The complexities of chemical composition and crystal structure are fundamental characteristics of minerals that have high relevance to the understanding of their stability, occurrence and evolution. This review summarises recent developments in the field of mineral complexity and outlines possible directions for its future elaboration. The database of structural and chemical complexity parameters of minerals is updated by H-correction of structures with unknown H positions and the inclusion of new data. The revised average complexity values (arithmetic means) for all minerals are 3.54(2) bits/atom and 345(10) bits/cell (based upon 4443 structure reports). The distributions of atomic information amounts, chemIG and strIG, versus the number of mineral species fit the normal modes, whereas the distributions of total complexities, chemIG,total and strIG,total, along with numbers of atoms per formula and per unit cell are log normal. The three most complex mineral species known today are ewingite, morrisonite and ilmajokite, all either discovered or structurally characterised within the last five years. The most important complexity-generating mechanisms in minerals are: (1) the presence of isolated large clusters; (2) the presence of large clusters linked together to form three-dimensional frameworks; (3) formation of complex three-dimensional modular frameworks; (4) formation of complex modular layers; (5) high hydration state in salts with complex heteropolyhedral units; and (6) formation of ordered superstructures of relatively simple structure types. The relations between symmetry and complexity are considered. The analysis of temporal dynamics of mineralogical discoveries since 1875 with the step of 25 years show the increasing chemical and structural complexities of human knowledge of the mineral kingdom in the history of mineralogy. In the Earth's history, both diversity and complexity of minerals experience dramatic increases associated with the formation of Earth's continental crust, initiation of plate tectonics and the Great Oxidation event.

Gachingite, Au(Te1–xSex), 0.2 ≈ x ≤ 0.5, is a new mineral discovered in the Gaching ore occurrence of the Maletoyvayam epithermal deposit, Kamchatka, Russia. Gachingite forms individual droplet-like grains of sizes from 2 to 10 μm included in native gold (Au–Ag), associated with calaverite, maletoyvayamite, watanabeite and Au–Sb oxides. The aggregates do not exceed 100 μm in diameter. In plane-polarised light, gachingite is grey with a bluish tint, has bireflectance (bluish-grey to deep grey), and strong anisotropy with rotation tints blue to dark blue to brown. Reflectance values for gachingite in air (Rmin, Rmax in %) are: 39.9, 40.3 at 470 nm; 41.6, 43.3 at 546 nm; 42.0, 43.7 at 589 nm; and 43.0, 44.0 at 650 nm. Eighteen electron-microprobe analyses of gachingite gave an average composition: Au 62.40, Ag 0.57, Se 9.78, Te 27.33 and S 0.01, total 100.09 wt.%, corresponding to the formula (Au0.96Ag0.02)Σ0.98(Te0.65Se0.37)Σ1.02 based on 2 apfu, the simplified formula is Au(Te0.65Se0.35); the average analyses of its synthetic analogue is Au 65.7, Se 13.1 and Te 21.1, total 99.9 wt.%, corresponding to Au1.00(Te0.50Se0.50). The calculated density is 10.47 g/cm3. The mineral is orthorhombic, space group Cmce (#64) with a = 7.5379 Å, b = 5.7415 Å, c = 8.8985 Å, V = 385.12 Å3 and Z = 8. The crystal structure was solved and refined from the single-crystal X-ray-diffraction data of synthetic Au1.00(Te0.50Se0.50). The crystal structure of gachingite represents a unique structure type, containing linear [Au–Au–Au] chains running along the b-axis indicating strong metallic interaction in one direction. The structural identity of gachingite and its synthetic analogue Au1.00(Te0.50Se0.50) was confirmed by electron back-scatter diffraction and Raman spectroscopy. The formation of gachingite requires an abundant source of Au and Se and a high oxidising environment. Gachingite is related to the gold-bearing productive stage of ore mineralisation, which is stable at 250°C in log$f_{{\rm S}{\rm e}_ 2}$ range of −12.4 and −5.7. The mineral is named after its type locality.

