The cerite supergroup is established and includes the cerite group (silicates) and merrillite group (phosphates). Cerite-group minerals are cerite-(Ce), ferricerite-(La), aluminocerite-(Ce) and taipingite-(Ce). The merrillite group is subdivided into two subgroups: merrillite (merrillite, ferromerrillite, keplerite and matyhite) and whitlockite (whitlockite, strontiowhitlockite, wopmayite and hedegaardite). Cerite-(La) has been renamed ferricerite-(La). The new nomenclature has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification.

The assayers’ ingots recovered in 2014 from the SS Central America contain geochemical signatures and anomalies which convey information about the culture of miners and assayers in the late 1850s. A significant number of ingots (21%) contain elevated copper and zinc in the same ratio as a common brass alloy of the period, suggesting that adulteration of placer gold dust was a common issue. The ingots with the brass-like signature also have a gold fineness much lower than measured for typical California placer gold. Trace elements like palladium in uniform concentrations in all Kellogg & Humbert ingots suggest the use of quality high-temperature tools, while lead and tin in all ingots suggest specific contamination and industrial hygiene issues common to the whole industry. Other trace constituents such as arsenic, antimony and bismuth are likely to have originated from dense minerals associated with placer gold, or from the natural placer gold alloy itself. Comparison of modern analysis of ingots vs. assay values stamped on the ingots themselves suggests some firms like Kellogg & Humbert did superior assay work, while others like the San Francisco office of Justh & Hunter probably had issues with quality control. The variations in assay ingot alloy chemistry provide a window into this important period in American history.

A green tourmaline sample from the Tzarevskoye uranium–vanadium deposit, close to the Srednyaya Padma deposit, Lake Onega, Karelia Republic, Russia, has been found to be the second world-occurrence of Cr-rich vanadio-oxy-dravite in addition to the Pereval marble quarry, Sludyanka crystalline complex, Lake Baikal, Russia, type-locality. From the crystal-structure refinement and chemical analysis, the following empirical formula is proposed: X(Na0.96K0.02□0.02)Σ1.00Y(V1.34Al0.68Mg0.93Cu2+0.02Zn0.01Ti0.01)Σ3.00Z(Al3.19Cr1.36V0.03Mg1.42)Σ6.00(TSi6O18)(BBO3)3V(OH)3W[O0.60(OH)0.23F0.17]Σ1.00. Together with the data from the literature, a compositional overview of Al–V–Cr–Fe3+-tourmalines is provided by using [6]Al–V–Cr–Fe3+ diagrams for tourmaline classification. These diagrams further simplify the tourmaline nomenclature as they merge the chemical information over the octahedrally-coordinated sites (Y and Z) by removing the issues of uncertainty associated with cation order–disorder across Y and Z. Results show the direct identification of tourmalines by using the chemical data alone.

The new mineral percleveite-(La) (IMA2019–037), ideally La2Si2O7, was found in polymineralic nodules of the Mochalin Log REE deposit, Chelyabinsk Oblast, South Urals, Russia. It is associated with allanite-(Ce), allanite-(La), bastnäsite-(Ce), bastnäsite-(La), ferriallanite-(Ce), ferriallanite-(La), ferriperbøeite-(Ce), ferriperbøeite-(La), fluorbritholite-(Ce), hydroxylbastnäsite-(Ce), perbøeite-(Ce), perbøeite-(La), törnebohmite-(Ce) and törnebohmite-(La). Percleveite-(La) occurs as isolated anhedral grains commonly up to 0.2 mm × 0.4 mm and very rarely up to 1 mm × 1 mm. The new mineral is transparent with greasy lustre. The mineral is very pale yellow to colourless in thin fragments to light yellow in aggregates. It is brittle, with imperfect {001} cleavage and an uneven fracture. Mohs’ hardness is ca. 6. Dcalc = 5.094 g cm–3. Under the microscope, percleveite-(La) is non-pleochroic, optically uniaxial (+), ω = 1.825(10) and ɛ = 1.835(10). The Raman spectrum is given. Chemical data (wt.%, electron-microprobe) are: La2O3 36.80, Ce2O3 31.22, Pr2O3 1.57, Nd2O3 2.96, SiO2 26.73, total 99.28. The empirical formula based on 7 O apfu is (La1.02Ce0.86Nd0.08Pr0.04)Σ2.00Si2.00O7. Percleveite-(La) is tetragonal, P41; the unit-cell parameters are: a = 6.8482(3), c = 24.8550(13) Å, V = 1165.64(11) Å3 and Z = 8. The strongest reflections in the powder X-ray diffraction pattern [d, Å(I)(hkl)] are: 4.194(18)(113), 3.564(16)(106), 3.349(16)(201,202), 3.157(100)(203,116,008), 3.043(22)(211), 2.934(39)(122), 2.893(29)(213) and 2.864(21)(117). The crystal structure of percleveite-(La) is solved from the single-crystal X-ray diffraction data [R = 0.0617 for 2831 unique reflections with I > 2σ(I)]. The new mineral is named as an analogue of percleveite-(Ce) with La predominance over the rare-earth elements.

