Akopovaite, ideally Li2Al4(OH)12(CO3)(H2O)3, is a new hydrotalcite-supergroup mineral from the Karasu–Karavshinskoye Sn deposit, Turkestan Range, Kyrgyzstan. It occurs as white or pale yellowish rosette-like aggregates that are composed of tiny curved plates up to 20–30 μm. Akopovaite is associated with gibbsite, quartz, albite, microcline, muscovite, montebrasite, siderite, schorl and birnessite-like Fe–Mn oxides. Akopovaite has a perfect cleavage along {001}, the mineral is transparent and very soft (VHN = 24 that corresponds to Mohs hardness of ca. 1). Dmeas = 2.12(2) g/cm3 and Dcalc = 2.106 g/cm3. The Raman spectra contain bands of carbonate groups and bands of O–H stretching vibrations. The chemical composition (wt.%, electron microprobe for Al and Fe; ICP-OES for Li; CHN method for CO2 and H2O) is Li2O 6.43, Al2O3 45.79, Fe2O3 0.27, CO2 10.09, H2O 36.1, total 98.68. The empirical formula based on (Li + Al + Fe) = 6 apfu is Li1.94(Al4.05Fe0.02)Σ4.07(OH)12(CO3)1.03(H2O)3.03. The crystal structure was refined by the Rietveld method with RB = 0.006 and Rwp = 0.014. Akopovaite is monoclinic, C2/m, a = 5.0953(6), b = 8.877(1), c = 7.806(1) Å, β = 102.572(6)°, V = 344.61(8) Å3 and Z = 1. The polytype should be denoted as 1M. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 7.66(100)(001), 4.397(27)(020), 3.821(45)(002,021), 2.4881(27)(200), 2.2273(16)(201) and 1.9027(18)(202). Akopovaite is the first naturally occurring hydrotalcite-supergroup carbonate species of Al and Li; its synthetic analogue is known.

The new leucophosphite-group mineral ammoniotinsleyite is found in a guano deposit located on the Pabellón de Pica Mountain, Iquique Province, Tarapacá Region, Chile. Associated minerals are halite, gypsum, salammoniac and clay minerals. Ammoniotinsleyite occurs as pink to pale violet globular aggregates up to 3 mm across with individual single crystals ~10–15 μm. The mineral is brittle. Its Mohs hardness is 4. Dmeas. = 2.42(2) g cm–3 and Dcalc. = 2.451 g cm–3. The IR spectrum shows the presence of NH4+ and PO43– groups and H2O molecules. Ammoniotinsleyite is optically biaxial (+), α = 1.557(2), β = 1.559 (calc.), γ = 1.563(2) (λ = 589 nm); and 2Vmeas. = 75(10)°. The chemical composition (K, Mg, Ca, Al, Fe and P from electron-microprobe data; H, C and N measured by gas chromatography on products of ignition at 1200°C; wt.%) is: (NH4)2O 7.25, K2O 1.50, MgO 0.42, CaO 0.34, Al2O3 29.91, Fe2O3 2.36, P2O5 43.97, H2O 14.89, CO2 below detection limit, total 100.64. The empirical formula is [(NH4)0.88K0.10Ca0.02)]Σ1.00(Al1.86Fe3+0.09Mg0.03)Σ1.98(PO4)1.96(OH)1.05⋅2.11H2O. The idealised formula is (NH4)2Al2(PO4)2(OH)⋅2H2O. The crystal structure of ammoniotinsleyite was refined based on powder X-ray diffraction data, using the Rietveld method. The final agreement factors are: Rp = 0.0071, Rwp = 0.0093 and Robs = 0.0167. The new mineral is isostructural with tinsleyite, spheniscidite and leucophosphite. It is monoclinic, space group P21/n, a = 9.5871(1) Å, b = 9.6089(1) Å, c = 9.6467(2) Å, β = 103.4461(8)°, V = 864.31(2) Å3 and Z = 4. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I,%)(hkl)] are: 7.56(23)($\bar{1}$01), 6.71(79)(011, 110), 5.947(100)(101, $\bar{1}$11), 4.676(36)(002, 200), 3.032(28)($\bar{1}$13, 031, 130), 2.958(25)($\bar{2}$22, 310, $\bar{1}$31) and 2.635(29)($\bar{2}$31).

