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.

Ranunculite is a rare supergene hydrated aluminium uranyl phosphate reliably reported only from the type locality – the Kobokobo pegmatite in the Sud-Kivu province, Democratic Republic of Congo; its structure has remained unknown until now. Based on 3D electron diffraction data, ranunculite is monoclinic, with a C-centred unit cell: a = 11.1812(7) Å, b = 17.9281(5) Å, c = 17.91548(16) Å, β = 98.350(4)°, and V = 3553.2(2) Å3 (Z = 4). The structure (C2/c) was refined kinematically to R1 = 0.4114 for 1697 unique observed reflections. The structure of ranunculite is based upon infinite uranyl-phosphate sheets of novel topology. The two-dimensional representation of the structural unit consists of hexagons (occupied by U6+), pentagons (occupied by U6+), squares (occupied by Al3+) and triangles (occupied by P5+). Those sheets are stacked perpendicular to c; the interplanar distance is ~9.5 Å. They result from the clusters of edge-sharing uranyl hexagonal and pentagonal bipyramids linked by Al-octahedra and PO4 tetrahedra. The decoration of the sheets is unique but somewhat resembles the arrangement (of U-clusters, squares and triangles) observed in bijvoetite and lepersonnite topologies; the ring symbol is 61514232. In the interlayer, there are two Al3+-hosting sites (one [6]- and [5]-coordinated; the pyramidal one is only partially occupied), as well as isolated H2O groups. There is an extensive network of hydrogen bonds; adjacent sheets are either held by hydrogen bonds only or by tetramers of Al-polyhedra when occupied (through shared O19). This arrangement most probably causes a poor crystallinity of ranunculite (which gives rise to stacking faults observed in the powder diffraction data).

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 pendevilleite-(Y) (IMA 2022-054), ideally Mg2Y3Al(UO2)2(CO3)7(OH)6(H2O)16, was found in the famous Kamoto-East open-cut, Lualaba province, Democratic Republic of Congo and named after Jean-Marie Pendeville (1936–2002), a specialist in collecting minerals of Congo. The new mineral occurs as extremely thin blades (up to ∼0.08–0.10 mm in length and only ∼1 μm thick), often forming lichen-like aggregates and crusts. It is associated with kamotoite-(Y), astrocyanite-(Ce) and shabaite-(Nd), uranophane and sklodowskite. Pendevilleite-(Y) crystals are whitish or greyish-white, locally pale-bluish white. The mineral is brittle; has an irregular fracture and a Mohs hardness of ∼2. Cleavage is perfect on {001}. Electron microprobe analyses provided (on the basis of 2 apfu U with CO32–, H2O derived from the structure and OH– to keep the electroneutrality) formula Mg1.78[(Y1.42Gd0.36Dy0.33Nd0.16Er0.14Sm0.13Eu0.12Tb0.05Ho0.04Yb0.04Ce0.03Tm0.03Pr0.01)Σ2.86 Ca0.11Pb0.01]Σ2.98Al0.88(UO2)2(CO3)7(OH)5.02(H2O)16. Pendevilleite-(Y) is triclinic, P$\overline 1 $, a = 11.9130(3) Å, b = 13.5252(11) Å, c = 16.1531(3) Å, α = 107.052(3)°, β = 92.7765(19)°, γ = 109.676(4)° and V = 2311.5(2) Å3 (Z = 2) at 97 K. The crystal structure (dynamical refinement against 3D ED data; R1 = 0.0948 for 1168 [I > 3σ(I)] reflections) possesses a large heteropolyhedral framework based on both a finite [(UO2)(CO3)3]4– cluster (UTC cluster) and a dimeric [(UO2)2(CO3)4(OH)2]6– unit formed due to olation of uranyl polyhedra. There are three M sites in the structure, occupied by Y3+ and Ln3+, with symmetry-related equivalents forming a polyoxometalate cluster of the general composition [(Y,Ln)6(OH)8(H2O)4(CO3)4]2+. Additionally, there is one Al site in the structure (symmetrically related equivalents forming a dimer of composition [Al2O2(OH)8]6–), and two Mg sites in octahedral coordination MgO2(H2O)4. In the sizeable channels of the framework (running parallel to c), there are at least eight independent partially occupied and disordered O sites of the H2O molecules.

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.

Bornite (Cu5FeS4) and digenite (Cu9–xFexS5; x = 0.4) have closely related cubic structures and are known for their range of superstructures derived from metal vacancies leading to larger unit cells expressed as n × a, where a = ∼5.5 Å and n is an integer. Such polymorphs can form during cooling from higher temperature bornite (Bn)–digenite (Dg) 1a solid solution (ss). The alleged basket-weave textures in natural bornite are investigated using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging and energy-dispersive X-ray spectrometry. These techniques, combined with crystal modelling and STEM simulations, are suitable for depicting changes in phases related to crystal-structural modularity as they collectively better reproduce atomic distributions in real space. Bornite associated with either chalcocite or chalcopyrite from the Olympic Dam Cu-U-Au-Ag deposit, South Australia has non-stoichiometric Cu/Fe ratios and displays nanoscale basket-weave textures between the main components Bn2a and anilite (Cu7S4); Dg1a is preserved throughout, albeit as a minor phase. Anilite is a derivative of digenite, whereby a = b = √2aDg and c = 2aDg. Two intermediate phases, Dg3a and Bn2a4a, are documented and an additional phase, Bn2a6a, is tentatively suggested to occur in Fe-rich nanodomains within Bn2a. Considering the epitaxial relationships between all phases, we infer that basket-weave textures record phase transitions via polymorphic transformations of parent Bn2a and Dg1a during cooling. Observed phase assemblages are thus linked to cooling of Bn–Dgss in the range 70–87 mol.% Bn along a Cu6.18Fe1.26S5 – Cu9.12Fe0.89S5 tie-line defined from measured compositions. We depict three associations: Bn2a + Dg1a, Bn2a4a + Dg3a, and Bn2a4a/Bn2a6a + anilite, formed during cooling. Polymorph associations like these are relevant for enrichment of critical/precious metals in copper ores because Bi, Pb, Ag, Te and, probably also Au, if dissolved in Bn–Dgss, could be incorporated into superstructures during Cu-Fe-sulfide phase transitions.

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.