Thunderbayite (IMA2020–042), ideally TlAg3Au3Sb7S6, is a new mineral from the Hemlo gold deposit, Marathon, Ontario, Canada. It occurs as very rare anhedral rims up to 70 μm across in contact with aurostibite and associated spatially with stibarsen, biagioniite and native gold in a calcite matrix. Thunderbayite is opaque with a metallic lustre and shows a black streak. In reflected light, thunderbayite is weakly bireflectant and faintly pleochroic from grey–blue to slightly greenish grey–blue. Under crossed polars, it is weakly anisotropic with bluish to light-blue rotation tints. Internal reflections are absent. Reflectance percentages for the four Commission on Ore Mineralogy wavelengths (Rmin, Rmax) are: 37.9, 38.4 (471.1 nm); 35.3, 36.0 (548.3 nm); 33.9, 34.4 (586.6 nm); and 32.0, 32.5 (652.3 nm), respectively. A mean of five electron-microprobe analyses gave Ag 14.91(16), Au 27.40(22), Tl 9.37(9), Sb 39.80(34) and S 8.61(7), for a total of 100.09 wt.%, corresponding, on the basis of a total of 20 atoms, to Tl1.00Ag3.01Au3.03Sb7.12S5.84. Thunderbayite is triclinic, space group P1, with a = 8.0882(5), b = 7.8492(5), c = 20.078(1) Å, α = 92.518(5), β = 93.739(5), γ = 90.028(6)°, V = 1270.73(9) Å3 and Z = 2. The five strongest powder-diffraction lines [d in Å (I/I0) (hkl)] are: 4.04 (100) (200); 3.92 (80) (020); 2.815 (50) (220/$\bar{2}$20); 2.566 (45) ($\bar{1}$17); and 2.727 (40) (0$\bar{1}$7). The crystal structure [R1 = 0.0220 for 5521 reflections with I > 2σ(I)] can be considered as a strongly deformed pyrite-type structure with several metal–metal bonds. Thunderbayite shows close similarities with criddleite, TlAg2Au3Sb10S10, from an optical, chemical and structural point of view. The new mineral is named for the Thunder Bay district, Ontario, in which the Hemlo gold deposit is located.

Galena–tetrahedrite intergrowth textures have been observed in some quenched run products of melting experiments conducted at 500 and 600°C in the systems ZnS–PbS–FeS–Cu2S–Sb2S3 and ZnS–PbS–FeS–Cu2S–Sb2S3–As2S3, using the evacuated silica-tube method. At 600°C the intergrowth formed an interface between sulfide melt and galena, whereas at 500°C it existed as inclusions partially embedded or completely embedded within tetrahedrite. At 600°C tetrahedrite was absent in PbS-bearing experiments, instead galena and melt were a part of the equilibrium phase assemblage. From the disposition of the galena–tetrahedrite intergrowths at 500°C it is evident that droplets of galena–tetrahedrite melt coexisted with tetrahedrite or tetrahedrite + galena and gave rise to these intergrowths upon quenching. The intergrowths coexisting with galena probably represent compositions on the galena-rich liquidus in the galena–tetrahedrite binary and those coexisting with tetrahedrite represent points on the tetrahedrite-rich liquidus. A eutectic at galena:tetrahedrite = ~30:70, very close to 500°C is apparent. It is clearly indicated that galena–tetrahedrite intergrowths can be formed by sulfide partial melting, and could be used as a potential indicator of partial melting in metamorphosed sulfide ore deposits.

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.