Brannerite (UTi2O6) is among the major uranium-bearing minerals found in ore deposits, however as it has been long considered as a refractory mineral for leaching it is currently disregarded in ore deposits. Brannerite is found in a variety of geological environments with the most common occurrences being hydrothermal and pegmatitic. On the basis of scanning electron microscopy observations coupled with electron probe micro-analyses and laser ablation inductively-coupled plasma mass spectrometer analyses, this study describes the morphological features and the major- and trace-element abundances of brannerite samples from five hydrothermal and five pegmatitic localities across the world. Mineral compositions are also compared with observations from transmission electron microscopy and Raman spectrometry showing that brannerite is amorphous. Significant results include the definition of substitution trends and REE patterns, which are characteristics of either an occurrence or genetic type (hydrothermal and pegmatitic). Hence, in combination, it is possible to obtain reliable constraints for establishing a geochemical classification of brannerite. Inferred fingerprints have direct implications for forensic science and the exploration industry; they also contribute to a better understanding of metallogenic processes and to optimising the extraction of uranium.

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.

Orthopyroxene-bearing felsic gneiss occurs as foliation-parallel layers and bands together with aluminous granulite, mafic granulite, and quartzofeldspathic granulite in the Chilka Lake migmatite complex of the Proterozoic Eastern Ghats Belt, India. The rock was classified previously as charnockite which underwent granulite-facies metamorphism. Field and textural features of this rock show evidence of the partial melting of a biotite-bearing greywacke protolith. Orthopyroxene with/without garnet and cordierite were produced with K-feldspar as peritectic phases of incongruent melting of presumed metaluminous sediments. Fluid-inclusion data suggest the presence of high-density CO2-rich fluids during peak metamorphism, which are similar to those found in associated aluminous granulite. Whole-rock major and trace element data show wide variability of the source materials whereas REE distributions show enriched LREE and flat HREE patterns. Zircon grains from representative samples show the presence of inherited cores having spot dates (SHRIMP) in the range c. 1790–3270 Ma. The overgrowth on zircon was formed predominantly during c. 780–730 Ma and sporadically during c. 550–520 Ma. Some neoblastic zircons with c. 780–730 Ma ages are also present. U-rich dark zones surrounding cores appear partially metamictised, but spot ages from this zone vary within c. 1000–900 Ma. The <1000 Ma ages represent metamorphism that mirrors the events in associated aluminous granulite. The sources of metaluminous sediments are speculative as the rock compositions are largely modified under granulite-facies metamorphism and partial melting. Considering the accretionary tectonic setting of the Eastern Ghats Belt during the c. 1000–900 Ma time frame, a greywacke-type protolith for the migmatite complex has been proposed.

The new mineral pseudomeisserite-(NH4) (IMA2018-166), (NH4,K)2Na4[(UO2)2(SO4)5]⋅4H2O, was found in the Blue Lizard mine, San Juan County, Utah, USA, where it occurs as light yellow prisms in a secondary assemblage with belakovskiite, blödite, changoite, ferrinatrite, gypsum, ivsite, metavoltine and tamarugite. The streak is very pale yellow and the fluorescence is bright lime green under 405 nm ultraviolet light. Crystals are transparent with vitreous lustre. The tenacity is brittle, the Mohs hardness is 2½, the fracture is curved or conchoidal and there is one perfect cleavage on {100}. The mineral is easily soluble in H2O and has a measured density of 3.22(2) g⋅cm–3. Pseudomeisserite-(NH4) is optically biaxial (–) with α = 1.536(2), β = 1.559(2) and γ = 1.565(2) (white light); 2Vmeas. = 53(1)°; dispersion is r > v, distinct; pleochroism: X colourless, Y light yellow and Z pale yellow (X < Z < Y); optical orientation: Z = b, Y ∧ c = 33° in obtuse β). Electron microprobe analyses (WDS mode) provided (NH4)1.49K0.60Na3.87U2.00S5.04O28H7.78. The five strongest X-ray powder diffraction lines are [dobs, Å(I)(hkl)]: 12.69(76)(100), 6.83(84)(012,102), 6.01(100)($\bar{2}$02), 3.959(67)($\bar{2}$21,$\bar{2}$14,$\bar{1}$23) and 3.135(76)($\bar{2}$06,223,$\bar{1}$16). Pseudomeisserite-(NH4) is monoclinic, P21/c, a = 13.1010(3), b = 10.0948(2), c = 19.4945(14) Å, β = 104.285(7)°, V = 2498.5(2) Å3 and Z = 4. The structural unit in the structure (R1 = 0.0254 for 3837 I > 2σI reflections) is a novel [(UO2)2(SO4)5]6– uranyl-sulfate band.

