The Open Access movement has gathered significant momentum over the last couple of years. This has been instigated largely by cOAlition S and those funders which support its aims. Is ‘Read and Publish’ the way forward? Will it work for all publishers? All authors? All subscribers? All readers? This article looks at the history of OA and updates a similar piece from 2013. A detailed glossary of terms is given at the end of the article.

Preferential removal of W relative to other trace elements from zoned, W–Sn–U–Pb-bearing hematite coupled with disturbance of U–Pb isotope systematics is attributed to pseudomorphic replacement via coupled dissolution reprecipitation reaction (CDRR). This hematite has been studied down to the nanoscale to understand the mechanisms leading to compositional and U/Pb isotope heterogeneity at the grain scale. High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF STEM) imaging of foils extracted in situ from three locations across the W-rich to W-depleted domains show lattice-scale defects and crystal structure modifications adjacent to twin planes. Secondary sets of twins and associated splays are common, but wider (up to ~100 nm) inclusion trails occur only at the boundary between the W-rich and W-depleted domains. STEM energy-dispersive X-ray mapping reveals W- and Pb-enrichment along 2–3 nm-wide features defining the twin planes; W-bearing nanoparticles occur along the splays. Tungsten and Pb are both present, albeit at low concentrations, within Na–K–Cl-bearing inclusions along the trails. HAADF STEM imaging of hematite reveals modifications relative to ideal crystal structure. A two-fold hematite superstructure (a = b = c = 10.85 Å; α = β = γ = 55.28°) involving oxygen vacancies was constructed and assessed by STEM simulations with a good match to data. This model can account for significant W release during interaction with fluids percolating through twin planes and secondary structures as CDRR progresses from the zoned domain, otherwise apparently undisturbed at the micrometre scale. Lead remobilisation is confirmed here at the nanoscale and is responsible for a disturbance of U/Pb ratios in hematite affected by CDRR. Twin planes can provide pathways for fluid percolation and metal entrapment during post-crystallisation overprinting. The presence of complex twinning can therefore predict potential disturbances of isotope systems in hematite that will affect its performance as a robust geochronometer.

Hagstromite, Pb8Cu2+(Te6+O6)2(CO3)Cl4, (IMA2019-093) is a new tellurate mineral from Otto Mountain near Baker, California, USA. It occurs on quartz in association with cerussite, fuettererite and thorneite. It is a secondary oxidation zone mineral and is presumed to have formed by oxidation of earlier formed tellurides, chalcopyrite and galena. Hagstromite occurs as light yellow–green blades, up to ~100 μm long. Crystals are transparent with adamantine to silky lustre. The mineral is brittle with two cleavages providing splintery fracture; the Mohs hardness is probably between 2 and 3. The calculated density is 7.062 g cm–3. Hagstromite is optically biaxial (+), with calculated indices of refraction for α = 2.045, β = 2.066 and γ = 2.102; 2Vmeas = 76(1)°; and optical orientation X = b, Y = a and Z = c. The Raman spectrum of hagstromite exhibits similarities with those of agaite and thorneite and confirms the presence of CO32–. The electron microprobe analyses provided the empirical formula Pb8.07Cu2+0.98Te6+1.96C1.17Cl3.83O15.34. Hagstromite is orthorhombic, space group Ibam, with a = 23.688(17), b = 9.026(8), c = 10.461(8) Å, V = 2237(3) Å3 and Z = 4. The crystal structure of hagstromite (R1 = 0.0659 for 284 I > 2σI reflections) contains a novel Cu2+Te6+2O12 chain assembled of corner-sharing Cu2+O4 squares and Te6+O6 octahedra. The O atoms in the chains form bonds with Pb2+ cations, which in turn bond to Cl– and CO32– anions, thereby creating a framework structure.

Witherite originates from the biochemical sedimentation of barium in sea water. Due to the complexity of the metallogenic environment, witherite appears in many morphologies. However, the relationship between its diverse morphologies and its mineralisation environment is not well understood. In this paper, Ca2+, a common substitute for Ba2+, and mixed protein (egg white) were used to simulate the inorganic and organic environments of witherite mineralisation, respectively. Comparison of samples prepared under different conditions showed that Ca2+ and egg white have relatively independent regulatory effects on the mineralisation of witherite particles. Egg white primarily limits the growth of the nanocrystals, while Ca2+ directs their non-isodiametric growth. Results shows that Ca2+ is distributed along a gradient in nanocrystalline witherite particles, with the Ca2+ content being proportional to the diameter of the nanocrystals. The results of this study shed light on the different roles of organic matter and inorganic ions in the formation of witherite and offer insight into the genesis of its various morphologies.

