Kristjánite, KNa2H(SO4)2, is a new mineral (IMA2022–131) found in a high-temperature fumarole on Fimmvörðuháls, Iceland. It is monoclinic, P21/n, a = 6.9625(1), b = 9.9953(1), c = 11.0928(2) (Å), β = 105.637(2)° and V = 743.40(2) Å3. Kristjánite forms colourless transparent crystals up to ~100 μm in size in compact aggregates with metathénardite, belomarinaite, aphthitalite, ivsite and an unknown mineral with tentative composition K2NaH(SO4)2, or grows in white globules of smaller grains on their surface. The eight strongest maxima in a powder X-ray diffraction diagram are [d, Å (Intensity)] 4.37 (59), 3.65 (32), 3.48 (78), 3.36 (100), 3.18 (76), 2.83 (43), 2.73 (38) and 2.405 (23). The mineral represents a novel crystal structure type. In it, K is coordinated by eight O atoms, and two symmetrically independent Na atoms by seven O atoms. Two symmetrically independent S atoms are in tetrahedral coordination by O atoms. A close to linear, very short hydrogen bond (2.44 Å), connects the two to a SO4–H–SO4 dimer. K and Na coordination polyhedra share vertices, edges and even faces forming with sulfate tetrahedra a tight structure with narrow [100] channels lined on two opposite sides by hydrogen bonds.

Sulfosalt assemblages in a specimen from the Boliden Au–Cu–(As) deposit in northern Sweden, comprise micrometre to nanometre scale intergrowths of Se-rich izoklakeite and tintinaite with average formulae and calculated homologue number (N) given as: (Cu1.88Fe0.18)2.06(Pb22.92Ag1.47Cd0.01Zn0.01)24.41(Sb13.12Bi8.69)21.8(S50.19Se6.43Te0.12)56.73, N = 3.83, and (Cu1.31Fe0.74)2.05(Pb10.58Ag0.18Cd0.05Zn0.02)10.83(Sb10.2Bi5.23)15.43(S32.22Se2.46)34.7, N = 2.05, respectively. Tintinaite coexists with (Bi, Se)-rich jamesonite. High-angle annular dark field scanning transmission electron microscopy (HAADF STEM) imaging reveals chessboard structures comprising PbS and SnS modules with the number of atoms in the octahedral (M) sites counted as: n1 = 18 and n2 = 8 for tintinaite and n1 = 30 and n2 = 16 for izoklakeite. The homologue number can be calculated using the formula: N = (n1/6)–1 and N = n2/4 for PbS and SnS modules giving NTti = 2 and NIz = 4. A new N = 3 homologue, defined by n = 12 and n = 24 SnS and PbS modules, respectively, is identified as single or double units within areas with intergrowths between kobellite and izoklakeite. HAADF STEM imaging also reveals features attributable to lone electron pair micelles within the Sb-rich kobellite homologues. Atomic-resolution EDS STEM chemical mapping of Pb–Bi–Sb-sulfosalts shows a correlation with crystal structural modularity. The maps also highlight sites in the SnS modules of tintinaite in which Sb > Bi. Coherent nanoscale intergrowths between tintinaite and izoklakeite define jigsaw patterns evolving from chessboard structures and are considered to have formed during co-crystallisation of the two phases. Displacement textures and crosscutting veinlets (a few nm in width) are interpreted as evidence for superimposed syn-metamorphic deformation and are associated with the redistribution of Bi and Se. Imaging and mapping using HAADF STEM techniques is well suited to characterisation of Pb–Sb–Bi-sulfosalt phases, offering largely untapped potential to unravel the evolution of chessboard structures with applications across mineralogy but also extending into allied fields.

Mineralogical features of two orebodies, lenses (C-3 and C-4), at the central part of the Oktyabr'sky deposit have been investigated. Multidirectional mineralogical zoning in the northern and southern orebodies is shown, confirming the hypothesis of their formation from various magmatic flows, which have individual features and their own modes of formation. The southern C-3 and northern C-4 orebodies differ in their mineralogical associations: C-3 is characterised by a high-sulfur association of sulfides; and C-4 contains minerals with a sulfur deficit (talnakhite, sugakiite). Variations in Fe and Ni ratios in pentlandite are controlled by the volatility of sulfur during ore crystallisation. Direct crystallisation zoning is observed in the disseminated ores of the C-4 orebody (borehole RT-107), where the fugacity of sulfur ($f_{S_2}$) increases from bottom to top. In contrast in orebody C-3 (borehole RT-30) $f_{S_2}$ decreases in the same direction. This reverse zoning coincides with the vectors of the evolution of ore systems in various blocks of the Main Ore Body of the Oktyabr'sky deposit. The difference in typomorphic features of disseminated ores of the southern and northern orebodies is confirmed by differences in the associations of platinum-group element minerals (PGMs). Disseminated ores in picritic gabbro-dolerites and massive pyrrhotite ores in the exocontact of the intrusion within the southern orebody differ in the specialisation of PGMs: the former is characterised by minerals of the Pd–Bi–Sb–Te system, the latter by only Pt minerals. The similarity of these types of ores lies in the similar reverse mineralogical and geochemical zoning from top to bottom along the sections, caused by the evolution of the sulfide melt in this direction. The formation of reverse zoning of disseminated ores (zones of ‘marginal reversion’) is probably due to the action of a mechanism of repeated influx of a melt of an increasingly primitive composition into the upper parts of the crystallising flow. Unidirectional trends in massive and disseminated ores are more likely to be due to the action of the same type of mechanism.