On the western part of the eastern Indian shield, Archaean basement (Bonai granite) is overlain by radioactive conglomerate. The conglomerate contains well-rounded, fractured Mg-poor (<0.18 wt.% MgO), Al-rich (up to 21.14 wt.% Al2O3) Zn-rich chromite (>35.0 wt.% Cr2O3) containing up to 15.5 wt.% ZnO. This is the first reported occurrence of detrital zinc-rich chromite having such unusually high ZnO from India, and to our knowledge, the third reported occurrence in radioactive quartz-pebble conglomerate after Witwatersrand, South Africa and Tarkwa, Ghana. Zinc-rich chromite grains are either clean or contain exsolution blebs/lamella of rutile. The conglomerates show evidence for post-depositional hydrothermal fluid influx and fluid-induced mineral alteration. The strong negative correlation between Zn and Fe2+, Al and Cr, and Al and Fe3+, and strong positive correlation between Zn and Al suggest secondary incorporation of Zn and Al by substitution of Fe2+ and Cr (and Fe3+), respectively, leading to partial transformation of (Fe)(Cr,Al,Fe3+)2O4 towards ZnAl2O4 composition. The chromite grains were possibly derived from komatiite. The timing of Zn enrichment, either at the provenance prior to sedimentation, or at the depositional site post-dating sedimentation, remains unresolved.

For this study, the rare Cu-bearing silicate fumarolic assemblages from the Somma–Vesuvius volcano, Italy, characterised by the rare mineral litidionite, CuKNaSi4O10, were investigated. We report new data about Cu- and Ti-bearing phases found in these mineralisations, in which Ti-bearing litidionite occurs together with kamenevite, perovskite and rutile. Ti-bearing litidionite appears on the latest stages of partial crystallisation of Ti-bearing silica glass. Incorporation of Ti4+ into the litidionite crystal structure was investigated in detail. The Raman spectra of Ti-bearing litidionite contains an intense band at 597 cm−1 related to anti-symmetric bending vibrations of Si‒O bonds or overlapping stretching vibrations of Ti‒O bonds. The bands in the range 350‒500 cm−1 correspond to symmetric bending vibrations of O‒Si‒O bonds and overlapping stretching vibrations of Ti‒O bonds. The crystal structure of Ti-litidionite has been refined in the P$\bar{1}$ space group, a = 6.9699(7), b = 7.9953(10), c = 9.8227(10) Å, α = 105.186(9), β = 99.458(8) and γ = 114.489(10) to R1 = 0.064 for 1726 unique observed reflections. The refinement of the site-occupation factors confirmed the presence of Ti at a five-coordinated M site. The mean bond distance of 2.125 Å for the M site agrees with its refined occupancy (Ti0.32Cu0.30Ca0.29Fe0.09)1.00. The incorporation of Ti into the litidionite structure is accompanied by the complex heteropolyhedral substitution according to the scheme VTi4+ + VII–VIII□ + IVAl3+ ↔ VCu2+ + VII-VIII(Na,K)+ + IVSi4+. Two possible configurations for the phase with maximal TiO2 content (12.06 wt.% or 0.56 Ti apfu) CuTiK□Na2Si7AlO20 (Z = 1) or CuTiK2Na□Si7AlO20 (Z = 1) have been proposed.

Svetlanaite, SnSe is a new mineral discovered from the high-sulfidation epithermal Au-deposit Ozernovskoe, Kamchatka peninsula, Russia. It forms tiny euhedral spindles (0.5–2 μm × 10–15 μm) in quartz, in close association with cassiterite, rutile, mohite, mawsonite, kiddcreekite, hemusite, tellurium, kostovite and Se-bearing ‘fahlores’ (Se-goldfieldite–Se(Bi)-tetrahedrite–Se-tennantite). In plane-polarised light, svetlanaite is light-grey, pleochroic from white to cream and strongly anisotropic in shades of light blue, dark blue, khaki and orange–brown; it exhibits no internal reflections. Reflectance values of synthetic analogue of svetlanaite in air (R1,R2 in %) are: 50.9, 56.5 at 470 nm, 50.2, 56.7 at 546 nm, 49.5, 55.3 at 589 nm and 48.7, 53.4 at 650 nm. Twelve electron-microprobe analyses of svetlanaite give an average composition: Sn 61.30, Se 37.22 and S 1.25 total 99.79 wt.%, corresponding to the empirical formula Sn1.01(Se0.92S0.07)Σ0.99 based on 2 atoms; the average of seven analyses on its synthetic analogue is: Sn 59.98 and Se 39.71, total 99.59 wt.%, corresponding to Sn1.00Se1.00. The density, calculated on the basis of the empirical formula, is 6.08 g/cm3. The mineral is orthorhombic, space group Pnma, with a = 11.500(2), b = 4.154(2), c = 4.445(2) Å, V = 212.34(14) Å3 and Z = 4. The crystal structure was solved and refined from the powder X-ray-diffraction data of synthetic SnSe. It crystallises in the GeS-structure type. It is isostructural with the mineral herzenbergite (SnS). The mineral name is in honour of Svetlana K. Smirnova, a Russian mineralogist, for her contributions to geology in the epithermal Au–Ag deposits of the Tien–Shan region.