Oxycalciomicrolite (IMA2019-110), (Ca,Na)2(Ta,Nb,Ti)2O6(O,F), is a new member of microlite-group mineral found in the saprolite of the weathered Fumal pegmatite, located close to the city of Nazareno, Minas Gerais state, Brazil. It occurs as an accessory mineral associated with quartz, albite, microcline, muscovite, columbite-subgroup minerals, cassiterite, hematite, ilmenite, monazite-(Ce), xenotime-(Y), zircon, beryl, spinel, epidote and garnet-group minerals. Oxycalciomicrolite is found as octahedral crystals, occasionally modified to rhombododecahedra, ranging from 0.2 to 0.5 mm in size. The crystals are brownish-yellow to brownish-red and translucent, with white streak and vitreous to resinous lustre. The tenacity is brittle, with a Mohs hardness of 5–5½. Cleavage and parting are not observed; the fracture is conchoidal. Electron microprobe analysis, Raman and infrared spectroscopies and X-ray powder diffraction were applied to characterise this mineral. Oxycalciomicrolite is isotropic, ncalc. = 2.037, and the calculated density is 6.333 g/cm3. The composition is (Ca1.57□0.26Na0.06Sn0.03Sr0.03U0.02Mn0.02Fe0.01Ce0.01)∑2.00(Ta1.79Nb0.18Ti0.03)∑2.00O6.00[O0.64F0.19□0.17]∑1.00 analysed by electron microprobe using wavelength dispersive spectrometry. The unit-cell parameters obtained by Pawley fitting from powder X-ray diffraction data are a = 10.4325(4) Å and V = 1135.46(14) Å3 with Z = 8.

The rhodonite-group mineral with the idealised, end-member formula MnMn4[Si5O15] and the crystal chemical formula VIIM(5)MnVIM(1–3)Mn3VIIM(4)Mn[Si5O15] (Roman numerals indicate coordination numbers) is defined as a valid mineral species named vittinkiite after the type locality Vittinki (Vittinge) mines, Isokyrö, Western and Inner Finland Region, Finland. Vittinkiite is an isostructural analogue of rhodonite, ideally CaMn4[Si5O15], with Mn2+ > Ca at the M(5) site. Besides Vittinki, vitiinkiite was found in more than a dozen rhodonite deposits worldwide, however, it is significantly less common in comparison with rhodonite. The mineral typically forms pink to light pink massive, granular aggregates and is associated with quartz, rhodonite, tephroite, pyroxmangite and Mn oxides. Vittinkiite is optically biaxial (+), with α = 1.725(4), β = 1.733(4), γ = 1.745(5) and 2Vmeas = 75(10)° (589 nm). The chemical composition of the holotype (wt.%, electron microprobe) is: MgO 0.52, CaO, 0.93, MnO 51.82, FeO 1.26, ZnO 0.11, SiO2 46.48, total 101.12. The empirical formula calculated based on 15 O apfu is Mn4.71Ca0.11Fe0.11Mg0.08Zn0.01Si4.99O15. Vittinkiite is triclinic, space group P$\bar{1}$, with a = 6.6980(3), b = 7.6203(3), c = 11.8473(5) Å, α = 105.663(3), β = 92.400(3), γ = 94.309(3)°, V = 579.38(7) Å3 and Z = 2. The crystal structure is solved on a single crystal to R1 = 3.85%. Polymorphism of MnSiO3 (rhodonite-, pyroxmangite-, garnet- and clinopyroxene-type manganese metasilicates) is discussed, as well as the relationship between vittinkiite and pyroxmangite, ideally Mn7[Si7O21], and the application of infrared spectroscopy for the identification of manganese pyroxenoids.

Kagomé nets of corner-connected triangles of atoms occur in diverse minerals, from the {111} anion arrays in perovskite-group minerals to natural metallic alloys like auricupride, AuCu3, to the cation layers in atacamite-group minerals. We review here two- and three-dimensional kagomé networks in minerals where the kagomé node atoms are octahedrally coordinated in hexagonal tungsten bronze (HTB) arrays. Octahedral tilting, coupled with capping of the apical anions of the triangular groupings of octahedra in the HTB layers, gives rise to several important mineral groups, including pyrochlores, alunite-supergroup minerals, zirconolite and weberite polytypes and spinel-group minerals, as a function of the magnitude and type of the octahedral tilting.