Genthelvite from pegmatite veins hosted by the albite-enriched granite (ca.1.8 Ga) corresponding to the Sn–Nb–Ta (F, REE, Li, Zr, U, Th) Madeira deposit, Amazonas, Brazil was studied. Genthelvite, the exclusive Be-bearing mineral within the deposit, occurs as massive crystals of up to 4.7 cm in size. Compositions are homogeneous within individual crystals, although there is moderate variation in the overall composition reflecting relatively limited substitutions within the helvine–genthelvite–danalite solid-solution series, with relatively high Zn contents (36.96 to 49.45 wt.% ZnO), lower Mn contents (0.61 to 3.03 wt.% MnO), and variable Fe contents (2.10 to 10.94 wt.% FeO), completing an existing compositional gap in this system. Genthelvite formed in an alkaline and subaluminous environment, under stable conditions within the late-evolved fluids, at relatively high temperature (>400°C), in a reducing environment. The extremely high concentration of fluorine in the magma and the crystallisation of magmatic galena resulted in an effective reduction of H2S fugacity. This resulted in the stabilisation of genthelvite during the transition from the late magmatic to early hydrothermal stages of the albite-enriched granite evolution. The variability in Fe content within genthelvite is associated primarily with localised variations in the mineral assemblage (e.g. the presence of riebeckite and polylithionite). Genthelvite was altered by low-temperature aqueous fluids rich in F which resulted in the incorporation of Fe, Mn, Mg, Pb, Ba, Na, K, U and REE into the Zn2+ structural site and the allocation of excess Si, Al, Ti and P in the IVSi and IVBe structural sites. The substantial content of U and REE substituting for Zn, together with Si substituting for Be, is charge balanced by the presence of vacancies at the A site.

Five celestine crystals were sampled from the (palaeo)surface intervening between the late Miocene to Pleistocene basaltic sequences of the Jabal Eghei (Nuqay) volcanic province in southern Libya and then characterised by applying a combination of the SEM–WDS, ICP/OES, PXRD and IR methods. Colour variations and related minerogenetic frameworks were also investigated. Three samples have greenish-blue-to-blue colour (480.4–482.5 nm), whereas the other two samples have blue–green colour (cyan; 489.1–494.1 nm). The colour purity ranges from 1.36–7.16. Their composition is similar, end-member celestine, in which only 1.6–4.1 at.% of Sr2+ content was substituted by Pb2+ (0.7–0.9 at.%), Ba2+ (0.5–0.7 at.%) and Ca2+ (0.2–0.8 at.%). Three samples contained vacancies, from 1.0 to 1.9 at.%. The content of other chemical elements is minor. The resulting unit-cell parameters have the ranges: a0 = 8.3578(9)–8.3705(6) Å; b0 = 5.3510(5)–5.3568(4) Å; c0 = 6.8683(7)–6.8767(2) Å and V0 = 307.17(5)–308.34(4) Å3. The PXRD and IR results are mainly in accordance with the SEM–WDS results, with a high level of correlation. However, a few discrepancies were found, producing several possible interpretations, the primary cause being a slight unit-cell axial anisotropy i.e. thermal expansion. As a consequence these results yield a new geothermometric tool that is based on the unit-cell axial anisotropy. The celestines investigated were formed during a Miocene intraplate volcanism with basaltic magmas, and associated brines lifted by the structural conduits (normal faults crosscutting the Sirt basin). The Sr-bearing fluids then poured into and over the faulted and fractured lagoon-type gypsum, anhydrite Eocene sediments. The celestine mineralisation formed within a ~368–430 K (~95–157°C) temperature range. The celestine formed at slightly elevated temperature and pressure conditions, close to the shallow subsurface environment (over 250 bars).