Combined microstructural, mineral chemical, X-ray maps and X-ray single-crystal diffraction analyses are used to reveal the behaviour of individual grains of magmatic allanite relicts hosted in variably deformed metagranitoids at Lago della Vecchia (inner part of the Sesia-Lanzo Zone, Western Alps, Europe), which experienced high-pressure and low-temperature metamorphism during the Alpine subduction. X-ray single-crystal diffraction shows that none of the allanite crystals, irrespective of the strain state of the host rock, record any evidence of plastic deformation (i.e. intracrystalline deformation), as indicated by the shape of the Bragg diffraction spots, the atomic site positions, and their displacement around the centre of gravity. On the contrary, strong plastic deformation affected matrix minerals, such as quartz, white mica and feldspar of the hosting rocks, during the development of the Alpine eclogitic- and blueschist-facies metamorphism. Despite the strain-free atomic structures of allanite, different patterns of chemical zoning, as a function of strain accumulated in the rock matrix, are observed. As allanite occurs in magmatic and metamorphic rocks and it is stable at high-pressure and low-temperature conditions, we infer that allanite could behave as one of the main carriers of light rare earth elements into the mantle wedge during subduction of continental crust. In particular, the release of light rare earth elements from allanite, under high-pressure conditions in subduction zones, is facilitated by high strain accumulated in the host rock.

Monteneroite (IMA2020-028), Cu2+Mn2+2(AsO4)2⋅8H2O, is a new vivianite-structure mineral from the Monte Nero mine, Rocchetta di Vara, La Spezia, Liguria, Italy. It is a secondary mineral that crystallised from As-, Cu- and Mn-rich fluids and it is associated with braunite, copper, cuprite, rhodochrosite and strashimirite. Monteneroite occurs as light green, thick blades up to ~2.5 mm long. The streak is white. Crystals are transparent with vitreous lustre. The mineral has Mohs hardness of 2, is somewhat sectile, exhibits two cleavages ({010} perfect and {001} fair) and has irregular stepped fracture. The measured density is 2.97(2) g cm–3. Monteneroite is optically biaxial (+), with α = 1.604(2), β = 1.637(2) and γ = 1.688(2), determined in white light; 2V = 80(1)°; slight dispersion is r < v, orientation: X = b; Z ^ c = 52° in obtuse β. Electron microprobe analyses provided the empirical formula (Cu2+0.88Mn2+0.11)Σ0.99Mn2+2.00(As1.00O4)2⋅8H2O. Monteneroite is monoclinic, C2/m, a = 10.3673(14), b = 13.713(2), c = 4.8420(8) Å, β = 105.992(8)°, V = 661.72(18) Å3 and Z = 2. Monteneroite has a vivianite-type structure (R1 = 0.0535 for 534 I > 2σI reflections). It is the first mineral with this structure type to be defined with ordered octahedral cation sites.

