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.

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.

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.

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.

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.

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.

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.

The new mineral almagreraite (IMA2025-025), Cu2+ZnMn4+3O8, was found on the dump of the República Romana mine, Sierra Almagrera, Cuevas del Almanzora, Almería, Andalusia, Spain, where it occurs as tapering prisms or blades up to ∼0.4 mm in length, commonly forming subparallel to divergent aggregates. It is a secondary mineral associated intimately with well-crystallized blades of malachite on a matrix consisting of well-crystallized goethite and massive friable hematite. Crystals are black and opaque with submetallic lustre. The streak is dark grey with a brownish tint. The tenacity is brittle and the fracture is splintery. There are probably two cleavages (possibly {100} and {010}). The Mohs hardness is ∼5. The calculated density is 5.174 g·cm–3 based upon the empirical formula. In reflected light, the mineral is bluish grey in colour with moderate bireflectance and slightly bluish grey to a lighter bluish grey pleochroism. Electron probe microanalyses provided (Mn4+3.05Cu2+0.93Zn0.91Al0.05)Σ4.94O8. Almagreraite is orthorhombic, Pmn21, a = 5.7298(4), b = 4.9715(4), c = 9.3479(7) Å, V = 266.28(3) Å3 and Z = 2. In the crystal structure (R1 = 0.0297 for 423 I > 2σI reflections), Mn4+O6 octahedra share edges forming an octahedral sheet parallel to {001}. One in every four octahedra in the sheet is vacant, providing the sheet formula [Mn4+3O8]4–. Successive sheets are connected via bonds to Cu2+ and Zn2+ located in a layer between the sheets. The structure is similar to that of minerals of the nolanite supergroup and most notably the kamiokite group. Almagreraite is a member of a family of mixed-metal oxides with the generic formula M2Mn3O8, which are an important subject of study for the development of new materials.

The new mineral, babunaite-(Nd) NdAsO4, was discovered in metasomatic rocks of the Mixed Series near the Nežilovo village in Northern Macedonia. These rocks are characterised by an unusual occurrence of Pb-Zn oxide ore mineralisation. This area forms part of the high-grade metamorphic region of the Upper Precambrian Pelagonian massif. Babunaite-(Nd) is an accessory mineral in pink schists, which mainly consists of Mn-bearing muscovite and quartz, with minor braunite. Accessory minerals are hematite, gahnite, almeidaite, långbanite, zircon, piemontite and piemontite-(Pb), nežilovite, Sb-bearing rutile, fluorapatite, As-bearing fluorapatite, gasparite-(La), chernovite-(Y), arsenoflorencite-(La). Babunaite-(Nd) forms single crystals measuring up to 70 μm in size. The transparent crystals exhibit an adamantine lustre and a pale-yellow colour. The microhardness of babunaite-(Nd) is VHN25 = 578(21) kg/mm2, equivalent to a hardness of 5 on the Mohs scale. The mineral is brittle and does not exhibit cleavage. The mean composition of the holotype crystal is as follows: (Nd3+0.39Ca0.14Th0.09Pr3+0.08La0.07Sm3+0.06Y0.06Gd0.05Ce3+0.02Eu3+0.01)Σ0.97(As5+0.95W6+0.05V5+0.01)Σ1.02O4. The calculated density is 5.918 g·cm–3. Babunaite-(Nd), with the general crystal chemical formula ABO4, has a scheelite-type structure and crystallises in the tetragonal I41/a space group: a = 5.1363(2) Å, c = 11.5764(8) Å, V = 305.41(3) Å3 and Z = 4. The main bands at 190, 348, 440 and 833 cm–1 are distinguished in the Raman spectrum of babunaite-(Nd). A synthetic analogue of babunaite-(Nd) is known, which forms under high-pressure and high-temperature conditions of 11 GPa and 1100–1300°C. The genesis of the Mixed Series rocks occurred under metamorphic conditions approaching the parameters of the kyanite–graphite subfacies, at which gasparite-(Nd) with the monazite structure should form. The formation of babunaite-(Nd) is associated with the stabilisation of its structure by a tungsten (W) impurity. It is the first arsenate phase containing rare earth elements (REE) in the scheelite group.

