Abstract. Lopatkaite, ideally Pb10As2Sb6S22 (Z=4), is a new arsenic-bearing sulfosalt found in the Madoc deposit, Taylor Pit, Ontario, Canada. Associated minerals in the holotype specimen are boulangerite, veenite, and sterryite, all embedded in a calcite matrix. Lopatkaite is greyish black and opaque, with metallic lustre and dark-grey streak. It is brittle without any discernible cleavage and parting and has a Mohs hardness of 3–3.5. In reflected light lopatkaite is greyish white, with distinct bireflectance and pleochroism from white to grey, especially in oil. Under crossed polarisers, anisotropism is distinct, with rotation tints in shades of grey. Reflectance measurements in air yield the following Rmin/Rmax values based on the standard wavelengths (Commission on Ore Mineralogy, COM): 37.0 % / 39.3 % (470 nm), 34.1 % / 36.9 % (546 nm), 33.1 % / 36.2 % (589 nm), and 31.3 % / 34.1 % at (650 nm). The average result of four electron probe microanalyses for the structurally investigated grain is as follows (in wt %): Pb 57.81(4), As 3.53(8), Sb 20.03(6), S 19.08(6), and total 100.46(22), corresponding to Pb10.28(3)As1.74(4)Sb6.06(3)S21.92(3) (based on 18Me + 22S = 40 atoms per asymmetric unit). The density calculated using the empirical formula is 6.168 Mg m−3. Single-crystal X-ray diffraction data show lopatkaite to be monoclinic, space group P21/c (no. 14), with a=8.0806(6), b=23.3597(18), c=21.4880(16) Å, β=100.7090(10)°, V=3985.4(5) Å3, and Z=4. The seven strongest lines in the (calculated) powder diffraction pattern are as follows (d in Å (intensity) (hkl)): 3.728(39) 211, 3.712(100) 035, 3.653(35) 062, 2.804(41) −261, 2.780(43) 260, 2.779(38) −262, and 2.020(47) −402. The ideal formula is in accordance with the results of the crystal structure analysis, Pb10.336As1.567Sb6.088S22 , and may be derived from the ideal boulangerite formula, Pb10Sb8S22 (Z=4), by means of substitution of two Sb atoms with two As atoms. Lopatkaite is an isotype of boulangerite, differing by dominant As occupancy at two crystallographically independent mixed (Sb, As) sites. This dominant-site substitution defines lopatkaite as the arsenic-dominant isotype of boulangerite and justifies its recognition as a distinct mineral species. Lopatkaite is also a new member of the rod-based family of sulfosalts.

Abstract. Calcite has been shown to incorporate trivalent actinides into its crystal lattice, highlighting its potential to contribute to radionuclide retention processes in the environment. Earlier studies on calcite's elemental uptake were conducted under controlled laboratory conditions, which do not fully capture the complexities of natural environments. To gain a deeper understanding of calcite's uptake capacity under natural conditions, a sample from the Wenzel ore mine, Germany, was analyzed, originating from a calcite vein that formed under conditions relevant to deep geological-waste repositories. The chemical analogy between rare earth elements (REEs) and trivalent actinides helps to evaluate the retention potential and incorporation mechanisms of trivalent radionuclides. A micro-X-ray fluorescence (µXRF) element map revealed that the investigated calcite consists of an euhedral crystal core exhibiting sector zoning, characterized by trace element heterogeneity across coevally grown crystal faces. High-resolution laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) further refined and quantified the elemental distribution, revealing preferential incorporation of specific elements, particularly REEs, within one sector of the calcite. Remarkably, this sector showed REE concentrations over 200 times higher than those in the depleted sector, indicating a significant potential for the retention of trivalent actinides. Furthermore, the data indicate that charge equilibration of incorporated trivalent cations occurs via two processes: coupled substitution with monovalent cations and vacancies in the crystal structure. Overall, these results demonstrate that sector-zoned calcite formed under repository-relevant conditions can maintain high retention potential for trivalent actinides, even in environments depleted in monovalent cations.