Luboržákite, ideally Mn2AsSbS5, is a new mineral from the Vorontsovskoe gold deposit, Northern Urals, Russia. It forms long-prismatic crystals up to 70 × 20 μm and anhedral grains of the same size embedded in the matrix of Mn-bearing dolomite and Mn-bearing calcite. Associated minerals include pyrite, orpiment, realgar, stibnite, aktashite, alabandite, boscardinite, chabournéite, coloradoite, clerite, écrinsite, gold, routhierite, sphalerite and twinnite. Luboržákite is black, opaque with metallic lustre and has a black streak. It is brittle and has an uneven fracture. No cleavage and parting have been observed. Mohs hardness is 4–4½. Dcalc = 4.181 g cm–3. In reflected light, luboržákite is tin-white, weakly anisotropic with rotation tints varying from dark grey to grey. The chemical composition of luboržákite is (wt.%; electron microprobe, WDS mode): Mn 21.23, Cu 0.29, Ag 0.56, Pb 1.90, As 15.25, Sb 27.03, S 33.23, total 99.49. The empirical formula based on the sum of all atoms = 9 apfu is Mn1.86Pb0.04Ag0.03Cu0.02As0.98Sb1.07S5.00. The new mineral is monoclinic, space group C2/m with a = 12.5077(6), b = 3.8034(2), c = 16.0517(8) Å, β = 94.190(4)°, V = 761.57(6) Å3 and Z = 4. The crystal structure of luboržákite was solved from the single-crystal X-ray diffraction data to R = 0.0383 for 712 observed reflections with I > 3σ(I). Luboržákite is a new member of the heterochemical isostructural series of ‘unit-cell twinned’ structures, named the pavonite series. The new mineral honours Lubor Žák, a prominent Czech crystallographer and the professor of the Charles University in Prague, Czech Republic.

The new mineral yarzhemskiite, K[B5O7(OH)2]⋅H2O, was found in a halite–sylvite evaporite rock at the Chelkar salt dome, Western Kazakhstan Region, Kazakhstan. It is also associated with carnallite, polyhalite, gypsum, strontioginorite, satimolite and quartz. Yarzhemskiite occurs as separate thick tabular, short prismatic or equant crystals up to 0.5 mm × 0.7 mm × 1 mm and grains having irregular outlines up to 1 mm × 1.5 mm × 2 mm. The mineral is transparent, colourless, with vitreous lustre. It is brittle, the Mohs’ hardness is ca 2½. Cleavage is perfect on {100}. Dmeas is 2.13(1) and Dcalc is 2.112 g cm–3. Yarzhemskiite is optically biaxial (+), α = 1.484(2), β = 1.508(2), γ = 1.546(2), 2Vmeas = 75(10)° and 2Vcalc = 80°. Chemical composition (wt.%, electron microprobe, H2O was calculated by stoichiometry) is: Na2O 0.01, K2O 17.84, CaO 0.07, B2O3 67.21, H2Ocalc 13.91, total 99.04. The empirical formula based on 10 O atoms per formula unit is K0.98B5.005O7(OH)2⋅H2O. Yarzhemskiite is monoclinic, P21/c, a = 9.47340(18), b = 7.52030(16), c = 11.4205(2) Å, β = 97.3002(17)°, V = 807.03(3) Å3 and Z = 4. The strongest reflections of the powder XRD pattern [d,Å(I,%)(hkl)] are: 9.39(86)(100), 4.696(41)(200), 3.296(18)($\bar{1}$13), 3.130(19)(022, 300), 2.935(42)(220), 2.898(100)($\bar{3}$02, $\bar{2}$21, 310), 2.832(56)(004) and 1.867(18)($\bar{2}$25). The crystal structure was solved based on single-crystal X-ray diffraction data, R1 = 3.36%. The structure contains infinite chains built by boron-centred polyhedra. The basic structural unit of the chain is a double ring B5O7(OH)2 consisting of one BO4 tetrahedron and four BO3 triangles. K+ cations centre ten-fold polyhedra which form, together with the borate chains [B5O7(OH)2]–∝, layers linked with each other only via H bonds. The mineral is named in honour of the Russian geologist, petrologist and mineralogist Yakov Yakovlevich Yarzhemskii (1901–?), a specialist in petrology of evaporite rocks and mineralogy and genesis of boron deposits related to evaporites.