The new eudialyte-group mineral amableite-(Ce), ideally Na15[(Ce1.5Na1.5)Mn3]Mn2Zr3□Si[Si24O69(OH)3](OH)2⋅H2O, was discovered in a peralkaline pegmatite at Saint-Amable Sill, Montérégie, Québec, Canada. The associated minerals are albite, microcline, aegirine, serandite, natrolite, yofortierite, and an unidentified titanosilicate forming minute grains. Amableite-(Ce) occurs as yellow equant or thick tabular crystals up to 2 mm across. The observed crystal forms are {0001}; the subordinate forms are {11$\bar{2}$0}, {10$\bar{1}$1}, and {10$\bar{1}$0}. Amableite-(Ce) is brittle, with a Mohs hardness of 5. D(meas) = 2.89(1), D(calc) = 2.899 g⋅cm–3. Amableite-(Ce) is optically anomalously biaxial (+) with α ≈ β = 1.603(2) and γ = 1.608(2). The chemical composition is (wt.%, electron microprobe, H2O measured by means of a modified Penfield method): Na2O 14.20, K2O 0.41, CaO 1.89, MnO 8.25, Fe2O3 2.40, La2O3 3.10, Ce2O3 4.19, Pr2O3 0.16, Nd2O3 0.59, SiO2 49.41, ZrO2 11.17, HfO2 0.24, TiO2 0.68, Nb2О5 1.54, Cl 0.26, H2O 1.70, –O≡Cl –0.06, total 100.13. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R1 = 0.0423. Amableite-(Ce) is trigonal, space group R3, with a = 14.1340(3) Å, c = 30.3780(11) Å and V = 5255.6(3) Å3. The crystal-chemical formula is (Na12.93K0.27Ce0.06)Σ13.26[(Mn2.49Ce0.30Ca0.21)Σ3.00(Ce1.14Na1.04Ca0.82)Σ3.00](Mn1.05Fe0.90□1.05)Σ3.00(Zr2.85Ti0.12Hf0.03)Σ3.00(□0.40Nb0.36Si0.24)Σ1.00(Si0.88□0.12)Σ1.00[Si24(O70.44(OH)1.56)Σ72.00][(OH)2.20(H2O)1.27]Σ3.47Cl0.22 (Z = 3). Infrared and Raman spectra are given. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %)(hkl)] are: 11.34 (51)(101), 7.06 (76)(110), 4.312 (63)(205), 3.783 (38)(033), 3.538 (43)(027, 220), 2.963 (84)($\bar{3}$45), 2.837 (100)(404). The mineral is named after the discovery locality.

Kalyuzhnyite-(Ce), ideally NaKCaSrCeTi(Si8O21)OF(H2O)3, is a new mineral from the Darai-Pioz alkaline massif, Tien-Shan mountains, Tajikistan. It occurs as equant grains up to 0.05 × 0.07 mm in a quartz–pectolite aggregate in a silexite-like peralkaline pegmatite. Associated minerals are quartz, fluorite, pectolite, baratovite, aegirine, leucosphenite, neptunite, reedmergnerite, orlovite, sokolovaite, mendeleevite-(Ce), odigitriaite, pekovite, zeravshanite, kirchhoffite and garmite. The mineral is colourless with a vitreous lustre and a white streak, and Dcalc. is 3.120 g/cm3. Kalyuzhnyite-(Ce) is monoclinic, P2/c, a = 18.647(4), b = 11.214(2), c = 14.642(3) Å, β = 129.55(3)° and V = 2360.9(11) Å3. The chemical composition of kalyuzhnyite-(Ce) is Nb2O5 0.53, TiO2 0.16, SiO2 43.85, Er2O3 0.13, Ho2O3 0.10, Gd2O3 0.09, Sm2O3 0.47, Nd2O3 6.22, Pr2O3 1.21, Ce2O3 6.34, La2O3 0.82, PbO 4.90, BaO 0.85, SrO 11.39, CaO 1.86, Cs2O 3.80, K2O 1.59, Na2O 2.99, H2O 5.24, F 1.55, O = F –0.65, total 100.31 wt.%. The empirical formula calculated on 26.11 (O + F) apfu is Na1.07K0.37Cs0.30Sr1.21Ca0.37Pb0.24Ba0.06(Ce0.43Nd0.41Pr0.08La0.06Sm0.03Gd0.01Er0.01Ho0.01)Σ1.04(Ti0.97Nb0.04)Σ1.01Si8.06O25.21F0.90H6.42, Z = 4. The simplified formula is (Na,□)(K,Сs)(Ca,Pb,Sr,Na)SrLn3+Ti(Si8O21)OF(H2O)3, where Ce is the dominant lanthanoid. The crystal structure was solved by direct methods and refined to an R1 index of 2.74%. In kalyuzhnyite-(Ce), the main structural units are a heteropolyhedral Na–Sr–Ce–Ti sheet, ideally [NaSrCeTiOF]7+, and a double (Si8O21)10– sheet parallel to (010). In the Si–O sheet, the Si tetrahedra form ten-membered rings. This is the first occurrence of such a double Si–O sheet in a mineral. The two sheets connect via common vertices of Na-, Sr-, Ce- and Ti-polyhedra and SiO4 tetrahedra to form a framework. The interstitial cations and H2O groups, ideally [(CaK)(H2O)3]3+, occur within the Si–O sheet. The mineral is named in honour of Vasily Avksentievich Kalyuzhny (1899–1993) in recognition of his contributions to the geology of ore deposits of Komi Republic (USSR) and the mineralogy of granitic pegmatites (Tajikistan).