A recent review paper on nepheline solid solutions dealt mainly with sodic varieties from nepheline syenites and differentiated alkali basaltic sills. Excel spreadsheets were provided for recalculating cell formulae in terms of the stoichiometric parameters ΔAlcavity cation charge and ΔTframework charge and molecular percentages of the end-members Ne, Ks, Ca-nepheline and excess Si (Qxs). Small amounts of tetrahedral divalent species (usually <0.10 wt.% though with up to 0.33 wt.% MgO) were related to a stuffed-tridymite end-member of ideal formula K8Mg4Si12O32 (denoted KsT2+-nepheline), although the equations used to calculate these end-members only considered Si, Al and Fe3+ as tetrahedral framework species. Recently, K-rich nepheline and kalsilite which contain up 0.54 wt.% MgO have been found in some feldspar-free mafic potassic volcanic rocks from central Brazil. Equations have been developed to deal rigorously with the presence of significant amounts of Mg (and Mn) in recalculating nepheline solid-solution parameters. In two related communications reworked spreadsheets are provided with examples of their use. High quality microprobe analyses of nepheline and kalsilite from magmatic rocks must include analyses for Mg and Mn and, if possible, estimates of Fe2+ contents.

Natural nepheline usually contains very small amounts of MgO (<0.1 wt.%), although these examples are mainly from Mg-poor alkaline igneous rocks such as nepheline syenites. However, this work shows that nepheline and kalsilite with much higher MgO concentrations can occur in the groundmass of strongly SiO2-undersaturated, feldspar-free, mafic volcanic rocks (i.e. olivine-rich foidites). Furthermore, a strong positive correlation is evident between their Mg and Fe contents. The occurrence of Mg-rich nepheline and kalsilite seems to be related to their derivation from Mg-rich magmas when compared to most of the host rocks investigated to date. Additionally, the physicochemical conditions of crystallisation seem to have an important role in the incorporation of ‘small’ divalent cations by these minerals. The prevalence of Mg-rich nepheline and kalsilite as late magmatic phases and the divergent Mg and Fe relationships for phenocrysts and ‘quenched’ groundmass crystals support this hypothesis. The positive correlation between Mg and Fe contents reflects their strong geochemical affinity and the entrance of Fe3+, Fe2+ and Mg2+ cations into the same crystallographic site of nepheline and kalsilite structures. The calculation of atomic formulae and stoichiometry parameters for nepheline-group minerals where data for the T2+ cations (e.g. Mg2+) are incorporated gives more reliable compositional parameters (see Paper 1). Calculated excess silica values (Si′) are affected significantly when the coupled substitution 2Al3+ = Mg2+ + Si4+ is considered. Thus, specific analyses of ‘small’ divalent cations are essential to obtain more realistic values of excess Si′, in particular, for nepheline and kalsilite that crystallised from Mg-rich, Si-poor, mafic–ultramafic alkaline lavas.