Kopernikite, K(Ti7Cr3+)O16 – a new mineral belonging to the priderite group with a hollandite-type structure, was discovered within a graphite–troilite nodule in the Morasko iron meteorite of the IAB-MG type. This meteorite fell in the territory of modern-day Poland around 5000 years ago. Kopernikite is confined to the external graphite zone of the nodule and intergrows with fluorapatite. The kopernikite structure was refined (I4/m, a = 10.0955(5) Å, c = 2.9534(2) Å, V = 301.01(4) Å3 and Z =1) for the grains with the composition (K1.12Ba0.23Ca0.03Na0.03)Σ1.42(Ti4+6.48Cr3+1.46Fe2+0.01 Al0.01)Σ7.96O16 based on 375 unique reflections to R1 value of 0.018. The colour of kopernikite changes from dark green in massive grains to yellow–green in thin grains, and the streak is light green. The mineral has a vitreous lustre and a hardness of ∼5.5 on the Mohs scale, as calculated on the basis of microhardness VHN25 = 646(17) kg/mm2. The Raman spectrum of kopernikite is similar to that of priderite. Kopernikite crystallised from drops of phosphate melt simultaneously with fluorapatite at a temperature of 1000°C, due to the local enrichment of the melt with K, Ba, Ti and Cr. The increased amount of tunnel cations in the kopernikite structure (∼1.4–1.5 apfu), as calculated from microprobe analyses, exceeds the theoretically possible value of 1.33 apfu for minerals of the priderite group. This increase can be attributed to formation of kopernikite under the increased pressure, as well as artefacts that appear when the mineral composition is measured using a microprobe analyser.

Mineral Cup is an annual global volunteer-run competition to determine the most popular mineral, decided by public voting and played out mainly on social media. Over nine years of Mineral Cup, nine distinct minerals or mineral groups have been crowned champion, in chronological order: olivine, garnet, ice, magnetite, quetzalcoatlite, fluorite, zircon, rhodochrosite and kyanite. Participants in Mineral Cup can vote for minerals for any reason, ranging from scientific and often esoteric to aesthetic. Factors influencing voting include name or nickname, colour, striking appearance, utility, unusual characteristics, cultural impact, and fun facts. Mineral Cup has a global reach, with over 30,000 voters a year in 2018–2025 encompassing voters from over 100 countries per year. The popular knockout format of Mineral Cup could also be used in a wider range of geoscience competitions, and in late 2025 the first such instance of a spin-off competition (Ore Cup) was held. In the context of declining enrolments in geoscience degrees, we discuss Mineral Cup as a potential tool to promote the joy of mineralogy widely, including to audiences that may otherwise have limited or no exposure to geoscience. Finally, we provide an outline of future directions of Mineral Cup, the reach of which as an entity is currently limited by volunteer time and availability in much the same way as professional society activities are limited by the availability of their members.

Montanite, named by Frederick August Genth over 150 years ago for the Montana Territory was the world’s first naturally occurring tellurium oxysalt, but was designated ‘Q’ (questionable) by the IMA-CNMNC until our study in 2022. Like many minerals described in the 19th century it had no designated type specimen. Interrogation of institutional archives, contemporaneous publications and specimen labels reveals three distinct early occurrences, two of which are interpreted as co-type localities. In lieu of being able to conclusively identify or access the original type specimens for research, suitable surrogates were defined as neotypes that respected the historical precedence of the initial discoveries enabling montanite to be formally established as a mineral species. Especially for this historically themed edition of Mineralogical Magazine, this paper presents the process and a case-study of the historical research and decision-making required for new minerals, redefinitions or discreditations that are often out-of-scope of the academic journals that publish the resulting scientific research.