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 line connecting rare earth elements (REE) in chondrite-normalised plots can be represented by a smooth polynomial function using λ shape coefficients as described by O'Neill (2016). In this study, computationally generated λ combinations are used to construct artificial chondrite-normalised REE patterns that encompass most REE patterns likely to occur in natural materials. The dominant REE per pattern is identified, which would lead to its inclusion in a hypothetical mineral suffix, had this mineral contained essential REE. Furthermore, negative Ce and Y anomalies, common in natural minerals, are considered in the modelled REE patterns to investigate the effect of their exclusion on the relative abundance of the remainder REE. The dominant REE in a mineral results from distinct pattern shapes requiring specific fractionation processes, thus providing information on its genesis. Minerals dominated by heavy lanthanides are rare or non-existent, even though the present analysis shows that REE patterns dominated by Gd, Dy, Er and Yb are geologically plausible. This discrepancy is caused by the inclusion of Y, which dominates heavy REE budgets, in mineral name suffixes. The focus on Y obscures heavy lanthanide mineral diversity and can lead to various fractionation processes to be overlooked. Samarium dominant minerals are known, even though deemed unlikely by the computational model, suggesting additional fractionation processes that are not well described by λ shape coefficients. Positive Eu anomalies only need to be moderate in minerals depleted in the light REE for Eu to be the dominant REE, thus identifying candidate rocks in which the first Eu dominant mineral might be found. Here, I present an online tool, called ALambdaR that allows interactive control of λ shape coefficients and visualisation of resulting REE patterns.

The metamorphic history of the Mesoarchean Amikoq Layered Complex within the Akia terrane of SW Greenland was characterised by electron microprobe mineral data and detailed petrography on 12 representative samples, integrated with zircon U–Pb geochronology and petrology. The complex intruded into a >3004 Ma supracrustal association now consisting of granoblastic metabasites with subordinate quartz-rich gneiss. Supracrustal host rocks contain a relict high-temperature assemblage of orthopyroxene–clinopyroxene (± pigeonite exsolution lamellae, exsolved at ~975–1010°C), which is interpreted to pre-date the Amikoq intrusion. Cumulate to granoblastic-textured rocks of the main Amikoq Layered Complex range modally from leuconorite to melanorite, orthopyroxenite to harzburgite/dunite and rare hornblende melagabbro. Observed mineralogy of main complex noritic lithologies is essentially relict igneous with orthopyroxene–biotite and hornblende–plagioclase thermometers yielding temperatures of ~800–1070°C. An anatectic zircon megacryst from a patchy quartzo–feldspathic leucosome hosted in an orthopyroxene-dominated Amikoq rock reflects local anatexis at peak metamorphic P–T conditions and yields an intrusion minimum age of 3004 ± 9 Ma. Field observations indicate local anatexis of orthopyroxene-dominated lithologies, possibly indicating a post-intrusion peak temperature of >900°C. The last preserved stages of retrogression are recorded in paragneiss plagioclase–garnet, biotite–garnet and host rock ilmenite–magnetite pairs (≤3 kbar and ~380–560°C).

The Amikoq Complex intruded a MORB-like crustal section and the former remained relatively undisturbed in terms of modal mineralogy. Preservation of igneous textures and mineralogy are related to an anhydrous, high-grade metamorphic history that essentially mimics igneous crystallisation conditions, whereas local high-strain zones acted as fluid pathways resulting in hydrous breakdown of igneous minerals. There is no evidence of equilibration of the intrusion at sub-amphibolite-facies conditions.

A new mineral species of the pyrochlore supergroup, hydroxyplumbopyrochlore (IMA2018-145), (Pb1.5,□0.5)Nb2O6(OH), has been discovered in the Jabal Sayid peralkaline granitic complex of the Arabian Shield, Saudi Arabia. It is associated with quartz, microcline, ‘biotite’, rutile, zircon, calcite, rhodochrosite, columbite-(Fe), goethite, thorite, bastnäsite-(Ce), xenotime-(Y), samarskite-(Y), euxenite-(Y), hydropyrochlore and fluornatropyrochlore. Hydroxyplumbopyrochlore usually shows euhedral octahedra, slightly rhombic dodecahedra and cubes or their combination (0.01–0.06 mm). The mineral is pale yellow to pale brown, transparent with white streak, and has adamantine to transparent lustre. It is brittle with conchoidal fracture. No cleavage or parting are observed. It is isotropic and non-fluorescent. The average microhardness is 463.4 kg mm–2. The calculated density is 6.474 g cm–3.