The new mineral fluorapophyllite-(NH4), ideally NH4Ca4(Si8O20)F⋅8H2O, was found at the Vechec andesite quarry located near Vechec village, Vranov nad Topľou Co., Prešov Region, Slovak Republic. It occurs in cavities of quartz–illite–saponite–tobelite xenolith embedded in pyroxene andesite. Fluorapophyllite-(NH4) is associated with calcite, tridymite, pyrite, chabazite-Ca and heulandite-Ca. It forms clusters, aggregates or crystalline crusts consisting of individual, well-developed crystals up to 4 mm in size, exhibiting the forms {110}, {101} and {001}. The mineral is colourless to light pink and translucent with white streak and vitreous to pearly lustre; it is non-fluorescent under ultraviolet radiation. The Mohs hardness is ~4½ to 5, tenacity is brittle, fracture is irregular, and there is perfect cleavage on {001}. The calculated density is 2.325 g cm–3. Fluorapophyllite-(NH4) is optically uniaxial (+) with ω = 1.5414(5) and ɛ = 1.5393(8) (λ = 589 nm). It is non-pleochroic. The empirical formula (based on 29 O + F apfu) is [(NH4)0.55K0.32Na0.07Ca0.06]Σ1.00(Ca4.01Mg0.02)Σ4.03Si7.97O20[F0.84(OH)0.16]Σ1.00⋅8H2O. Fluorapophyllite-(NH4) is tetragonal, space group P4/mnc, a = 8.99336(9) Å, c = 15.7910(3) Å, V = 1277.18(3) Å3 and Z = 2. The seven strongest X-ray powder diffraction lines are [dobs in Å,(I,%)(hkl)]: 7.897(32)(002), 7.812(13)(101), 4.547(14)(103), 3.946(100)(004), 2.985(39)(105), 2.4841(11)(215) and 1.5788(12)(00.10). The crystal structure of fluorapophyllite-(NH4), refined to R1 = 0.0299 for 743 unique (I > 3σI) observed reflections, confirmed that the atomic arrangement is very similar to that of the other members of the apophyllite group. The new mineral is named according to the current nomenclature scheme for apophyllite-group minerals and is an NH4 dominant analogue of fluorapophyllite-(K), fluorapophyllite-(Na) and fluorapophyllite-(Cs).

Khademite, ideally Al(SO4)F(H2O)5, from the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy, has been characterised through quantitative electron microprobe analysis, micro-Raman spectroscopy and single-crystal X-ray diffraction. Khademite occurs as colourless to whitish tabular crystals, up to 5 mm. Electron microprobe analysis (in wt.%, average of 20 spot analyses) gave: SO3 35.43, Al2O3 21.27, F 6.92, H2Ocalc 39.73, sum 103.35, –O = F 2.92, total 100.43. On the basis of 10 anions per formula unit, assuming the occurrence of 5 H2O groups and 1 (F+OH) atom per formula unit, its chemical formula can be written as Al0.96S1.02O4[F0.84(OH)0.16]Σ1.00⋅5H2O. The Raman spectrum of khademite is characterised by the occurrence of vibrational modes of SO4 groups and by broad and strong bands due to the O–H stretching modes. Khademite is orthorhombic, space group Pcab, with unit-cell parameters a = 11.1713(2), b = 13.0432(3), c = 10.8815(2) Å, V = 1585.54(5) Å3 and Z = 8. The crystal structure refinement converged to R1 = 0.0293 on the basis of 2359 unique reflections with Fo > 4σ(Fo) and 152 refined parameters. The crystal structure of khademite is characterised by the alternation, along b, of two distinct kinds of {010} layers, one formed by [001] rows of isolated Al-centred octahedra, connected to each other through H bonds, and the other showing isolated SO4 groups. Along b, oxygen atoms belonging to SO4 groups act as acceptor of H bonds from H2O groups coordinating Al atoms. The new data improved the description of the H bonds in khademite and led us to discuss about the possible existence of its (OH)-analogue, rostite. In addition, Raman spectroscopic data were collected on the same crystal used for the crystal-chemical characterisation, allowing a comparison with previous results.

