Volume 107, Issue 11

We compare the stable isotope compositions of Zn, S, and Cl for Apollo mare basalts to better constrain the sources and timescales of lunar volatile loss. Mare basalts have broadly elevated yet limited ranges in δ 66 Zn, δ 34 S, and δ 37 Cl SBC+WSC values of 1.27 ± 0.71, 0.55 ± 0.18, and 4.1 ± 4.0‰, respectively, compared to the silicate Earth at 0.15, –1.28, and 0‰, respectively. We find that the Zn, S, and Cl isotope compositions are similar between the low- and high-Ti mare basalts, providing evidence of a geochemical signature in the mare basalt source region that is inherited from lunar formation and magma ocean crystallization. The uniformity of these compositions implies mixing following mantle overturn, as well as minimal changes associated with subsequent mare magmatism. Degassing of mare magmas and lavas did not contribute to the large variations in Zn, S, and Cl isotope compositions found in some lunar materials (i.e., 15‰ in δ 66 Zn, 60‰ in δ 34 S, and 30‰ in δ 37 Cl). This reflects magma sources that experienced minimal volatile loss due to high confining pressures that generally exceeded their equilibrium saturation pressures. Alternatively, these data indicate effective isotopic fractionation factors were near unity. Our observations of S isotope compositions in mare basalts contrast to those for picritic glasses (Saal and Hauri 2021), which vary widely in S isotope compositions from –14.0 to 1.3‰, explained by extensive degassing of picritic magmas under high- P / P Sat values (>0.9) during pyroclastic eruptions. The difference in the isotope compositions of picritic glass beads and mare basalts may result from differences in effusive (mare) and explosive (picritic) eruption styles, wherein the high-gas contents necessary for magma fragmentation would result in large effective isotopic fractionation factors during degassing of picritic magmas. Additionally, in highly vesiculated basalts, the δ 34 S and δ 37 Cl values of apatite grains are higher and more variable than the corresponding bulk-rock values. The large isotopic range in the vesiculated samples is explained by late-stage low-pressure “vacuum” degassing ( P / P Sat ~ 0) of mare lavas wherein vesicle formation and apatite crystallization took place post-eruption. Bulk-rock mare basalts were seemingly unaffected by vacuum degassing. Degassing of mare lavas only became important in the final stages of crystallization recorded in apatite—potentially facilitated by cracks/ fractures in the crystallizing flow. We conclude that samples with wide-ranging volatile element isotope compositions are likely explained by localized processes, which do not represent the bulk Moon.

The mineral apatite, Ca 10 (PO 4 ) 6 (F,OH,Cl) 2 , incorporates sulfur (S) during crystallization from S-bearing hydrothermal fluids and silicate melts. Our previous studies of natural and experimental apatite demonstrate that the oxidation state of S in apatite varies systematically as a function of oxygen fugacity ( f O2 ). The S oxidation states –1 and –2 were quantitatively identified in apatite crystallized from reduced, S-bearing hydrothermal fluids and silicate melts by using sulfur K -edge X‑ray absorption near-edge structure spectroscopy (S-XANES) where S 6+ /ΣS in apatite increases from ~0 at FMQ-1 to ~1 at FMQ+2, where FMQ refers to the fayalite-magnetite-quartz f O2 buffer. In this study, we employ quantum-mechanical calculations to investigate the atomistic structure and energetics of S(-I) and S(-II) incorporated into apatite and elucidate incorporation mechanisms. One S(-I) species (disulfide, S22− ) $\left. \text{S}_{2}^{2-} \right)$and two S(-II) species (bisulfide, HS − , and sulfide, S 2− ) are investigated as possible forms of reduced S species in apatite. In configuration models for the simulation, these reduced S species are positioned along the c- axis channel, originally occupied by the column anions F, Cl, and OH in the end-member apatites. In the lowest-energy configurations of S-incorporated apatite, disulfide prefers to be positioned halfway between the mirror planes at z = 1/4 and 3/4. In contrast, the energy-optimized bisulfide is located slightly away from the mirror planes by ~0.04 fractional units in the c direction. The energetic stability of these reduced S species as a function of position along the c- axis can be explained by the geometric and electrostatic constraints of the Ca and O planes that constitute the c- axis channel. The thermodynamics of incorporation of disulfide and bisulfide into apatite is evaluated by using solid-state reaction equations where the apatite host and a solid S-bearing source phase (pyrite and Na 2 S 2(s) for disulfide; troilite and Na 2 S (s) for sulfide) are the reactants, and the S-incorporated apatite and an anion sink phase are the products. The Gibbs free energy (Δ G ) is lower for incorporation with Na-bearing phases than with Fe-bearing phases, which is attributed to the higher energetic stability of the iron sulfide minerals as a source phase for S than the sodium sulfide phases. The thermodynamics of incorporation of reduced S is also evaluated by using reaction equations involving dissolved disulfide and sulfide species [ HnS2(aq)(2−n) and HnS(aq)(2−n);n=0,1 , $\left[ {{\text{H}}_{\text{n}}}\text{S}_{2(\text{aq})}^{(2-\text{n})}\text{ }\!\!~\!\!\text{ and }\!\!~\!\!\text{ }{{\text{H}}_{\text{n}}}\text{S}_{(\text{aq})}^{(2-\text{n})};\text{n}=0,1 \right.,$and 2] as a source phase. The Δ G of S-incorporation increases for fluorapatite and chlorapatite, and decreases for hydroxyl-apatite, as these species are protonated (i.e., as n changes from 0 to 2). These thermodynamic results demonstrate that the presence of reduced S in apatite is primarily controlled by the chemistry of magmatic and hydrothermal systems where apatite forms (e.g., an abundance of Fe; solution pH). Ultimately, our methodology developed for evaluating the thermodynamics of S incorporation in apatite as a function of temperature, pH, and composition is highly applicable to predicting the trace and volatile element incorporation in minerals in a variety of geological systems. In addition to solid-solid and solid-liquid equilibria treated here at different temperatures and pH, the methodology can be easily extended to different pressure conditions by just performing the quantum-mechanical calculations at elevated pressures.