Hydroxyplumbopyrochlore belongs to the cubic crystal system and exhibits the space group Fd$\bar{3}$m with unit-cell parameters a = 10.5456(6) Å, V = 1172.8(2) Å3 and Z = 8. Electron microprobe analysis gave (6-point average composition, wt.%): CaO 0.32, SrO 0.16, FeO 0.17, Ce2O3 0.07, Pr2O3 0.02, PbO 51.69, Nb2O5 40.06, SiO2 0.05, TiO2 1.68, Ta2O5 4.74, H2Ocalc 0.95, total 99.90, yielding the empirical formula (Pb1.34Ca0.03Fe0.01Sr0.01□0.61)Σ2(Nb1.75Ti0.12Ta0.12Si0.01)Σ2O6(OH0.53O0.08□0.39)Σ1, where □ = vacancy. The Raman spectrum of hydroxyplumbopyrochlore contains the characteristic bands of O–H vibrations and no bands for H2O vibrations.

The occurrence and characterisation of a new member of the dundasite group are reported. Grguricite, ideally CaCr2(CO3)2(OH)4⋅4H2O, is the Cr analogue of alumohydrocalcite, CaAl2(CO3)2(OH)4⋅4H2O and occurs as lilac crusts of very fine-grained crystalline aggregates in the Pb–Ba–V mineralisation found at the Adeghoual Mine, Mibladen, Morocco (32°46′0″N, 4°37′59″W). The identification was based upon a close match with the powder X-ray diffraction data for alumohydrocalcite, the confirmation of anion components identified by Raman spectroscopy and the cation composition determined by electron-probe microanalysis. The empirical formula based upon 14 oxygen atoms per formula unit is Ca0.84Pb0.03Cr1.65Al0.39Mg0.02(CO3)2(OH)4⋅4H2O, with carbonate, hydroxyl and water contents set to those of the alumohydrocalcite stoichiometry. The fine-grained nature of the crystals (c. 0.5 μm × 0.1 μm × 5 μm) precluded a single-crystal X-ray study and both density and optical determinations. Grguricite is triclinic with space group P${\bar 1}$. Unit-cell parameters refined from the powder diffraction data are: a = 5.724(2), b = 6.5304(9), c = 14.646(4) Å, α = 81.682(1), β = 83.712(2), γ = 86.365(2)°, V = 537.8(2) Å3 and Z = 2. The five strongest peaks in the powder pattern are [dhkl, Å (I/Imax)(hkl)]: 6.222(100)(011), 3.227(87)(020), 6.454(63)(010), 2.883(58)(005, 023, 121) and 7.208(45)(002). The mineral is named after Australian geologist Ben Grguric.

New occurrences of derbylite, Fex2+Fe3+4–2xTi4+3+xSb3+O13(OH), and graeserite, Fex2+Fe3+4–2xTi4+3+xAs3+O13(OH), have been identified in the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy. Derbylite occurs as prismatic to acicular black crystals in carbonate veins. Iron and Ti are replaced by V (up to 0.29 atoms per formula unit, apfu) and minor Cr (up to 0.04 apfu). Mössbauer spectroscopy confirmed the occurrence of Fe2+ (up to 0.73 apfu), along with Fe3+. The Sb/(As+Sb) atomic ratio ranges between 0.73 and 0.82. Minor Ba and Pb (up to 0.04 apfu) substitute. Derbylite is monoclinic, space group P21/m, with unit-cell parameters a = 7.1690(3), b = 14.3515(7), c = 4.9867(2) Å, β = 104.820(3)° and V = 495.99(4) Å3. The crystal structure was refined to R1 = 0.0352 for 1955 reflections with Fo > 4σ(Fo). Graeserite occurs as prismatic to tabular black crystals, usually twinned, in carbonate veins or as porphyroblasts in schist. Graeserite in the first kind of assemblage is V rich (up to 0.66 apfu), and V poor in the second kind (0.03 apfu). Along with minor Cr (up to 0.06 apfu), this element replaces Fe and Ti. The occurrence of Fe2+ (up to 0.68 apfu) is confirmed by Mössbauer spectroscopy. Arsenic is dominant over Sb and detectable amounts of Ba and Pb have been measured (up to 0.27 apfu). Graeserite is monoclinic, space group C2/m, with unit-cell parameters for two samples: a = 5.0225(7), b = 14.3114(18), c = 7.1743(9) Å, β = 104.878(3)°, V = 498.39(11) Å3; and a = 5.0275(4), b = 14.2668(11), c = 7.1663(5) Å, β = 105.123(4)° and V = 496.21(7) Å3. The crystal structures were refined to R1 = 0.0399 and 0.0237 for 428 and 1081 reflections with Fo > 4σ(Fo), respectively. Derbylite and graeserite are homeotypic. They share the same tunnel structure, characterised by an octahedral framework and cuboctahedral cavities, hosting (As/Sb)O3 groups and (Ba/Pb) atoms.

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.