Thermal behaviour of fibrous tremolite from Maryland, USA has been investigated in situ up to breakdown temperature. Tremolite can be found both as primary constituent and as contaminant in Asbestos Containing Materials (ACMs). The products of breakdown are subcalcic diopside and calcium-rich clinoenstatite in a 2:1 ratio, traces of hematite plus minor silica-rich amorphous material. Thermal expansion follows a regular trend up to 723 K before the onset of Fe2+ oxidation/OH– deprotonation which is completed at 1023 K. At 923 K the Fe3+ migration starts towards M(1) and the corresponding counter-migration of Mg to M(2) and M(3). At T close to structure breakdown, M(2) shows a significant site-scattering reduction possibly consistent with the occurrence of minor vacancies. In fully oxidised tremolite, Fe3+ is allocated prevalently at M(1) and subordinately at M(3). As it is well-known that M(1), along with M(2), is the most exposed octahedral site at the surface of amphiboles, most of the Fe3+ is available for participating in the Fenton-like reactivity of oxidised tremolite, potentially making it dangerous for human health. This point should be properly taken into account in the evaluation of the safety of thermally decomposed tremolite-containing ACMs, in particular in the case of accidentally incomplete treatments.

The high-silica rhyolitic Joe Lott Tuff was erupted at 19.2 ± 0.4 Ma from the Mount Belknap caldera, SW Utah. Certain units in the tuff contain two species of wakefieldite, the Nd- and Y-dominant types. They occur in disseminated streaks and patches in association with rhodochrosite, calcite, Fe oxide, cerite-(Ce), and a Mn silicate (caryopilite?), thought to have been deposited from hydrothermal fluids. The wakefieldites contain the highest levels of As (≤15.34 wt.% As2O5) and P (≤5.7 wt.% P2O5) yet recorded in this mineral, indicating significant solid solution towards chernovite-(Y) and xenotime-(Y). Thorium levels are also unusually high (≤14.2 wt.% ThO2). The source of the hydrothermal fluid(s) is unknown but might be related to uranium mineralisation in the region, in that As, V and U are commonly associated in such deposits.

Oxybismutomicrolite, ideally [(Bi3+,#)2]Σ4+Ta2O6O, where # = subordinate substituents, such as Na+, Ca2+ and vacancy (□), is a microlite-group, pyrochlore-supergroup mineral discovered at the Solnechnaya (‘Sunny’) pegmatite vein, Malkhan pegmatite field, Zabaykalskiy Kray, Central Transbaikalia, Russia. It forms rough octahedral crystals up to 1 mm across and equant grains up to 2 mm across embedded in an albite–lepidolite–elbaite complex. Other associated minerals are Bi-rich fluornatromicrolite, bismutotantalite and stibiotantalite. The new mineral is black, with resinous lustre; the streak is greyish white. It is non-fluorescent under ultraviolet light. Oxybismutomicrolite is brittle, with Mohs’ hardness of ~5. Cleavage is not observed, fracture is uneven. Dmeas. = 6.98(2) g/cm3 and Dcalc. = 7.056 g/cm3. The mineral is optically isotropic. The mean refractive index calculated from the Gladstone–Dale equation is 2.184. The infrared spectrum shows the absence of H2O molecules and OH groups. The chemical composition is (electron microprobe, wt.%): Na2O 3.45, CaO 2.88, MnO 0.31, PbO 0.76, Bi2O3 29.81, ThO2 0.18, TiO2 3.89, SnO2 1.77, Nb2O5 4.50, Ta2O5 51.08, F 1.17, O = F –0.49, total 99.31. The empirical formula, on the basis of 2 cations at the B site, is (Bi0.79Na0.68Ca0.32Mn0.03Pb0.02□0.16)Σ2.00(Ta1.42Ti0.30Nb0.21Sn0.07)Σ2.00O6.00(O0.52F0.38□0.10)Σ1.00. The crystal structure refinement (R = 0.019) gave the following data: cubic, Fd–3m, a = 10.4746(11) Å, V = 1149.2(4) Å3 and Z = 8. The eight strongest lines of the powder X-ray diffraction pattern [d, Å(I, %)(hkl)] are: 6.051(12)(111), 3.160(10)(311), 3.026(100)(222), 2.621(32)(400), 1.854(33)(440), 1.581(27)(622), 1.514(7)(444) and 1.203(7)(662). Type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, registration number 5409/1.