The dynamic properties and melting behavior of the Earth’s mantle are strongly influenced by the presence of volatile species, including water, carbon dioxide, and halogens. The role that halogens play in the mantle has not yet been fully quantified: their presence in only small quantities has dramatic effects on the stability of mantle minerals, melting temperatures, and in generating halogen-rich melts such as lamproites. Lamproites are volumetrically small volcanic deposits but are found on every continent on the planet: they are thought to be melts generated from volatile-rich mantle sources rich in fluorine and water. To clarify the mantle sources of lamproites, we present experimentally determined mineral/melt partition coefficients for fluorine and barium between phlogopite and lamproite melts. Both fluorine and barium are compatible in phlogopite [ DF(Phl/Melt) 0.96±0.02−3.44±0.33,DBa(phl/Melt) 0.52±0.05−3.68±0.43 ] $\left[ D_{\text{F}}^{\text{(Phl/Melt) }\!\!~\!\!\text{ }}0.96\pm 0.02-3.44\pm 0.33,D_{\text{Ba}}^{\text{(phl/Melt) }\!\!~\!\!\text{ }}0.52\pm 0.05-3.68\pm 0.43 \right]$at a range of pressures (5–30 kbar), temperatures (1000–1200 °C), and fluid compositions (C-O-H mixtures). Using our partition coefficients, we model the melt compositions produced by potential lamproite sources, including phlogopite garnet lherzolite, phlogopite harzburgite, and hydrous pyroxenite. The results demonstrate that hydrous pyroxenites and phlogopite garnet lherzolite can produce melts with F and Ba contents similar to lamproites, but only hydrous pyroxenites fully reproduce other geochemical characteristics of lamproites including high K 2 O, low CaO contents, and high F/H 2 O ratios.

Both continental crust and depleted mantle are characterized by subchondritic Nb/Ta, leading to a mass imbalance when compared to the bulk Earth. Even though several potential high-Nb/Ta reservoirs in Earth’s core and undepleted mantle have been proposed, little attention has been given to those in the crust. Here we present bulk-rock and rutile geochemical data for samples from a lower crustal pelitic granulite, North China Craton, which exhibit systematic variation in their Nb and Ta contents. High-temperature (HT) and ultrahigh-temperature (UHT) granulite residues exhibit Nb/Ta ratios that are close to chondritic and subchondritic, respectively, whereas leucosomes from UHT granulite mostly have suprachondritic Nb/Ta. These variations are best explained via competition for Nb and Ta between biotite and rutile during metamorphism, although initial bulk-rock Nb/Ta values also have an effect. As biotite preferentially incorporates Nb over Ta, the early stages of biotite dehydration melting produce a high-Nb/Ta residue and a low-Nb/Ta melt; however, geochemical modeling suggests that once biotite is depleted, the Nb/Ta ratio of the system is instead controlled by rutile growth, which promotes the formation of a lower Nb/Ta residue and a higher Nb/ Ta melt, even though the volume of melt produced at this stage may be small. We propose that in situ and in-source leucosomes and leucocractic veins in UHT terranes may retain a high-Nb/Ta geochemical signature. However, residual crustal-derived A2-type granites that experience significant fractionation of Nb- or Ta-bearing minerals during crystallization or contamination from other low-Nb/Ta sources cannot retain this high-Nb/Ta ratio, even though these ratios are generally higher than that of S-type granites. Anhydrous partial melting of metapelite can generate Nb-rich melts, such that high-temperature leucosomes, in addition to related A2-type granites, may represent significant Nb deposits.