Jahnsite-(CaMnZn), CaMn2+Zn2Fe3+2(PO4)4(OH)2⋅8H2O, is a new jahnsite-group mineral associated with alteration of phosphophyllite at the Hagendorf-Süd pegmatite, Bavaria. It forms as thin yellow crusts and brown epitactic growths on altered phosphophyllite, both of which comprise lath-like crystals in orthogonal orientation, up to 100 μm long. The crystals contain intergrowths of jahnsite-(CaMnZn) and jahnsite-(CaMnMn) on a scale of ~50 μm. The calculated density is 2.87 g cm−3 based on the empirical formula. Optically it is biaxial (–), with α = 1.675(2), β = 1.686(2) and γ = 1.691(2) (white light). The calculated 2V is 68°. Dispersion could not be observed, and the optical orientation is Z = b. Pleochroism was imperceptible. Electron microprobe analyses together with results from Mössbauer spectroscopy gives the formula (Ca0.59Mn0.24)Σ0.83Mn(Zn0.74Mn2+0.48Mg0.18Fe2+0.13Fe3+0.47)Σ2Fe3+2(P0.995O4)4(OH)2.03(H2O)7.97.

Jahnsite-(CaMnZn) is monoclinic, P2/a, with a = 15.059(1), b = 7.1885(6), c = 10.031(2) Å, β = 111.239(8)° and V = 1012.1(2) Å3. The recent International Mineralogical Association approved nomenclature system for jahnsite-group minerals was applied to establish jahnsite-(CaMnZn) from the empirical formula. The structural flexibility of jahnsite-group minerals to accommodate cations of quite different sizes in the X and M1 sites is discussed in terms of rotations about the 7 Å axis of two independent octahedra centred at the M3 sites.

Biogenic nanoscale vanadium magnetite is produced by converting V(V)-bearing ferrihydrites through reductive transformation using the metal-reducing bacterium Geobacter sulfurreducens. With increasing vanadium in the ferrihydrite, the amount of V-doped magnetite produced decreased due to V-toxicity which interrupted the reduction pathway ferrihydrite–magnetite, resulting in siderite or goethite formation. Fe L2,3 and V L2,3 X-ray absorption spectra and data from X-ray magnetic circular dichroism analysis revealed the magnetite to contain the V in the Fe(III) Oh site, predominately as V(III) but always with a component of V(VI), present a consistent V(IV)/V(III) ratio in the range 0.28 to 0.33. The bacteriogenic production of V-doped magnetite nanoparticles from V-doped ferrihydrite is confirmed and the work reveals that microbial reduction of contaminant V(V) to V(III)/V(IV) in the environment will occur below the Fe-redox boundary where it will be immobilised in biomagnetite nanoparticles.

A new mineral glikinite, ideally Zn3O(SO4)2, was found in high-temperature exhalative mineral assemblages in the Arsenatnaya fumarole, Second scoria cone of the Great Tolbachik Fissure Eruption (1975–1976), Tolbachik volcano, Kamchatka Peninsula, Russia. Glikinite is associated closely with langbeinite, lammerite-β, bradaczekite, euchlorine, anhydrite, chalcocyanite and tenorite. It is monoclinic, P21/m, a = 7.298(18), b = 6.588(11), c = 7.840(12) Å, β = 117.15(3)°, V = 335.4(11) Å3 and R1 = 0.046. The eight strongest lines of the powder X-ray diffraction pattern [d in Å (I) (hkl)] are: 6.969(56)(00$\bar{1}$), 3.942(52)(101), 3.483(100)(00$\bar{2}$), 3.294(49)(020), 2.936(43)(120), 2.534(63)(201), 2.501(63)(20$\bar{3}$) and 2.395(86)(02$\bar{2}$). The chemical composition determined by electron-microprobe analysis is (wt.%): ZnO 42.47, CuO 19.50, SO3 39.96, total 101.93. The empirical formula calculated on the basis of O = 9 apfu is Zn2.07Cu0.97S1.98O9 and the simplified formula is Zn3O(SO4)2. Glikinite is a Zn,Cu analogue of synthetic Zn3O(SO4)2. The crystal structure of glikinite is based on OZn4 tetrahedra sharing common corners, thus forming [Zn3O]4+ chains. Sulfate groups interconnect [Zn3O]4+ chains into a 3D framework.