Abstract. In surface soils and sediments, iron monosulfide (FeS) species, including nanocrystalline mackinawite, tend to quickly form in the presence of iron and sulfide in anoxic conditions. As such, FeS species are the main precursors for the formation of other iron sulfides such as Fe3S4 greigite and FeS2 pyrite, which are ubiquitous in surface sedimentary environments. It is known that, under prolonged aging under reducing conditions in a sulfidic aqueous medium, FeS species can evolve into crystalline mackinawite. However, the possible influence of pH on the evolution of mackinawite under such anoxic low-temperature conditions relevant to sedimentary (sub)surface environments has not been investigated yet. In this study, we used Rietveld refinement and pair distribution function analysis (PDF) of synchrotron-based X-ray powder diffraction (XRD) patterns to derive the mean coherent domain (MCD) size of mackinawite after aging under various pH conditions and X-ray absorption near-edge structure (XANES) spectroscopy at the S and Fe K-edges to study the structural and electronic properties. Moreover, in order to strengthen our interpretations, we confirmed the shape and relative energy of pre-edge features in the S K-edge XANES spectra of mackinawite (FeS) and pyrite (FeS2) model compounds via first-principle calculations. Our results show that, after FeS has precipitated from aqueous Fe(II) and H2S/HS- in a saline medium at pH 7.1, aqueous aging at the same pH over 47 d results in the formation of nanocrystalline mackinawite (MCDab=11.5±0.1 nm; MCDc=7.1±0.1 nm). When Na2S is added into the solution to reach pH 9.7 after FeS has precipitated at pH 7.1, no other Fe sulfide is observed during the aging phase, and mackinawite particles are of smaller size (MCDab=7.9±0.1 nm; MCDc=4.6±0.1 nm). In this sample, an additional weak and broad peak appears at d=10.5 Å that could be interpreted as being due to either lattice expansion at the particle boundaries or a double-cell super-structure. When H+ is added as HCl to reach pH 5.1 before the aging phase, the size of mackinawite particles increases (MCDab=13.0±0.2 nm; MCDc=8.1±0.2 nm), and a fraction transforms into greigite (Fe3S4). This reaction is accompanied by a pH increase to 6.4, likely because of H+ consumption, which suggests that Fe(II) in FeS would serve as an electron donor and that H+ would serve as an electron acceptor. The calculated electronic structure of mackinawite shows partly filled Fe-3d states, which supports the fact that acidic aging conditions are favorable for Fe(II) to act as an electron donor. We propose and further discuss the fact that the formation of greigite from nanocrystalline mackinawite could result in H2 production as, for instance, observed for anoxic corrosion of zero-valent Fe at higher temperatures. Greigite has been designated in the literature either as an intermediate towards pyrite formation or as a mineralogical endmember in another reaction route. Our observations raise the question of the existence of such a reaction producing Fe3S4 and H2 in reducing sedimentary (micro)environments across geological times. In addition, the metallic character of mackinawite suggests that Fe(II) oxidation to Fe(III) by H+ in this mineral species could proceed without the need for another oxidizing agent. Although the possible formation of pyrite from greigite would require further studies on extended aging time and/or under more acid-sulfidic conditions, our findings could have implications for the understanding of the initial steps of the H2S pathway to pyrite.

Abstract. Gem-quality corundum varieties of ruby and sapphire are one of the most valuable and desired gemstones. Due to their rarity, new methods of synthesis and treatment were developed over the last decades, complicating the reliable identification between natural, treated, and synthetic specimens. Among the geochemical methods used for identification, trace element analysis using laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) is widely applied. However, solely relying on LA-ICP-MS trace element analysis for differentiation between natural and synthetic corundum origins, especially when grown by the hydrothermal method, can potentially lead to misidentifications. To further enable geochemical tracing of corundum, this study explores secondary ion mass spectrometry (SIMS) oxygen isotope analysis. High-spatial-resolution SIMS δ18O analysis of hydrothermally synthesized corundum yielded values between -7.84±0.13 ‰ and -14.54±0.13 ‰ (relative to Vienna standard mean ocean water, VSMOW; 1 standard error) that are atypical for natural corundum. For flame fusion corundum, SIMS δ18O analyses are in the range of -6.73±0.13 ‰ to -17.46±0.13 ‰ for sapphires of blue, yellow, and orange colour and +28.51±0.11 ‰ to +30.47±0.10 ‰ for ruby, which, in both cases, are again atypical for natural corundum. SIMS δ18O analysis of corundum thus has strong potential to distinguish synthetic and natural corundum.

2.

Abstract. Loparite is a natural perovskite (ABO3) with a complex composition, essentially a solid solution between loparite(-Ce) (Na0.5Ce0.5TiO3), lueshite (NaNbO3), and perovskite (CaTiO3), with small amounts of many other elements. The majority of reported loparite crystals are spinel-type interpenetration twins with compositional zoning. Associated with the high variability of compositions, different crystal structures of loparite were described, and the origin of twinning has not been addressed so far. In this work, we studied a loparite twin composed of two symmetrically intergrown cuboctahedra. Microprobe analyses revealed that the cubic and octahedral growth sectors have slightly different compositions. According to atomic-scale analyses, the cubic sectors with a lower Na:LREE ratio have a disordered orthorhombic structure, while the octahedral sectors with a higher Na:LREE ratio show partial ordering of Na and light rare-earth elements along A-type lattice planes in the [001] direction. The degree of ordering was evaluated by quantitative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Atomic-scale analyses of (111) and {112} twin boundary (TB) contacts have shown that all TBs contain a 1–2 nm thin layer of an Si-rich amorphous phase. Our observations suggest that the loparite twin was established in the nucleation stage of crystallisation, followed by independent crystallisation of both twin domains from the melt. The initial crystal form was cubic; octahedral sectors evolved when the crystal size was still in the nanometre range, as a result of slow crystal growth. Differences in structural ordering between adjacent growth sectors developed during slow cooling due to the compositional variations. Our results imply that compositionally zoned crystals might show different structural ordering within single domains, which should be considered when interpreting bulk diffraction data of compositionally zoned perovskites.