Arsenoústalečite is a new mineral discovered in a sample collected from the abandoned Ústaleč deposit near Horažďovice, SW Bohemia, Czech Republic. It occurs as rare anhedral grains, up to 40 μm in size, in a calcite gangue, associated with stibioústalečite, hakite-(Hg), berzelianite and uraninite. Arsenoústalečite is dark grey, with a metallic lustre. Mohs hardness is ca. 3½–4; calculated density is 5.730 g/cm3. In reflected light, arsenoústalečite is pale grey with a yellowish shade; it is isotropic. Internal reflections were not observed. Reflectance values for the four COM wavelengths in air [R (%) λ (nm)] are: 33.3 (470); 33.1 (546); 33.0 (589); and 32.9 (650). The empirical formula of arsenoústalečite is (Cu5.81Ag0.17)Σ5.98(Cu5.95Fe0.02Zn0.02Hg0.01)Σ6.00(As1.40Sb0.87Te1.73)Σ4.00(Se10.30S2.32)Σ12.61. The ideal formula is Cu12(As2Te2)Se13, which requires (in wt.%) Cu 34.76, As 6.83, Te 11.63, Se 46.78, total 100.00. Arsenoústalečite is cubic, I$\bar{4}$3m, with unit-cell parameters a = 10.6580(19) Å, V = 1210.7(6) Å3 and Z = 2. The strongest reflections of the calculated powder X-ray diffraction pattern [d, Å (I, %) hkl] are: 3.077 (100) 222, 2.848 (10) 321, 1.946 (12) 521, 1.884(52) 440 and 1.608(21) 622. According to the single-crystal X-ray diffraction data (R1 = 0.0285 on the basis of 334 unique reflections with Fo > 4σFo and 24 refined parameters), arsenoústalečite is isotypic with other tetrahedrite-group minerals. The crystal structure of co-existing stibioústalečite, with an empirical formula of (Cu5.69Ag0.07)Σ5.76(Cu5.80Zn0.13Fe0.06Hg0.01)Σ6.00(Sb1.82As0.42Te1.76)Σ4.00(Se9.52S3.10)Σ12.62 and unit-cell parameters a = 10.6975(16) Å, V = 1224.2(5) Å3 and Z = 2, was refined to R1 = 0.0191 on the basis of 267 unique reflections with Fo > 4σFo and 24 refined parameters. Structural relationships and crystal-chemistry of both members of the ústalečite series are discussed. Arsenoústalečite is named after its type locality, the Ústaleč deposit and its chemical composition. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2022-116).

Watanabeite from the Pefka epithermal deposit, northeastern Greece, was examined using single-crystal X-ray diffraction and electron microprobe analysis. The empirical formula of watanabeite is Cu3.93Fe0.10Ag0.01Pb0.23As1.55Sb0.19S4.99. This mineral is orthorhombic, space group Amm2, with unit-cell parameters a = 10.9601(5), b = 14.6498(8), c = 10.3001(5) Å, V = 1653.82(14) Å3 and Z = 8. The crystal structure was solved and refined to R1 = 0.0471 for 2108 unique reflections with Fo > 4σ(Fo) and 123 refined parameters. The crystal structure of watanabeite can be described as a three-dimensional framework of Cu-centred tetrahedra; cavities of the tetrahedral scaffolding host Cu6S and As2(Pb,Sb,As)2S7 clusters. On the basis of structural data, the formula of watanabeite could be written as [III]Cu3[IV]Cu5As3(Pb,Sb,As)S10 (Z = 4), considering the three independent three-fold Cu sites and the three independent tetrahedrally coordinated Cu sites as aggregated positions. The occurrence of Pb2+ in watanabeite is probably related to the substitution Cu+ + (As,Sb)3+ = 2Me2+, where Me = Pb, Fe, Zn and formally divalent Cu. The relationships with tetrahedrite-group minerals are discussed on the basis of the refined structural model, highlighting possible crystal chemical implications of such relationships.

The impact of temperature on the migration of cations within layers of clay minerals is of profound significance for the design and practical application of materials derived from clay minerals. This study focuses on Li+ and Na+ as representative cations, together with illite (Ilt) and montmorillonite (Mnt) as representative clay minerals. The study investigates the behaviour of cation migration and occupation within clay minerals across varying temperatures. A series of samples were prepared meticulously by immersing illite and montmorillonite in Li+ and Na+ solutions, subsequently subjecting them to different temperatures (unheated, 100, 150, 200, 250 and 300°C) for 24 h. Through the use of techniques such as X-ray diffraction (XRD), cation exchange capacity (CEC), Fourier-transform infrared spectroscopy (FTIR), magic angle rotating solid nuclear magnetic resonance spectroscopy (MAS NMR), and X-ray photoelectron spectroscopy (XPS), the study discerns structural transformations in illite and montmorillonite, and tracks the migration and occupation of Li+ and Na+. The findings reveal that following heating, Na+ and Li+ do not infiltrate the lattice of illite. For montmorillonite, Na+ also does not migrate into the montmorillonite lattice, however, in contrast, Li+ does exhibit migration into this lattice. Notably, the migration and occupation of interlayer Li+ within montmorillonite exhibit discernible temperature dependence. Specifically, upon reaching 150°C, interlayer Li+ migrate to ditrigonal cavities within the tetrahedral layers. As the temperature elevates to 200°C, Li+ further permeate vacant octahedral sites through the ditrigonal cavities, culminating in the formation of a localised trioctahedral structure.