We report the crystal structures of four synthetic members of the variscite group (space group type Pbca) and of bonacinaite, the first naturally occurring scandium arsenate member of the metavariscite group. All structures were determined using single-crystal X-ray intensity data. The following members were all synthesised under either mild hydrothermal conditions or by wet-chemical methods (<90°C). CrAsO4⋅2H2O (deep green): a = 8.894(2), b = 9.946(2), c = 10.206(2) Å and V = 902.8(3) Å3; R1 = 2.14%. Tl3+PO4⋅2H2O (colourless): a = 10.2848(7), b = 8.8578(6), c = 10.3637(7) Å and V = 944.14(11) Å3 (data at –173°C); R1 = 2.56%. MnSeO4⋅2H2O (pale pink): a = 10.441(2), b = 9.2410(18), c = 10.552(2) Å and V = 1018.1(3) Å3; R1 = 2.19%. A different method of preparation of MnSeO4⋅2H2O yielded crystals with very similar unit-cell parameters, a = 10.4353(5), b = 9.2420(5) and c = 10.5349(6) Å; R1 = 2.25%. CdSeO4⋅2H2O (colourless) has a = 10.481(1), b = 9.416(1), c = 10.755(1) Å and V = 1061.4(2) Å3; R1 = 1.53%. The thermal behaviour of the two selenate members was studied by a combination of DSC and TG, supplemented by PXRD. Bonacinaite (IMA2018-056), metavariscite-type natural (Sc,Al)(As,P)O4⋅2H2O (ideally ScAsO4⋅2H2O), crystallises in the space group P21/n, with a = 5.533(1), b = 10.409(2), c = 9.036(2) Å, β = 91.94(3)° and V = 520.11(18) Å3; R1 = 3.66%. The structural formula, supported by chemical analysis, is (Sc0.807(1)Al0.193)(As0.767(7)P0.233)O4⋅2H2O. All structures are based on frameworks built by corner-sharing of TO4 tetrahedra (T = P5+, As5+ or Se6+) with MO4(H2O)2 (M = Mn2+, Cd2+, Cr3+, Sc3+ or Tl3+) octahedra. The flexible frameworks are reinforced by partly bifurcated, strong to weak hydrogen bonds.

The crystal chemistry of all known synthetic and natural members of the variscite and metavariscite groups is discussed and compared, and the relative stabilities are evaluated. With the aid of the COMPSTRU program (Bilbao Crystallographic Server), a quantitative comparison of the crystal structures in both groups is given. Calculations of the structural and topological complexity reveal that the metavariscite structure type is structurally and topologically simpler than that of variscite. It is suggested that metavariscite and phosphosiderite are metastable kinetically stabilised phases, in contrast to thermodynamically stable variscite and strengite, respectively. The 3D frameworks of the members of both groups have been shown to be potential electrode materials for rechargeable Li ion batteries.

Tetrahedrite-(Hg), Cu6(Cu4Hg2)Sb4S13, has been approved as a new mineral species using samples from Buca della Vena mine (hereafter BdV), Italy, Jedová hora (Jh), Czech Republic and Rožňava (R), Slovakia. It occurs as anhedral grains or as tetrahedral crystals, black in colour, with metallic lustre. At BdV it is associated with cinnabar and chalcostibite in dolomite veins. At Jh, tetrahedrite-(Hg) is associated with baryte and chalcopyrite in quartz–siderite–dolomite veins; at R it is associated with quartz in siderite–quartz veins. Tetrahedrite-(Hg) is isotropic, greyish-white in colour, with creamy tints. Minimum and maximum reflectance data for Commission on Ore Mineralogy wavelengths in air (BdV sample), R in %) are 32.5 at 420 nm; 32.9 at 546 nm; 33.2 at 589 nm; and 30.9 at 650 nm. Chemical formulae of the samples studied, recalculated on the basis of 4 (As + Sb + Bi) atoms per formula unit, are: (Cu9.44Ag0.07)Σ9.51(Hg1.64Zn0.36Fe0.06)Σ2.06Sb4(S12.69Se0.01)Σ12.70 (BdV), Cu9.69(Hg1.75Fe0.25Zn0.06)Σ2.06(Sb3.94As0.06)S12.87 (Jh) and (Cu9.76Ag0.04) Σ9.80(Hg1.83Fe0.15Zn0.10)Σ2.08(Sb3.17As0.58Bi0.25)S13.01 (R). Tetrahedrite-(Hg) is cubic, I$\overline 4 $3m, with a = 10.5057(8) Å, V = 1159.5(3) Å3 and Z = 2 (BdV). Unit-cell parameters for the other two samples are a = 10.4939(1) Å and V = 1155.61(5) Å3 (Jh) and a = 10.4725(1) Å and V = 1148.55(6) Å3 (R). The crystal structure of tetrahedrite-(Hg) has been refined by single-crystal X-ray diffraction data to a final R1 = 0.019 on the basis of 335 reflections with Fo > 4σ(Fo) and 20 refined parameters. Tetrahedrite-(Hg) is isotypic with other members of the tetrahedrite group. Mercury is hosted at the tetrahedrally coordinated M(1) site, along with minor Zn and Fe. The occurrence of Hg at this position agrees both with the relatively large M(1)–S(1) bond distance (2.393 Å) and the refined site scattering. Previous occurrences of Hg-rich tetrahedrite and tetrahedrite-(Hg) are reviewed, and its relations with other Hg sulfosalts are discussed.