Zolotarevite (IMA2020-076), ideally Na5Zr[Si6O15(ОН)3]⋅2–3H2O, is а new mineral of the lovozerite group. The mineral was found in a leucocratic nepheline syenite from Kedykverpakhk Mt., Lovozero alkaline massif, Kola Peninsula, Russia. It occurs as anhedral grains up to 1 mm across and is associated with microcline–perthite, nepheline, sodalite, aegirine, lamprophyllite, lueshite, umbozerite, lomonosovite, nastrophite, a mineral of the kazakovite–tisinalite series, sphalerite and löllingite. Zolotarevite is cherry red, with a vitreous lustre and white streak. The mineral is brittle, with a Mohs hardness of 5. Cleavage was not observed; the fracture is uneven. The measured density is 2.75(5) g⋅cm–3, the density calculated using the empirical formula and single-crystal unit-cell parameters is 2.85 g⋅cm–3. Zolotarevite is anomalously biaxial (–), α = 1.580(2), β = 1.600(2) and γ = 1.602(2) (for λ = 589 nm); 2Vmeas < 10°. Chemical data (electron microprobe, wt.%) are: Na2O 20.41, СаО 0.42, MnO 3.49, Fе2O3 0.55, SiO2 52.46, ТiO2 1.34, ZrO2 11.33, H2O (calculated from the structural formula) 10.20, total 100.20. The empirical formula based оn 6 Si atoms per formula unit is Na4.53Zr0.63Mn0.34Ti0.11Ca0.05Fe3+0.05Si6O14.43(ОН)3.56(H2О)2.11. Zolotarevite is trigonal, space group R${\bar 3}$m, with a = 10.294(6) Å, c = 13.115(8) Å, V = 1203.7(16) Å3 and Z = 3. The crystal structure was solved from single-crystal data and refined to R1 = 0.049. The strongest lines of the powder X-ray diffraction pattern [d in Å (I) (hkl)] are: 7.37 (69) (101); 5.26 (56) (012); 3.686 (64) (202); 3.330 (79) (113); 3.265 (99) (211), 2.640 (100) (024) and 2.576 (60) (220). Zolotarevite is unique in that it is a highly hydrated lovozerite-group mineral with the B site occupied by variable amounts of H2O.

The Mineo pallasite is a relatively poorly known meteorite, which shows interesting features that are not fully understood, such as the occurrence of iron oxide regions bordering both the olivine grain boundaries and the (Fe,Ni) metal. In this study, the Fe oxides have been characterised by Raman spectroscopy, electron microprobe analysis, field emission scanning electron microscopy, transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and 3D electron diffraction (3D ED). The combination of TEM–EDS and 3D ED yields a reliable identification of the chemical and crystallographic features of the cryptocrystalline portion of the sample investigated, enabling the Fe-oxide regions to be positively identified as goethite, FeO(OH).

The occurrence of goethite was unambiguously associated with terrestrial alteration, also confirmed by the presence of calcite, detected by TEM-EDS and 3D ED. Goethite contains minor elements such as Na, Si and Ca, probably coming from alumino-silicates in the terrestrial environment, and Ni associated with the (Fe,Ni) metal. The observation of goethite along olivine grain boundaries, as an alteration product of the (Fe,Ni) metal diagenesis, is also very intriguing as it might be related to the (Fe,Ni) metal intruded into the sub-micrometric olivine fragments during pallasite formation. Further work is needed to extensively analyse the texture and composition of olivine/metal boundaries.

The Mount Mather Creek sodalite-bearing carbohydrothermal breccia dyke is located ~35 km northwest of Golden, British Columbia, Canada, within the Foreland Belt on the eastern side of the Mississippian-to-Devonian British Columbia Alkaline Province. The dyke occurs in situ on the western side of the Mount Mather Creek gully, cropping out over a distance of ~80 m with a thickness of up to 10 m, intruding a syncline of Middle and Upper Cambrian Chancellor Group carbonate rocks. To date, no parental alkaline complex has been found adjacent or proximal to the dyke. The breccia dyke is flow-banded and matrix-supported and consists of host rock clasts, medium- to coarse-grained, anhedral-to-poikilitic carbonate-rich syenite segregations that occur as veins and pods, and fine-grained banded sodalite plus carbonate fragments in a carbonate-rich matrix. The sodalite-carbonate segregations consist dominantly of sodalite, ferroan dolomite, calcite and microcline, with an extensive suite of trace minerals enriched in rare earth elements (REE), Na, Ba and Sr, including albite, analcime, ancylite-(Ce), chabazite-Na, fluorapatite, baryte, barytocalcite, cancrinite, galena, goethite, gonnardite, harmotome, edingtonite, a potentially new Mg-bearing edingtonite-like mineral, natrolite, nordstrandite, pyrite, quintinite and sphalerite. Alteration of the primary silicate–carbonate assemblage has resulted in a secondary assemblage of hydrothermal or carbohydrothermal REE–Ba–Sr–Na phases including albite, analcime, cancrinite, gonnardite and nordstrandite after sodalite, together with complex intergrowths of ancylite-(Ce), barytocalcite, edingtonite, and a potentially-new Mg-bearing edingtonite-like mineral. Remobilisation of Ba and Sr from barytocalcite resulted in crystallisation of late-stage baryte and Sr-rich calcite. Mount Mather Creek is only the fifth Canadian occurrence of the Ba-zeolite, edingtonite, and the second Canadian occurrence of the rare layered double hydroxide quintinite. The Mount Mather Creek breccia is a carbohydrothermal deposit, the product of a low temperature (<450°C), highly evolved, alkaline, SiO2-undersaturated, Na–Ba–REE–Cl-rich, residual carbonated silicate melt whose parental origins remain unknown.