Okruginite, Cu2SnSe3 is a new mineral discovered from the high-sulfidation epithermal Au Ozernovskoye deposit, Kamchatka peninsula, Russia. It occurs as distinct Se-rich zones in Se-bearing mohite crystals or forms aggregates of small crystals 10–15 μm in size in quartz. In plane-polarised light, okruginite appears brownish grey. Pleochroism and bireflectance are discernible, anisotropy is weak, with rotation tints pale blue-grey to pale grey-brown; it exhibits no internal reflections. Reflectance values of the synthetic analogue of okruginite in air (R1, R2 in %) are: 25.9, 26.5 at 470 nm, 27.5, 26.5 at 546 nm, 27.8, 28.4 at 589 nm and 27.7, 28.4 at 650 nm. Twenty seven electron-microprobe analyses of okruginite give an average composition: Cu 29.48, Sn 28.10, Se 33.40 and S 8.75, total 99.73 wt.%, corresponding to the empirical formula Cu1.99Sn1.02(Se1.82S1.17)Σ2.99 based on 6 atoms; the average of seven analyses on its synthetic analogue is: Cu 23.62, Sn 24.37 and Se 49.09, total 97.08 wt.%, corresponding to Cu1.86Sn1.03Se3.11. The density, calculated on the basis of the empirical formula, is 5.126 g/cm3. The mineral is monoclinic, space group Cc, with a = 6.9906(2), b = 12.0712(4) Å, c = 6.9723(2) Å, β = 109.350(10)°, V = 555.1(2) Å3 and Z = 4. The crystal structure was solved and refined from the powder X-ray-diffraction data of synthetic Cu2SnSe3. Okruginite is the selenium-end member of the Cu2SnS3–Cu2SnSe3 solid solution. The mineral name is in honour of Dr. Victor Mikhailovich Okrugin, a Russian mineralogist, for his contributions to mineralogy and geology of epithermal deposits, in particular of the Au–Ag deposits in Kamchatka.

Bortolanite, a rare mineral of the rinkite group, seidozerite supergroup occurs in two different associations in the Lovozero massif in the Kola Peninsula, Russia: (1) together with ferri-katophorite and phlogopite, it forms porous or mesh aggregates (symplectitic accretions) with euhedral contours in the contact zone of a volcano–sedimentary xenolith and eudialyte lujavrite at Kuamdespakhk Mt and (2) in intergrowths with titanite and fluorcaphite in the poikilitic feldspathoid syenites at Sengischorr Mt. In both cases, bortolanite was found in association with rosenbuschite that is close to it in chemical composition, but unlike bortolanite, it contains no REEs. The mineral is triclinic, space group P$\bar{1}$, a = 9.5807(5), b = 5.6943(4), c = 7.2813(4) Å, α = 89.891(5)°, β = 100.959(4)°, γ = 101.241(5)°, V = 382.25(4) Å3 and Z = 1.

The Lovozero bortolanite differs from the Brazilian holotype sample from de Caldas alkaline massif, Minas Gerais, due to the presence of (OH)-groups in its composition, which is indicated by Raman data. A combination of single-crystal X-ray diffraction data and electron microprobe data provides the following crystal-chemical formula: (Ca1.97Ce0.01Nd0.01Th0.01)Σ2(Ca1.39Zr0.61)Σ2(Na0.72Ca0.28)Σ1(Na1.36Ca0.56Mn0.03Zn0.01)Σ1.96(Ti0.78Zr0.08Nb0.05Mg0.05Fe0.04)Σ1Si4O14((OH)0.92O0.87F0.21)Σ2F2.

This study describes a new variety of chalcedony with a unique inhomogeneous bluish green hue, named aquaprase. It was discovered in Africa and is considered to be a valuable addition to the gem trade. A multi-methodological approach was used to examine its chemistry, mineralogy and microstructure, which were then compared to those of chrysoprase and agate, two of the most popular varieties of chalcedony. Optical microscopy revealed a complex microstructural heterogeneity in the different colour intensity areas/bands of aquaprase and agate, whereas chrysoprase exhibited a more homogeneous coexistence of micro- and cryptocrystalline quartz. High-resolution synchrotron XRD was essential for highlighting the complex assemblage of various types of α-quartz in aquaprase and agate (which differ in terms of crystal size and/or cell parameters). Micro-Raman spectroscopy revealed α-quartz and moganite in all three varieties of chalcedony and the presence of the nickel-bearing layered silicate mineral, willemseite, in chrysoprase, which is responsible for its green colouration. The chemical analysis displayed a homogeneous composition of agate, as well as high levels of nickel content in the chrysoprase variety. Aquaprase showed significant amounts (ppm by weight) of trace elements (Al, Mg, Na, K, Ca, Ti, U and Fe) characteristic of its formation environment, as well as high values of Cr, which are thought to be the cause of its bluish green colouration.