Abstract. Götzenite and wöhlerite were found as part of a fissure assemblage in the Fohberg phonolite (Kaiserstuhl, SW Germany), in close association with natrolite and clinopyroxene (aegirine–augite). Crystal grains were separated and investigated by single-crystal X-ray diffraction (SXRD), electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), showing the presence of two intimately intergrown phases, götzenite and wöhlerite. SXRD analyses showed that both minerals are twinned. Götzenite (Na1.50Ca5.18Sr0.13Fe0.032+Mn0.01Zr0.06La0.08Ce0.11Nd0.02Ti0.81Nb0.19(Si2O7)2O1.2F2.8) shows rotation twinning on [001] according to -a-1/2c, −b, c, with contributions of 40 % and 60 % from the two twin domains, respectively. Applying the twin law to the diffraction analysis, the crystal structure was refined to R1 (Fo >4σ (Fo)) = 3.0 %, with a=9.6191(3) Å, b=5.7342(2) Å, c=7.3386(2) Å, α=89.986(1)°, β=101.040(1)°, γ=100.485(1)°, and V=390.40(3) Å3 in space group P1‾. Wöhlerite (Na1.63Ca4.37Sr0.04Zr0.63Fe0.232+Mn0.09Ce0.01Ta0.01Nb0.79Ti0.20(Si2O7)2O2.6F1.4) shows reflection twinning on (100) according to -a-c, b, c, with contributions of 31 % and 69% from the two twin domains, respectively, and with lattice parameters of a=10.842(1) Å, b=10.249(1) Å, c=7.2673(8) Å, β=109.343(4)°, and V=761.9(2) Å3 in the monoclinic space group P21, refined to R1 = 1.3 %. Refractive indices of götzenite were measured using the immersion method yielding nx=1.662(2), ny=1.663(2), nz=1.670(2), and 2V=61(2)°. Optical measurements on the twinned crystal were possible because of the coincidence of the two indicatrices related to each other by rotation about nx being parallel to [001], simulating a unique extinction behavior. Wöhlerite could not be optically examined because of the polysynthetic twinning not showing this effect.

Abstract. Two new minerals, zhenruite, ideally (MoO3)2⋅H2O, and tianhuixinite, ideally (MoO3)3⋅H2O, were discovered, respectively, from the Freedom #2 mine in the central part of the Marysvale volcanic field, Utah, USA, and an unnamed short adit on the Summit group of claims near Cookes Peak, Luna County, New Mexico, USA. Zhenruite occurs as acicular or prismatic crystals (up to 0.06×0.01×0.01 mm). Associated minerals include alunogen, anhydrite, coquimbite, fluorite, liangjunite, quartz, and raydemarkite. Zhenruite is colorless in transmitted light and transparent with a white streak and vitreous luster. It is brittle with a Mohs hardness of 1 1/2–2; cleavage is perfect on {001}. The calculated density is 4.081 g cm−3. Tianhuixinite occurs as nanometric crystal aggregates, 10–70 µm in size, intergrown with virgilluethite. Associated minerals include barite, fluorite, ilsemannite, jordisite, powellite, pyrite, quartz, raydemarkite, sidwillite, and virgilluethite. Tianhuixinite is dark blue-green and translucent in transmitted light. It has a white streak and vitreous luster. Tianhuixinite is brittle with a Mohs hardness of ∼2; no cleavage was observed. The calculated density is 4.131 g cm−3. At room temperature, neither zhenruite nor tianhuixinite is soluble in water or hydrochloric acid. Electron microprobe analyses yielded an empirical formula (Mo1.00O3)2⋅H2O for zhenruite and (Mo1.00O3)3⋅H2O for tianhuixinite, calculated on the basis of 7 and 10 O apfu, respectively. Zhenruite and tianhuixinite are the natural counterparts of synthetic (MoO3)2⋅H2O and hexagonal (MoO3)3⋅H2O, respectively. Zhenruite is monoclinic with space group P21/m and unit-cell parameters a=9.6790(6), b=3.70653(19), c=7.1029(4) Å, β=102.391(5)°, V=248.89(2) Å3, and Z=2. Its crystal structure is characterized by two kinds of topologically identical octahedral double chains extending along [010], one consisting of edge-sharing Mo1O6 octahedra only and the other Mo2O5(H2O) octahedra only. These two kinds of chains are linked together alternately through sharing corners to form layers parallel to (001), which are interconnected by hydrogen bands along [001]. Tianhuixinite is hexagonal with space group P63/m and unit-cell parameters a=10.5963(12), c=3.7216(4) Å, V=361.88(9) Å3, and Z=2. Its crystal structure is composed of double chains of edge-sharing MoO6 octahedra extending along [001], which are corner-connected with one another to form hexagonal channels with H2O residing at the center. The double chains of edge-sharing MoO6 octahedra in zhenruite and tianhuixinite are topologically identical to those in molybdite and raydemarkite, and zhenruite can be regarded as a combination of molybdite and raydemarkite both structurally and chemically. The discovery of tianhuixinite implies the likelihood of finding the ammonia analogue, (MoO3)3⋅NH3, in nature.