Reznitskyite, ideally CaMg(VO4)F, is a new mineral species of the tilasite group from the Arsenatnaya fumarole, Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It occurs as zones (usually up to 0.05 mm thick) in crystals of V- and P-containing tilasite or as homogeneous grains up to 0.1 mm across. It was found in polymineralic sublimate encrustations in association with minerals of the svabite–fluorapatite–pliniusite system and the schäferite–berzeliite, tilasite–isokite, wagnerite–arsenowagnerite and udinaite–arsenudinaite series. The mineral assemblage also includes calciojohillerite, diopside, forsterite, titanite, rhabdoborite-(V), rhabdoborite-(W), rhabdoborite-(Mo), ludwigite, powellite, scheelite, hematite, baryte and magnesioferrite. Reznitskyite is transparent or semi-transparent, colourless, with vitreous lustre. Dcalc. = 3.453 g cm–3. Under the microscope, in reflected light reznitskyite is grey, non-pleochroic, with very weak bireflectance (ΔR589 nm = 0.5%) and distinct anisotropy. Reznitskyite is the first vanadate with a titanite-type structure. It is monoclinic, space group C2/c, a = 6.6912(7), b = 8.9395(7), c = 7.0587(8) Å, β = 113.078(13)°, V = 388.43(8) Å3 and Z = 4. The strongest reflections of the powder X-ray diffraction pattern are [d in Å(I)(hkl)]: 3.082(100)(200), 3.250(66)($\bar{1}$12, 002), 2.631(44)(022), 2.854(34)($\bar{2}$02), 2.683(33)(130), 3.686(26)(021), 2.531(25)(220), 1.749(25)($\bar{3}$32, $\bar{2}$04) and 2.344(24)(131). Electron microprobe analysis gave (wt.%) MgO 20.44, CaO 26.83, P2O5 6.24, V2O5 21.09, As2O5 18.97, SO3 0.47, F 9.42, –O=F –3.97, with a total of 99.49. The empirical formula of reznitskyite, calculated on the basis of O+F = 5 atoms per formula unit, is: Ca0.97Mg1.03(V0.47As0.33P0.18S0.01)Σ0.99O3.99F1.01. Reznitskyite is a vanadate analogue of tilasite CaMg(AsO4)F and isokite CaMg(PO4)F. The mineral is named in honour of the outstanding Russian mineralogist Leonid Zinovievich Reznitsky (born 1938) who has made significant contribution to the mineralogy of vanadium.

The crystal structure of hiärneite has been refined from single-crystal X-ray diffraction data (λ = 0.71073 Å) on type material from Långban, Värmland, Sweden. The refinement converged to R1 = 0.046 based on 1073 reflections with F2 > 4σ(F2). The tetragonal unit cell, space group I41/acd, has the parameters a = 15.2344(6) Å and c = 10.0891(6) Å with Z = 8. The mineral is isostructural with calzirtite, ideally Ca2Zr5Ti2O16, with a structural topology derived from fluorite. In hiärneite, Mn3+ is ordered at a 4- to 8-fold coordinated site (with a distorted polyhedral coordination figure), without the atom splitting encountered at the corresponding Zr-dominated site of calzirtite. The end-member formula for hiärneite is established as Ca2Zr4Mn3+SbTiO16. The calzirtite group, with calzirtite, hiärneite and tazheranite (cubic ZrO2-x), has been approved by the IMA–CNMNC.