Organic acids are commonly found in soils and sediments, playing an important role in the alteration and weathering of minerals and influencing a series of geochemical processes such as soil fertility, metal cycling and pollutant migration. In order to better comprehend the reaction mechanisms of different layered silicate minerals with organic acids, three minerals with various structure types, namely montmorillonite, kaolinite and muscovite, were investigated in this work. In particular, the effects of interfacial reactions with oxalic acid on the crystal structure, chemical composition, morphology and specific surface area of minerals were compared. The composition and structure of montmorillonite, kaolinite and muscovite during the interfacial reaction with oxalic acid were characterised using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and inductively coupled plasma-optical emission spectroscopy (ICP-OES) methods. It was shown that Si4+ and Al3+ were dissolved gradually during the interfacial reactions and that the changes in the properties of minerals depended on structural characteristics. After 300 days of the interfacial reactions with oxalic acid, the dissolution percentages of Si4+ and Al3+ in montmorillonite, kaolinite and muscovite were 12.7%, 8.4%, 3.8% and 62.1%, 30.7%, 6.1%, respectively. Moreover, the lamellar morphology of montmorillonite was destroyed upon the interfacial reaction with oxalic acid, and irregular particles with sizes of ~100–500 nm were formed on the surface. The diameter of kaolinite flake particles decreased from 400–1500 nm to 50–400 nm, and the surface of rod-shaped particles was ruptured. The small particles disappeared from the muscovite surface, and the initially sharp edges became blunted. The specific surface area and the total pore volume of montmorillonite and kaolinite increased after the interfacial reaction with oxalic acid, whereas the opposite results were obtained for muscovite. The differential dissolution of the minerals during their interfacial reaction with oxalic acid was mainly related to the differences between cation occupancies, structural types, chemical bond strengths and specific surface areas.

Heflikite, the first Sc-dominant epidote-supergroup mineral, was discovered in two occurrences. The holotype was found in a granitic pegmatite associated with rodingite-like calc-silicate rocks and metasomatised granitic bodies exposed in a serpentinite quarry at Jordanów Śląski near Sobótka, Lower Silesia, SW Poland. The cotype comes from the Heftetjern pegmatite, Tørdal region, Norway. The holotype is composed of (in wt.%): 35.69 SiO2, 0.22 TiO2, 21.98 Al2O3, 6.12 Sc2O3, 0.07 V2O3, 1.10 Fe2O3, 0.11 Y2O3, 1.55 La2O3, 4.05 Ce2O3, 0.31 Pr2O3, 1.53 Nd2O3, 0.40 Sm2O3, 0.11 EuO, 0.56 Gd2O3, 0.14 MnO, 3.56 FeO, 0.16 MgO, 19.16 CaO and 1.78 H2O(+)calc.; total 98.60. The cotype contains: 34.92 SiO2, 0.44 TiO2, 0.82 SnO2, 19.13 Al2O3, 4.79 Sc2O3, 1.96 Fe2O3, 2.55 La2O3, 7.39 Ce2O3, 0.48 Pr2O3, 0.67 Nd2O3, 0.12 EuO, 0.61 Gd2O3, 0.13 MnO, 5.97 FeO, 17.66 CaO and 1.73 H2O(+)calc.; total 99.37. The compositions correspond to the following empirical formulae: (Ca1.729Ce0.125La0.048Nd0.046Gd0.016Sm0.012Pr0.010Y0.005Eu2+0.003)Σ1.994[(Al2.182Sc0.449Fe3+0.070V3+0.005)Σ2.706(Fe2+0.251Mg0.020Mn0.010)Σ0.281Ti0.014]Σ3.001(Si3.006O11)O(OH) and (Ca1.644Ce0.235La0.082Nd0.021Gd0.018Pr0.015Eu2+0.004)Σ1.019[(Al1.958Sc0.362Fe3+0.128)Σ2.448(Fe2+0.434Mn0.009)Σ0.443(Ti0.029Sn0.029)Σ0.058]Σ2.949(Si3.033O11)O(OH), respectively, and to the ideal formula Ca2(Al2Sc)(Si2O7)(SiO4)O(OH). The crystal structure of the holotype was refined in the monoclinic system with an R1 index of 8.62%. The crystal-structure refinement indicates exclusively Si occupied T sites, Al occupied M1 and M2 sites, and a Ca occupied A1 site. The M3 site is filled predominantly by trivalent cations, mainly Sc3+, with divalent cations (mainly Fe2+) as minor occupants. The A2 site is filled mostly by Ca with minor amounts of rare earth elements (REE). The holotype heflikite crystallised from metasomatic fluids that infiltrated a contact between the granitic pegmatite and the surrounding rodingite-type calc-silicate rocks and serpentinites. The fluids that introduced Sc into the pegmatite could have been either hydrothermal or related to low-grade regional metamorphism that postdated the formation of the pegmatite. The cotype heflikite formed during the late-stage hydrothermal crystallisation of the Sc-enriched granitic pegmatite.