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.

Fire agates from Deer Creek are highly appreciated gemstones due to the presence of optical phenomena and rainbow colours that cause fiery effects to be observed on their characteristic brown base. The specific morphology of poorly ordered chalcedony (crystallinity index = 0.1–1.5) with an admixture of mogánite (av. 6.6%), micro-quartz and opal-C forming a colloform texture seems to be responsible for the presence of fire effect in these agates. The multi-layered silica spheroidal forms (‘bubble’-like structure), already noted in hand specimens, could be the centres of reflection and interference of white light. Numerous, microscopic inclusions of Fe and Ti compounds randomly scattered within some silica zones, together with microstructural features of agate, could determine the colour and size of the domains with the optical effect. Deer Creek fire agates form veins within their host volcanic rocks. The silica mineralisation filling the network of fissures in the host rocks was supplied cyclically with aqueous fluids of varying composition, enriched periodically in CO2, Fe, Ti, Mn, Zn and Ca. As a result, the red-brown colour of fire agates was created by scattered pigments of tiny iron oxides (magnetite, maghemite) and titanium oxides (rutile, anatase) within the silica matrix. The precipitation of strongly disordered silica with a characteristic colloform texture is diagnostic for boiling processes in this area.

A new mineral falgarite, K4(V+4O)3(SO4)5 was discovered at the tract of Kukhi-Malik, Fan-Yagnob coal deposit, ca. 75 km N of Dushanbe, Tajikistan. The new mineral is named after the Falgar, an ancient Sogdian name for an area around the Zeravshan riverhead. Falgarite is a fumarolic mineral formed directly from a gas emitted by a natural underground coal fire. Associated minerals are anhydrite, baryte, molybdite, an unidentified Tl-vanadyl sulfate, K–Mg sulfate and an anhydrous Mg-sulfate. Falgarite forms small isometric or pseudo-octahedral individual crystals (10–60 μm) of turquoise colour and spherical aggregates up to 0.5 mm in diameter. Mohs hardness is ~ 2.5, Dmeas = 2.87(2) and Dcalc = 2.89 g/cm3. Refractive indices are: α = 1.588(3), β(calc.) = 1.600(3) and γ = 1.609(2) (590 nm). In transmitted light falgarite is transparent green with a weak pleochroism. The mineral is non-soluble in H2O and 5% HNO3 at room temperature. Infrared spectra support the absence of H2O and OH–. The chemical composition determined by electron-microprobe analysis is (wt.%): Na2O 0.55, K2O 20.76, Tl2O 1.83, VO2 29.38 and SO3 46.78, total 99.29. The empirical formula (based on 23 O apfu) is: (K3.76Na0.15Tl0.07)Σ3.98V3.02S4.99O23.0. The strongest lines of the powder X-ray diffraction pattern are [d,Å(I,%)(hkl)]: 3.20(70)(202); 3.17(80)024; 3.14(70)$\bar{2}$04; 3.01(50)$\bar{1}$51; and 2.88(100)151. Falgarite is monoclinic, P21/n, a = 8.7209(5), b = 16.1777(6), c = 14.4614(7) Å, β = 106.744(5)°, V = 1953.77(17) Å3, Z = 4 and R1 = 0.05. VO6 octahedra and SO4 tetrahedra link together by sharing corners thus forming a [(VO)3(SO4)5]4– framework. K+, Na+ and Tl+ cations are located in the channels of the framework. The synthetic K4(VO)3(SO4)5 analogue is known.

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.