Argentotetrahedrite-(Zn), Ag6(Cu4Zn2)Sb4S13, has been approved as a new mineral species by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA-CNMNC) using samples from Kremnica, Slovak Republic (hereafter KR), and Lengenbach, Switzerland (LE). Additionally, it was also identified at the small deposit of Zvěstov (Stříbrnice), Czech Republic (ZV). At the Slovak locality, it occurs as anhedral grains up to 0.1 mm in size, steel-grey in colour, with a metallic lustre, in association with ‘argentotennantite-(Fe)’ and tiny chalcopyrite grains in quartz gangue. At Lengenbach, it was found as domains of tristetrahedral crystal of tetrahedrite-(Zn), up to 1 mm in size, associated with dolomite. At Zvěstov, it occurs as irregular aggregates, up to 0.2 mm in size, partly rimmed by kenoargentotetrahedrite-(Fe). Argentotetrahedrite-(Zn) is isotropic, grey in colour, with blue-greenish tints. Reflectance data for COM wavelengths in air (KR sample) are [λ (nm), R (%)]: 470, 30.1; 546, 29.8; 589, 29.8; and 650, 28.3. Chemical formulae of the samples studied, recalculated on the basis of ΣMe = 16 apfu (atoms per formula unit), are: (Ag3.27Cu2.69)Σ5.96[Cu4.00(Zn1.69Fe0.23Cu0.05Cd0.02Hg0.01)Σ2.00](Sb3.86As0.17)Σ4.03S12.73 (KR), (Ag3.17Cu2.79)Σ5.96[Cu4.00(Zn1.55Cd0.23Fe0.16Cu0.05Hg0.01)Σ2.00](Sb3.71As0.32)Σ4.03S12.77 (LE) and (Ag3.27Cu2.67)Σ5.94[Cu4.00(Zn1.39Fe0.50Cu0.07Cd0.03Hg0.01)Σ2.00](Sb4.03As0.04)Σ4.07S13.08 (ZV). Argentotetrahedrite-(Zn) is cubic, I$\overline 4$3m, with a = 10.5505(10) Å, V = 1174.4(3) Å3 and Z = 2 (KR); a = 10.5155(13) Å and V = 1162.8(4) Å3 (LE); and a = 10.5663(12) Å and V = 1179.7 Å3 (ZV). The crystal structure of argentotetrahedrite-(Zn) has been refined by single-crystal X-ray diffraction data to a final R1 = 0.035 on the basis of 327 unique reflections with Fo > 4σ(Fo) and 22 refined parameters (sample KR). Argentotetrahedrite-(Zn) is isotypic with other members of the tetrahedrite group. The structural relationship between argentotetrahedrite-(Zn) and other members of the freibergite series are discussed and previous findings of this species are briefly reviewed.

Tennantite-(Cu), Cu12As4S13, was approved as a new mineral species from the Layo epithermal deposit, Castilla Province, Arequipa Department, Peru, where it occurs as black metallic anhedral grains, up to 0.1 mm across, replacing enargite and associated with chalcopyrite and vinciennite. In reflected light, tennantite-(Cu) is isotropic, grey with a bluish shade. Reflectance data for the four COM wavelengths in air are [λ (nm): R (%)]: 470: 29.1; 546: 28.4; 589: 27.4; and 650: 25.0. Electron microprobe analysis for holotype material gave (in wt.% – average of 10 spot analyses): Cu 49.32(27), Fe 2.20(12), Zn 0.09(2), Sn 0.03(5), As 19.45(43), Sb 1.94(10), Te 0.02(5), S 27.75(43), total 100.80(20). On the basis of (As + Sb + Te) = 4 atoms per formula unit (apfu), the empirical formula of tennantite-(Cu) is (Cu11.27Fe0.57Zn0.02)Σ11.86(As3.77Sb0.23)Σ4.00S12.57. Tennantite-(Cu) is cubic, I$\overline 4$3m, with unit-cell parameters a = 10.1710(10) Å, V = 1052.2(2) Å3 and Z = 2. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R1 = 0.0178 on the basis of 263 unique reflections with Fo > 4σ(Fo) and 24 refined parameters. Tennantite-(Cu) is isotypic with other tetrahedrite-group minerals. Previous findings of tennantite-(Cu) are reported and some nomenclature issues, related to the Fe and Cu oxidation states, are discussed. At the Layo epithermal deposit, tennantite-(Cu) is the result of the replacement of enargite under decreasing $f_{{\rm S}_ 2}$ conditions.