The paper opens a series devoted to the mineral diversity of the Mochalin Log rare earth element (REE) deposit, South Urals, Russia. There is a brief outline of the history of studies and geology of the deposit as well as the description of two new isostructural gatelite-group minerals, ferriperbøeite-(La) (CaLa3)(Fe3+Al2Fe2+)[Si2O7][SiO4]3O(OH)2 and perbøeite-(La) (CaLa3)(Al3Fe2+)[Si2O7][SiO4]3O(OH)2. Both minerals occur in polymineralic nodules and are associated with one another and allanite-(Ce), allanite-(La), bastnäsite-(Ce), bastnäsite-(La), ferriallanite-(Ce), ferriallanite-(La), ferriperbøeite-(Ce), perbøeite-(Ce), törnebohmite-(Ce) and törnebohmite-(La). The new minerals form isolated anhedral grains up to 0.3 mm [ferriperbøeite-(La)] and 0.5 mm [perbøeite-(La)] across and granular aggregates up to 1.5 mm × 0.5 mm [ferriperbøeite-(La)] and 3 mm × 1 mm [perbøeite-(La)]. Both minerals are brownish-black with vitreous lustre. Dcalc = 4.510 [ferriperbøeite-(La)] and 4.483 [perbøeite-(La)] g cm–3. They are optically biaxial (+); ferriperbøeite-(La): α = 1.788(5), β = 1.790(5), γ = 1.810(5) and 2Vmeas = 40(10)°; perbøeite-(La): α = 1.778(8), β = 1.783(8), γ = 1.805(8) and 2Vmeas = 40(10)°. Chemical data [wt.%, electron-microprobe; Fe3+:Fe2+ ratio from charge-balance requirements, H2O by stoichiometry; ferriperbøeite-(La)/perbøeite-(La)] are: CaO 4.91/4.81, ThO2 0.00/0.32, La2O3 23.75/22.16, Ce2O3 19.69/20.05, Pr2O3 0.85/1.09, Nd2O3 1.48/2.18, MgO 1.47/1.38, Al2O3 10.68/11.25, MnO 1.07/0.92, FeO 3.04/3.35, Fe2O3 5.31/3.78, TiO2 0.19/0.19, SiO2 27.47/27.35, F 0.11/0.23, H2O 1.61/1.54, –O = F –0.05/–0.10, total 101.58/100.50. The empirical formulae based on O20(OH,F)2 are for ferriperbøeite-(La): (Ca0.95La1.58Ce1.30Nd0.10Pr0.06)Σ3.99(Al2.27Fe3+0.72Fe2+0.46Mg0.40Mn0.16Ti0.03)Σ4.04Si4.96O20[(OH)1.94F0.06]; and for perbøeite-(La): (Ca0.94Th0.01La1.49Ce1.34Nd0.14Pr0.07)Σ3.99(Al2.42Fe3+0.52Fe2+0.51Mg0.38Mn0.14 Ti0.03)Σ4.00Si4.99O20[(OH)1.87F0.13]. Both minerals are monoclinic, P21/m; the unit-cell parameters [ferriperbøeite-(La)/perbøeite-(La)] are: a = 8.9458(2)/8.9652(4), b = 5.72971(13)/5.7306(2), c = 17.6192(3)/17.6770(9) Å, β = 115.9497(19)/116.053(6)°, V = 812.06(3)/815.88(6) Å3 and Z = 2/2. The crystal structures are solved on a single crystal and converged to R = 0.0355 [ferriperbøeite-(La)] and 0.0750 [perbøeite-(La)]. Ferriperbøeite-(La) and perbøeite-(La) are named as the La-analogues of ferriperbøeite-(Ce) and perbøeite-(Ce), respectively.