Tetrahedrite-(Cu), Cu12Sb4S13, has been approved as a new mineral species (IMA2022–078) from the Bankov magnesite deposit near Košice, Slovak Republic where it occurs as anhedral grains, up to 0.4 mm across, associated with skinnerite, chalcostibite, famatinite, tetrahedrite-(Fe) and zoned aggregates of tennantite-(Cu) to tennantite-(Fe). Tetrahedrite-(Cu) is steel-grey, with a metallic lustre. Mohs hardness is ca. 3½–4 and calculated density is 5.029 g.cm–3. In reflected light, tetrahedrite-(Cu) is isotropic and grey with a bluish shade. Reflectance data for the four COM wavelengths in air are [λ (nm): R (%)]: 470: 31.1; 546: 30.1; 589: 29.9; and 650: 28.1. The empirical formula, based on electron-microprobe data (mean of 17 spot analyses), is Cu11.42Zn0.26Fe0.19(Sb4.06As0.08)Σ4.14S12.99. The ideal formula is Cu6(Cu4Cu2)Sb4S13, which requires (in wt.%) Cu 45.76, Sb 29.23 and S 25.01, total 100.00. Tetrahedrite-(Cu) is cubic, I$\bar{4}$3m, with unit-cell parameters a = 10.3296(15) Å, V = 1102.2(5) Å3 and Z = 2. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R1 = 0.0347 on the basis of 261 unique reflections with Fo > 4σ(Fo) and 22 refined parameters. Tetrahedrite-(Cu) is isotypic with other tetrahedrite-group minerals. Previous findings of tetrahedrite-(Cu) are reported and some nomenclature issues, related to the Fe and Cu oxidation states, are discussed. At the Bankov deposit, tetrahedrite-(Cu) is related to hydrothermal, most probably Alpine, solutions strongly enriched in Cu, Sb and S.

The new columbite-supergroup mineral dmitryvarlamovite, ideally Ti2(Fe3+Nb)O8, was discovered in weathered alkaline metasomatic assemblages formed after late Riphaean sedimentary carbonate rocks of the Verkhne-Shchugorskoe deposit, Middle Timan Mts., Russia. The associated minerals are columbite-(Fe), pyrochlore-group minerals, monazite-(Ce), xenotime-(Y), baryte, pyrite, drugmanite and plumbogummite. Dmitryvarlamovite occurs as isolated anhedral equant grains up to 0.5 mm across. The colour of dmitryvarlamovite is black, the streak is black and the lustre is submetallic. The new mineral is brittle, with the mean VHN hardness of 753 kg mm–2 corresponding to the Mohs’ hardness of 6. No cleavage is observed. The fracture is conchoidal. The calculated density is 4.891 g⋅cm–3. In reflected light, dmitryvarlamovite is light grey; no pleochroism is observed. The reflectance values (Rmin, % / Rmax, % / λ, nm) are: 19.8/20.3/470, 18.3/18.9/546, 17.8/18.5/589 and 17.3/17.8/650. The chemical composition is (electron microprobe data, with iron divided into Fe2O3 and FeO based on the charge balance, wt.%): MnO 0.11, FeO 1.51, V2O3 0.89, Cr2O3 0.28, Fe2O3 19.26, TiO2 37.72, Nb2O5 40.08, total 99.85. The IR and Raman spectra indicate the absence of H-, C- and N-bearing groups. The empirical formula is (Fe2+0.08V3+0.05Cr3+0.01Fe3+0.92Ti1.79Nb1.15)Σ4.00O8. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.048. Dmitryvarlamovite is orthorhombic, space group P21212, a = 4.9825(6), b = 4.6268(4), c = 5.5952(6) Å and V = 5.5952(6) Å3 (Z = 1). The structure is related to those of wolframite-group minerals but differs in the scheme of cation ordering. The crystal-chemical formula derived based on the structural data is (Ti0.57Nb0.21Fe3+0.15Fe2+0.04V0.02Cr0.01)2(Nb0.36Ti0.33Fe3+0.31)2O8. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 3.58 (40) (011), 2.911 (100) (111), 2.809 (40) (002), 2.497 (38) (020), 2.447 (29) (103), 1.7363 (32) (103) and 1.7047 (29) (220). Dmitryvarlamovite is named after Dmitry Anatol'evich Varlamov (b. 1965).

The new mineral theuerdankite, ideally Ag3AsO4, was found in the Alter Theuerdank Mine, Beerberg, St. Andreasberg, Goslar District, Lower Saxony, Germany. Theuerdankite occurs as aggregates of anhedral grains up to 3 mm in size, growing in cavities of strongly supergene-weathered material consisting of native silver and chlorargyrite (but with calcite present). It is dark violet, changing to reddish and black when exposed to the air and light. It has a grey to violet grey streak; when readily fresh, its streak is brownish-red. The Mohs hardness is ~2. It is brittle with no observable cleavage or parting and with a conchoidal fracture. The calculated density is 6.620 g⋅cm–3. In reflected light, theuerdankite is dark grey with a pinkish tint, with no observable bireflectance, pleochroism, or anisotropy. It shows dark red internal reflections. The reflectance values for wavelengths recommended by the Commission on Ore Mineralogy of the International Mineralogical Association are (R, %): 13.3 (470 nm), 12.8 (546 nm), 12.7 (589 nm) and 12.5 (650 nm). The empirical formula (based on 4 apfu) is Ag3.00As1.00O4. Theuerdankite is cubic, space group P$\bar{4}$3n, a = 6.144(2) Å, V = 231.93(13) Å3 and Z = 2. The six strongest powder X-ray diffraction lines are [dobs in Å, (I) hkl]: 3.0736, (22) 200; 2.7502, (100) 210; 2.5106, (55) 211; 1.7050, (36) 320; 1.6249, (44) 321; and 1.3412, (17) 421. The crystal structure of theuerdankite (R1 = 1.69% for 519 reflections having I > 3σ(I)), is isotypic to those of synthetic Ag3AsO4 and Ag3PO4. The Gram–Charlier development describing the higher-order tensors representing the atomic displacement parameters of the silver atom was implemented, documenting that silver tends to behave anharmonically in the theuerdankite structure at room temperature.