Abstract. Keutschite, Cu2AgAsS4, is a new mineral from the Ag–Pb–Zn deposit at Uchucchacua, Oyon District, Catajambo, Lima Department, Peru. The mineral occurs as metallic, highly lustrous, blocky, free-standing crystals measuring up to 3 mm. These crystals exhibit a grey colour with a slight green-brassy tint and a grey-black streak, and they are present on both manganoquadratite and proustite. It was observed that keutschite was brittle, and no fractures or cleavages were identified. In plane-polarised light, keutschite exhibits a grey hue devoid of any discernible internal reflections. It demonstrates a minimal manifestation of pleochroism and exhibits a negligible degree of bireflectance. Between crossed polars, the mineral is weakly anisotropic with rotation tints in shades of greenish grey to grey. Reflectance measurements in air yield the following Rmin/Rmax values for wavelengths recommended by the Commission on Ore Mineralogy of the International Mineralogical Association: 25.2/26.1 (470 nm), 29.6/29.4 (546 nm), 29.4/29.2 (589 nm), and 28.5/28.6 (650 nm). Keutschite crystallises in a tetragonal geometry and is classified as space group I4‾2m. The unit cell parameters are as follows: a=5.5834(15), c=10.021(3) Å, V=312.40(14) Å3, a:b:c=1:1:0.897, and Z=2. The crystal structure was refined to R1=0.0199 for 286 reflections with I>3σ(I). The structure of keutschite is derived from that of sphalerite by ordered substitution of Zn atoms, analogous to the substitution pattern for deriving stannite from sphalerite. The crystal structure of the mineral can be derived from that of luzonite through the complete substitution of one of the two copper sites with silver. The five strongest intensities in the X-ray powder diagram are [d in Å (intensity) hkl]: 3.101 (100) 110; 2.792 (11) 200; 1.974 (20) 220; 1.665 (34) 204; and 2.846 (27) 312. The chemical formula, as determined by electron microprobe analysis, is Cu2.05Ag0.96(As0.95Sb0.04)Σ0.99S4.00 (based on eight atoms). The ideal formula, derived from the crystal structure, is Cu2AgAsS4. The name honours Frank Keutsch (born 1971) for his contribution to the mineralogy of the Uchucchacua deposit.

Abstract. The crystal structure of launayite, ideally Cu2Pb20(Sb,As)26S60 (Z=4) from Taylor Pit, Madoc, Ontario, Canada, has been solved for the first time using the single-crystal X-ray diffraction (SCXRD) method. The mineral is composed of distinct superstructures that can be derived from the same parent structure. The structure of the main component is monoclinic and has been solved in the space group P2/a, with cell parameters a=42.6466(14), b=8.0381(2), c=34.3957(10) Å, β=64.684(2) °, and V=10 658.4(6) Å3 from an untwined crystal. The asymmetric unit of launayite contains 48 cation sites and 60 sulfur sites. Final refinement resulted in an R1 value of 0.0955 for 11 741 unique reflections. The structural formula obtained from SCXRD study is Cu2Pb20.330Sb23.024As2.689S60, Z=4, in agreement with the formula Cu2.078Ag0.059Tl0.057Pb20.404Sb22.830As2.772S59.80 from microprobe analysis. The structure of launayite can be viewed both as a boxwork structure and as a rod-based structure. The modular description of the launayite structure reveals a very close relationship with the structure of rouxelite: the parent structures of both can be regarded as merotypes. A full comparison of the crystal chemistry and modular description of both structures is presented.