Bulachite specimens from Cap Garonne, France, comprise two intimately mixed hydrated aluminium arsenate minerals with the same Al:As ratio of 2:1 and with different water contents. The crystal structures of both minerals have been solved using data from low-dose electron diffraction tomography combined with synchrotron powder X-ray diffraction. One of the minerals has the same powder X-ray diffraction pattern (PXRD) as for published bulachite. It has orthorhombic symmetry, space group Pnma with unit-cell parameters a = 15.3994(3), b = 17.6598(3), c = 7.8083(1) Å and Z = 4, with the formula [Al6(AsO4)3(OH)9(H2O)4]⋅2H2O. The second mineral is a higher hydrate with composition [Al6(AsO4)3(OH)9(H2O)4]⋅8H2O. It has the same Pnma space group and unit-cell parameters a = 19.855(4), b = 17.6933(11) and c = 7.7799(5) Å i.e. almost the same b and c parameters but a much larger a parameter. The structures are based on polyhedral layers, parallel to (100), of composition [Al6(AsO4)3(OH)9(H2O)4] and with H-bonded H2O between the layers. The layers contain [001] spiral chains of edge-shared octahedra, decorated with corner connected AsO4 tetrahedra that are the same as in the mineral liskeardite. The spiral chains are joined together by octahedral edge-sharing to form layers parallel to (100). Synchrotron PXRD patterns collected at different temperatures during heating of the specimen show that the higher-hydrate mineral starts transforming to bulachite when heated to 50°C, and the transformation is complete between 75 and 100°C.

The new alluaudite-group mineral badalovite was found in the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with hematite, tenorite, cassiterite, johillerite, nickenichite, calciojohillerite, bradaczekite, metathénardite, aphthitalite, langbeinite, calciolangbeinite, sanidine, fluorophlogopite, fluoborite, tilasite, anhydrite, pseudobrookite, sylvite, halite, lammerite, urusovite, ericlaxmanite, arsmirandite, svabite, krasheninnikovite, euchlorine, wulffite and alumoklyuchevskite. Badalovite forms oblique-angled prismatic crystals up to 1 mm × 1 mm × 5 mm, typically combined in groups or crusts up to several hundred cm2 in area. The mineral is transparent, green, grey, yellow or colourless, with vitreous lustre. It is brittle, the Mohs hardness is 3½. Cleavage was not observed, the fracture is uneven. Dcalc is 4.02 g cm–3. Badalovite is optically biaxial (–), α = 1.753(3), β = 1.757(3), γ = 1.758(3) and 2Vmeas. = 50(10)°. Chemical composition (wt.%, electron-microprobe; holotype) is: Na2O 9.23, K2O 0.19, CaO 2.04, MgO 13.78, MnO 0.31, CuO 0.12, ZnO 0.24, Al2O3 0.06, Fe2O3 12.77, TiO2 0.01, SiO2 0.06, P2O5 0.33, V2O5 0.05, As2O5 61.51, SO3 0.02, total 100.72. The empirical formula based on 12 O apfu is Na1.67Ca0.20K0.02Mg1.92Zn0.02Mn0.02Cu0.01Fe3+0.90Al0.01(As3.01P0.03Si0.01)Σ3.05O12. The simplified formula is Na2Mg2Fe3+(AsO4)3. Badalovite is monoclinic, C2/c, a = 11.9034(3), b = 12.7832(2), c = 6.66340(16) Å, β = 112.523(3)°, V = 936.59(4) Å3 and Z = 4. The strongest reflections of the powder XRD pattern [d,Å(I)(hkl)] are: 6.41(38)(020), 5.505(20)(200), 3.577(23)($\bar{1}$31), 3.523(25)(310), 3.211(46)($\bar{1}$12), 2.911(28)($\bar{2}$22, $\bar{3}$12), 2.765(100)(240, 400) and 2.618(26)($\bar{1}$32). The crystal structure was solved from single-crystal XRD data with an R1 of = 2.49%. Badalovite is isostructural with other alluaudite-group minerals. Its simplified crystal chemical formula is A(1)NaA(1)’□A(2) □A(2)’NaM(1)MgM(2)(Mg0.5Fe3+0.5)2(AsO4)3 (□ – vacancy) and the end-member formula is NaNaMg(MgFe3+)(AsO4)3. The mineral is named in honour of the outstanding mineralogist and geochemist Stepan Tigranovich Badalov (1919–2014).