Charmarite, Mn4Al2(OH)12CO3⋅3H2O, is a hydrotalcite supergroup member (or layered double hydroxide, LDH) with a previously unknown crystal structure and a Mn2+-analogue of quintinite (commonly erroneously reported as ‘2:1 hydrotalcite’). The single-crystal X-ray diffraction (XRD) data were obtained from the specimen from Mont Saint-Hilaire, Québec, Canada and are best processed in the space group P$\bar{3}$, a = 10.9630(4), c = 15.0732(5) Å and V = 1568.89(12) Å3. The crystal structure has been solved by direct methods and refined to R1 = 0.0750 for 3801 unique reflections with Fo > 2σ(Fo). The charmarite structure has long-range periodicity in the xy plane due to $2\sqrt 3$a’ × $2\sqrt 3$a’ scheme (or 11 × 11 Å) determined for LDHs for the first time (where a’ is a subcell parameter ≈ 3.2 Å). This periodicity is produced by the combination of two superstructures formed by: (1) Mn2+ and Al3+ ordering in the metal-hydroxide layers [Mn4Al2(OH)12]2+ according to the $\sqrt 3$a’ × $\sqrt 3$a’ pattern and (2) the (CO3)2– ordering according to the 2a’ × 2a’ pattern in the [CO3(H2O)3]2– interlayer sheet in order to avoid close contacts between adjacent carbonate groups. The $2\sqrt 3$a’ × $2\sqrt 3$a’ superstructure is an example of the adaptability of the components of the interlayer space to the charge distribution of the metal-hydroxyl layers. The Mn2+ and Al3+ cations have a large difference in size, which apparently leads to the considerable degree of their order as di- and trivalent cations resulting in a higher degree of statistical order of the interlayer components. Both powder and single-crystal XRD data show that the samples studied belong to the hexagonal branch of two-layer polytypes (2T or 2H) with d00n ≈ 7.57 Å. The chemical composition of the samples studied is close to the ideal formula. The Raman spectrum of charmarite is reported and the band assignment is provided.

Cotype material of stibiogoldfieldite from the Mohawk mine, Goldfield, Nevada, USA, has been examined in order to collect single-crystal X-ray diffraction data of Te-rich stibiogoldfieldite and to characterise the associated Ag–Bi–(S,Se) phase. Tellurium-rich stibiogoldfieldite, with empirical formula (Cu11.30Ag0.03)Σ11.33(Sb0.80As0.57Bi0.06Te2.57)Σ4.00(S12.83Se0.20)Σ13.03, is cubic, space group I$\bar{4}$3m, with unit-cell parameters a = 10.2947(3) Å and V = 1091.04(10) Å3. Its crystal structure has been refined to R1 = 0.0161 for 397 unique reflections with Fo > 4σ(Fo) and 25 refined parameters. The structure refinement confirmed the occurrence of a vacancy at the M(2) site, in agreement with the substitution M(2)Cu+ + X(3)(Sb/As)3+ = M(2)□ + X(3)Te4+. The Ag–Bi–(S,Se) phase was identified as the 6P homologue of the pavonite series, namely dantopaite. Its empirical formula is Cu1.36Ag4.39Pb0.12Bi12.62Sb0.06(S14.01Se7.91Te0.08), showing an exceptionally high Se content. Unit-cell parameters of Se-bearing dantopaite are a = 13.518(2), b = 4.0898(6), c = 18.984(3) Å, β = 106.816(6)°, V = 1004.7(3) Å3 and space group C2/m. The crystal structure was refined to R1 = 0.0504 for 1230 unique reflections with Fo > 4σ(Fo) and 82 refined parameters. The metal excess (~0.55 atoms per formula unit) of this pavonite homologue is mainly due to the accumulation of Ag and Cu in the thin slab of the crystal structure, whereas the high Se content is related to the partial replacement of S occurring preferentially in the thick PbS-like slab. Domains richer in Se and Pb in dantopaite, with empirical formula Cu0.89Ag4.50Pb0.49Bi12.53Sb0.07(S11.26Se10.74), were also identified, as grains up to 30 μm in size intimately intergrown with bohdanowiczite, indicating the possibility of a wide Se-to-S substitution in dantopaite.