In this work we explore the low-energy complexions of the symmetrical tilt grain boundary (GB) 60.8°//[100](011) in forsterite through molecular dynamics and first principles calculations. Using a conservative sampling, we find six stoichiometric complexions with energies ranging from 0.66 to 1.25 J/m 2 . We investigate the segregation of MgO vacancy pairs, and find that in most cases it is more favorable for the vacancies to lie within the GBs than in the surrounding crystals, leading to new atomic structures. From these results we infer that at finite temperature when vacancies are present in the system, GBs are likely to absorb them and to be non-stoichiometric. We find many GB complexions containing a free oxygen ion, which may have profound implications for geological processes.

Raman shifts of the symmetric stretch of silicate Q 2 species vary over a range of ~90 cm −1 in crystals and glasses containing alkali and alkaline earth oxides. The shifts display a striking, sympathetic relationship with the electronegativity of the alkali and alkaline earth metals (M), with the highest frequency observed for Mg-silicate glasses and crystals and the lowest frequency for Cs-bearing glasses. Frequencies are determined primarily by the electron density on constituent Si and O atoms of the Q 2 tetrahedra, as measured by Si 2p and O 1s X‑ray photoelectron spectra (XPS). The electron density is, in turn, determined by the extent to which electronic charge is transferred from the modifier metal “M” to the NBO of the Q 2 tetrahedron. The charge transferred to NBO is redistributed (delocalized) over all atoms of the tetrahedron by the four equivalent Si sp 3 orbitals. Although negative charge accumulates on all atoms of the tetrahedron, it accumulates preferentially on Si. Coulombic interactions among Si and all O atoms are thus weakened, resulting in decreased force constants and lowered symmetric stretch frequencies of Q 2 species. Density functional theoretical (DFT) calculations on six staggered and eclipsed M 6 Si 2 O 7 (M = Li, Na, K) molecules corroborate the findings. Charge is transferred from the metal atoms to NBO and delocalized over tetrahedra in accordance with Li, Na, and K electronegativities. Calculated Si-O force constants and Raman shifts decrease with decreasing electronegativity of the cation but surprisingly, calculated Si-NBO bond lengths are largely unaffected, with all being similar at 1.665 ± 0.003 Å.

Carbonates in the system Na 2 CO 3 -CaCO 3 are nowadays suggested as having a wide stability field at conditions of the mantle transition zone. Our structural analysis of nyerereite, which has limited stability fields at ambient conditions, and its similarities with already known carbonates that are stable at high-pressure conditions, allowed us to propose that nyerereite likely undergoes phase transitions at both high-pressure and high-temperature conditions. This supports the hypothesis that nyerereite takes part in carbon transportation from the mantle/deep crust toward the surface, with important implications for the deep carbon cycle associated with carbonatites. K-free nyerereite [Na 2 Ca(CO 3 ) 2 ] was synthesized both at hydrothermal conditions and from the melt. The structure of nyerereite was refined as a three-component twinned structure in the centrosymmetric Pbca space group with ratios of the three twinning components 0.221(3):0.287(3):0.492(3). Twinning at micro- and nano-level can introduce some minor structural deformations that influence the likely occurrence of the inversion center as one of the symmetry elements in the nyerereite structure. Based on the automated topological algorithms, we show that nyerereite has a unique crystal structure, not having analogs among the known structures, except for the structure with a similar composition K 2 Ca(CO 3 ) 2 fairchildite. A comparison between the centrosymmetric Pbca nyerereite structure and that of aragonite (CaCO 3 , Pmcn space group) reveals two main scenarios for the high-pressure form of Na 2 Ca(CO 3 ) 2 : (1) polysomatic relations as the interlayering of the high-pressure polymorph Na 2 CO 3 and CaCO 3 -aragonite, and (2) high-pressure structure with ninefold-coordinated Na and Ca sites resembling that of aragonite. Our discussion heightens the interest in the high-pressure behavior of the nyerereite structure and strengthens the hypothesis about the possibility for nyerereite to be stable at high-pressure/high-temperature conditions.

