic glasses rather than from direct samples of the mantle. We present the first direct measurement of D/H ratios in nominally anhydrous orthopyroxene and olivine inclusions hosted within diamonds. The diamond hosts were recovered from placer deposits in Guyana, South America. The diamond-hosted inclusions preserve a population with δD of –31 ± 59‰ (normalised to Vienna Standard Mean Ocean Water). This value is D-enriched relative to that expected for a homogeneous upper mantle and is similar to values measured for arc basalts. We introduce three explanations for the observed D-enrichment in the anhydrous silicate inclusions. The hydrogen isotopic composition of the mantle might simply be heterogeneous in space and/or time. The measured D-enrichment could instead preserve local heterogeneity produced by fluids genetically related to the crystallisation of the diamond hosts. Local and mantle-wide processes might also have operated in concert, and overprint each other. Diamond-hosted silicate inclusions preserve an underexplored record of Earth’s water cycle.

The new mineral argentopearceite (IMA2020-049) was found at the mine dump of the abandoned Lehnschafter mine, Mikulov–Hrob district (holotype), and later at museum samples (cotypes) from the Moldava fluorite deposit, both in the Krušné hory Mountains, Czech Republic. Argentopearceite is associated with proustite in quartz gangue (Mikulov) or acanthite and proustite in fluorite gangue (Moldava). The new mineral occurs as tabular (pseudo)hexagonal crystals up to 0.8 mm (Mikulov) and 3 mm (Moldava), and as groups and aggregates up to 1 cm. Argentopearceite from Mikulov is steel grey to black. Mohs hardness is ca. 3; the calculated density is 6.29 g.cm–3. In reflected light, argentopearceite is grey with a greenish shade. Bireflectance was not observed and pleochroism is very weak. Anisotropy under crossed polars is moderate with weak greenish and green–blue tints. Internal reflections were not observed. Reflectance values of argentopearceite in air (Rmin/Rmax, %) are: 27.3/30.0 at 470 nm, 26.6/29.3 at 546 nm, 26.2/28.8 at 589 nm, and 25.9/28.1 at 650 nm). The empirical formula for argentopearceite, based on electron-microprobe analyses (n = 15), is (Ag15.95Cd0.02)Σ15.97(As1.82Sb0.11)Σ1.93(S11.03Cl0.05Te0.01)Σ11.09. The ideal formula is Ag16As2S11, which requires (in wt.%) Ag 77.45, As 6.72 and S 15.83, total of 100.00. Argentopearceite is trigonal, P321, a = 14.8583(5), c = 12.3038(15) Å, with V = 2352.38(15) Å3 and Z = 4. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R1 = 0.0773 on the basis of 6594 unique reflections with Fo > 3σ(F) and 242 refined parameters. The structure of argentopearceite mostly conforms to the general architecture of the As-dominant members of the pearceite–polybasite family of minerals.

Anomalously high amounts of orthopyroxene are an unexpected feature of some depleted peridotite xenoliths from the cratonic lithosphere. This investigation presents new petrographic, geochemical and thermodynamic modelling data for silica-rich spinel harzburgites and one garnet–spinel harzburgite from the Kaapvaal Craton, South Africa, to constrain the pressure–temperature (P–T) conditions of orthopyroxene formation. At the time of sampling by the host kimberlite, the garnet–spinel harzburgite resided on the geotherm at 25 kbar, 765°C, wheras the compositionally similar garnet-free samples probably equilibrated at broadly comparable P–T conditions. The samples are subdivided into ‘bronzite type’ peridotites characterised by orthopyroxene with micrometre scale exsolutions of spinel ± clinopyroxene, and ‘normal type’ peridotites in which orthopyroxene does not contain visible exsolved phases. The presence of these exsolved phases indicates unmixing from a more Al-Cr-(Ca)-rich precursor orthopyroxene i.e. an original orthopyroxene that precipitated from, and was, in equilibrium with the Si-enriching agent at high temperature, before cooling and undergoing sub-solidus re-equilibration to form the exsolved orthopyroxene we observe now. Major and trace element compositions, geochemical interpretation and thermodynamic modelling of the samples show that the precursor orthopyroxene coexisted with melt at high pressure and temperature (~40–50 kbar, 1600–1700°C) within the garnet stability field. We consider these to be the conditions of orthopyroxene formation, and thus silica enrichment. These conditions, together with the refractory composition of the host peridotite (olivine forsterite contents > 92) suggest that silica enrichment is a secondary process potentially resulting from interaction with komatiitic melts. Our observations demonstrate that orthopyroxene enrichment in spinel-facies peridotites is not limited to low-pressure processes but can also occur via high-pressure melt–rock interaction, offering new insights into the thermal and chemical evolution of the sub-cratonic lithospheric mantle.

Single-crystal synchrotron X-ray diffraction data were collected up to 10 GPa at room temperature on a natural omphacite with composition close to Jd43Di57, at the Xpress beamline at Elettra Synchrotron, using a diamond anvil cell. A second-order Birch-Murnaghan equation of state (EoS) fit to the unit-cell volumes determined at 20 pressure points yielded V0 = 422.85(15) Å3, and K0 = 121.3(1.2) GPa. These elastic parameters are consistent with the general trend of the diopside–jadeite join. The structural evolution with pressure was determined from both ab initio simulations and structure refinements to the X-ray intensity data. The consistency between experimental findings and local geometrical distortions identified through ab initio calculations is discussed. A distortion variation at the M1 polyhedron occurs at ∼3 GPa, which correlates with the TILT angle of the T2 tetrahedron which stabilises at a similar pressure, coinciding with a decrease in the rate of M1 deformation under pressure.

These results revealing the structural evolution with pressure correlate with changes observed previously in some Raman shifts in the same pressure range in the same material.

Twenty extra-framework S- and C-bearing species including anions (SO42–, SO32–, S2–, S52–, HS–, CO32–, C2O42– and HCO4–), radical anions (S2•–, S3•–, cis- and trans-S4•– and SO4•–) and neutral molecules (CO2, COS, H2S, cis- and trans-S4 and S6) have been identified in natural tectosilicates (members of the sodalite, cancrinite and scapolite groups) using a novel multimethodic approach based on several spectroscopic methods, single-crystal and powder X-ray diffraction, and electron microprobe and wet chemical analyses. Various polysulfide groups were detected in lazurite, haüyne, vladimirivanovite, sapozhnikovite, slyudyankaite, kyanoxalite, balliranoite, marinellite, tounkite, bystrite, sulfhydrylbystrite and meionite. Experimental data on mutual conversions of S-bearing species and structure modulations of these minerals under different temperatures, reducing and oxidizing environment as well as radiation-induced conversions provide a key for their use as markers of the crystallization or transformation conditions in Nature and during synthesis and modification.

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.

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.

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.

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.