Maletoyvayamite, Au3Se4Te6, is a new mineral discovered in a heavy-mineral concentrate from the Gaching occurrence of the Maletoyvayam deposit, Kamchatka, Russia (60°19′51.87″N, 164°46′25.65″E). It forms anhedral grains (10 to 50 μm in size) and is found in intergrowths with native gold (Au–Ag), Au tellurides (calaverite), unnamed phases (AuSe, Au2TeSe and Au oxide), native tellurium, sulfosalts (tennantite, tetrahedrite, goldfieldite and watanabeite) and supergene tripuhyite. Maletoyvayamite has a good cleavage on {010} and {001}. In plane-polarised light, maletoyvayamite is grey, has strong bireflectance (grey to bluish grey), and strong anisotropy; it exhibits no internal reflections. Reflectance values for maletoyvayamite in air (Rmin,Rmax in %) are: 38.9, 39.1 at 470 nm; 39.3, 39.5 at 546 nm; 39.3, 39.6 at 589 nm; and 39.4, 39.7 at 650 nm. Sixteen electron-microprobe analyses of maletoyvayamite gave an average composition: Au 34.46, Se 16.76, Te 47.23 and S 0.84, total 99.29 wt.%, corresponding to the formula Au2.90(Se3.52S0.44)Σ3.96Te6.14 based on 13 atoms; the average of eleven analyses on synthetic analogue is: Au 34.20, Se 19.68 and Te 45.42, total 99.30 wt.%, corresponding to Au2.90Se4.16Te5.94. The calculated density is 7.98 g/cm3. The mineral is triclinic, space group P1, with a = 8.901(2), b = 9.0451(14), c = 9.265(4) Å, α = 97.66(3), β = 106.70(2), γ = 101.399(14)°, V = 685.9(4) Å3 and Z = 2. The crystal structure of maletoyvayamite represents a unique structure type resembling a molecular structure. There are cube-like [Au6Se8Te12] clusters linked via van der Waals interactions. The structural identity of maletoyvayamite with the synthetic Au3Se4Te6 was confirmed by powder X-ray diffraction and Raman spectroscopy.

The crystal structure of arsenopalladinite, Pd8As2.5Sb0.5, from the Kaarreoja River, Inari commune, Finnish Lapland, Finland, was solved to R1 = 0.0451 on the basis of single-crystal X-ray diffraction data. The mineral is triclinic, space group P$\bar{1}$. The unit-cell parameters are: a = 7.3344(7), b = 7.3870(8), c = 7.5255(7) Å, α = 98.869(8), β = 102.566(8), γ = 119.096(11)°, V = 331.19(7) Å3 and Z = 2. The crystal structure of arsenopalladinite consists of an alternation of layers made by pnictogen (As, Sb) and layers made by palladium atoms stacked along the c axis. Arsenic and (As, Sb) nets exhibit a triangular topology (A and D nets), whereas palladium layers show triangular or pentagon–triangular nets (B and C nets). The unit-cell contains 6 layers, with the ABCDCBA stacking sequence. Although arsenopalladinite shows characteristics very similar (nets of the same topology) to the closely-related mineral mertieite-II, Pd8Sb2.5As0.5, it has a different stacking sequence.

This work presents a detailed study of platinum-group mineral (PGM) assemblages from the Malaya Kamenushka River placer, whose formation is associated with the weathering of the Kamenushensky Uralian–Alaskan type massif, Middle Urals, Russian Federation. The deposit is characterised by the dominance of isoferroplatinum, together with significant numbers of inclusions of Os–Ir–Ru alloys and platinum-group element (PGE) sulfides. A study of the Os–Ir–Ru alloys permitted recognition of two types of iridium with different morphology and composition. The similarity of the PGM assemblages from the Malaya Kamenushka River placer and the lode mineralisation of the Kamenushensky massif is demonstrated. A comparison of PGM assemblages from the Malaya Kamenushka River placer with other placers and massifs of the Ural platinum belt demonstrates significant differences in the number of Os–Ir–Ru inclusions. Such differences for minerals of refractory elements cannot be explained by the vertical zoning of the lode mineralisation. Most probably this is associated with the enrichment of the primary substrate with Os, Ir and Ru and/or the degree of melting, depending on the chosen model of formation of the Uralian–Alaskan type massifs.