Ferrofettelite, ideally [Ag6As2S7][Ag10FeAs2S8], is a new mineral (IMA2021-094) from the Glasberg quarry, Nieder-Beerbach, Odenwald, south-western Germany. It occurs as anhedral to subhedral flakes and grains up to 80 μm, associated with proustite and xanthoconite, on arsenolite, calcite and prehnite. Ferrofettelite is opaque with a metallic lustre and possesses a dark reddish-grey streak. It is brittle with an uneven fracture; the Vickers microhardness (VHN20) is 122 kg/mm2 (range 111–131). The calculated density is 5.74 g/cm3 (on the basis of the empirical formula). In plane-polarised reflected light, ferrofettelite is greyish white. Between crossed polars it is weakly anisotropic with red internal reflections. Electron-microprobe analyses give the chemical formula Ag16.04(Fe0.55Hg0.40Cu0.02)Σ0.97(As3.94Sb0.03)Σ3.97S15.02 on the basis of total atoms = 36.

Ferrofettelite is monoclinic, space group C2, with a = 26.011(2), b = 15.048(1), c = 15.513(1) Å, β = 90.40(1)° and V = 6071.9(7) Å3 for Z = 8. The six strongest Bragg peaks in the powder X-ray diffraction pattern (d, Å (I, %) (hkl) are: 3.18 (50) ($\bar{8}$01), 3.104 (100) (005), 3.004 (60) ($\bar{8}$02), 2.755 (40) ($\bar{4}$43), 2.501 (30) ($\bar{4}$44) and 1.880 (30) ($\overline {12}$40). The crystal structure can be described as the alternation of two kinds of layers along the c-axis: layer A with general composition [Ag6As2S7]2– and layer B with a general composition of [Ag10FeAs2S8]2+. In the structure, the Ag atoms adopt various coordinations extending from quasi linear to quasi tetrahedral, the AsS3 groups form pyramids as are typically observed in sulfosalts, and mixed (Fe,Hg) links two sulfur atoms in a linear coordination. Ferrofettelite is the first reported inorganic phase showing a linear coordination for Fe2+. The high-temperature behaviour of ferrofettelite was studied up to 410 K and compared to that of fettelite.

Guite (IMA2017-080), Co3O4, is a new mineral species and an important economic mineral found in the Sicomines copper-cobalt mine, located ~11 km southwest of Kolwezi City, Democratic Republic of Congo. The mineral occurs as a granular agglomerate, 50 to 500 μm in size, and is associated closely with heterogenite in a quartz matrix. Guite is opaque, has a dark grey colour with metallic lustre and a black streak. In reflected light microscopy, it is white with no internal reflections. The reflectance values (in air, R in %) are: 27.0 (470 nm); 25.6 (546 nm); 25.2 (589 nm), and 24.6 (650 nm). The average of 20 electron-microprobe analyses is Co 71.53, Cu 0.58, Mn 0.67, Si 0.25, O 26.78, total 99.82 wt.%, corresponding to the empirical formula calculated on the basis of 4 O apfu: (Co2+0.92Cu2+0.02Si4+0.02)Σ0.96(Co3+1.98Mn3+0.03)Σ2.01O4.00, with Co2+ and Co3+ partitioned using charge balance. The ideal formula is Co2+Co3+2O4. Guite is cubic with space group Fd$\bar{3}$m. The unit cell parameters refined from the single crystal X-ray diffraction data are: a = 8.0898(1) Å, V = 529.436(11) Å3 and Z = 8. The calculated density of guite is 6.003 g/cm3. The eight strongest observed powder X-ray diffraction lines [d in Å (I/I0) (hkl)] are: 4.6714 (16.7) (111), 2.8620 (18.4) (220), 2.4399 (100) (311), 2.3348 (10.4) (222), 2.0230 (24.8) (400), 1.5556 (26.3) (511, 333), 1.4296 (37.7) (440) and 1.0524 (10.1) (731, 553). The crystal structure of guite was determined by single-crystal X-ray diffraction and refined to R = 0.0132 for 3748 (69 unique) reflections. Guite has a typical spinel-type structure with Co2+ in tetrahedral coordination with a Co2+–O bonding length of 1.941(1) Å, and Co3+ in octahedral coordination with a Co3+–O bonding length of 1.919(1) Å. The structure is composed of cross-linked framework of chains of Co3+–O6 octahedra sharing the equilateral triangle edges (2.550 Å) in three directions [0 1 1], [1 1 0], [1 0 1] with Co2+ filling the tetrahedral interstices among the chains. Guite is named in honour of Prof. Xiangping Gu (1964–).