Brucite [Mg(OH) 2 ] has been extensively studied as a simple yet important analog for studying physical and chemical properties of hydrous minerals, and fluorine substitution (OH – = F – ) is common in hydrous minerals since the radius of F – is similar to that of O 2− . We synthesized two F-bearing brucite samples, Mg(OH) 1.78 F 0.22 and Mg(OH) 1.16 F 0.84 , at 9.5 GPa and 1373 K. Single-crystal X‑ray diffraction measurements indicate that both phases still crystallize in the space group of P 3 m 1, and fluorine substitution significantly reduces the unit-cell volume, axial lengths, and averaged Mg-O(F) bond lengths. The averaged O···H distances get slightly shortened, and the H-O-H angles become smaller due to the fluorine efect. Additional IR-active OH-stretching bands are observed at 3660, 3644, and 3513 cm − 1 for the F-bearing samples, besides the original one at 3695 cm − 1 . In situ high-temperature and high-pressure Raman and Fourier transform infrared (FTIR) spectra were collected on the F-bearing brucite samples, and comparisons were made with the natural one with 0.7 mol% F – . The temperature dependence [(∂ v i /∂ T ) P ] of the OH-stretching modes is inversely correlated to the vibrational frequencies from 3500 to 3700 cm − 1 , whereas (∂ v i /∂ P ) T is in positive correlation with v i . In addition, the dehydration temperatures of the F-bearing brucites are 100–150 K higher than that for the F-free sample at ambient pressure. By creating new proton positions in lower energies, fluorine substitution stabilizes hydrous minerals (like brucite) to higher temperatures and significantly affects their thermodynamic properties, which has significant implications in mineral physical and geochemical studies.

Topaz, Al 2 SiO 4 (OH) x F (2– x ) , may play a significant role in transporting water and fluorine into the Earth’s interior at subduction zones. Seismological detection of topaz gives us insights into the transport mechanisms of water and fluorine but requires a thorough understanding of its elastic properties. The influence of OH content on elastic constants of topaz has not been fully understood, though experimental and theoretical studies have been done on topaz with various OH contents. We thus determined elastic constants of topaz for five natural single-crystal specimens with different OH contents ( x = 0.28~0.72) via the sphere resonance method at an ambient condition. Our determined C 11 , C 22 , C 44 , C 66 , C 12 , C 23 , and C 31 increase with OH content while C 33 and C 55 decrease. For the change in OH molar content from 0.0 to 1.0, relatively large changes (>3.0%) are seen in C 33 [8.0(6)%], C 55 [4.9(6)%], and C 22 [3.1(7)%]. The OH content dependence of elastic constants is qualitatively similar to that of theoretically determined values except for C 11 . The theoretical value of C 11 decreases as the OH molar content increases from 0.0 to 1.0, whereas the experimental value of C 11 slightly increases. Our elastic constants are significantly higher (>3%) than theoretically determined values, especially in diagonal components ( C ii ). The theoretical lower values must be related to the used lattice parameters, which are systematically larger than the measured lattice parameters. The theoretical approach should be modified to reproduce measured lattice parameters and lead to the agreement of theoretical and experimental elastic constants. Our results provide a clue to a better understanding of the elasticity of topaz and a basis for the seismological detection of subducted oceanic sediments.

Oxygen fugacity is an important but difficult parameter to constrain for primitive arc magmas. In this study, the partitioning behavior of Fe 3+ /Fe 2+ between amphibole and glass synthesized in piston-cylinder and cold-seal apparatus experiments is developed as an oxybarometer, applicable to magmas ranging from basaltic to dacitic composition. The partitioning of Fe 2+ is strongly dependent on melt polymerization; the relative compatibility of Fe 2+ in amphibole decreases with increasing polymerization. The Fe 2+ /Mg distribution coefficient between amphibole and melt is a relatively constant value across all compositions and is, on average, 0.27. The amphibole oxybarometer is applied to amphibole in mafic enclaves, cumulates, and basaltic tephra erupted from Shiveluch volcano in Kamchatka with measured Fe 3+ /Fe Total . An average Fe 3+ /Fe 2+ amphibole-glass distribution coeficient for basalt is used to convert the Fe 3+ /Fe Total of amphibole in samples from Shiveluch to magmatic oxygen fugacity relative to NNO. The f O2 of primitive melts at the volcano is approximately NNO+2 and is faithfully recorded in amphibole from an amphibole-rich cumulate and the basaltic tephra. Apparently, higher f O2 recorded by amphibole in mafic enclaves likely results from partial dehydrogenation of amphibole during residence in a shallow andesite storage region. We identify three pulses of mafic magma recharge within two weeks of, a month before, and two to three months before the eruption and find that, at each of these times, the host andesite was recharged by at least two magmas at varying stages of differentiation. Application of the amphibole oxybarometer not only gives insight into magmatic f O2 but also potentially details of shallow magmatic processes.

Magadiite from Lake Magadi was structurally analyzed based on X‑ray powder diffraction data. The idealized chemical composition of magadiite is Na 16 [Si 112 O 224 (OH) 16 ]∙64H 2 O per unit cell. The XRD powder diffraction pattern was indexed in orthorhombic symmetry with lattice parameters a 0 = 10.5035(9) Å, b 0 = 10.0262(9) Å, and c 0 = 61.9608(46) Å. The crystal structure was solved from a synthetic magadiite sample in a complex process using 3D electron diffraction combined with model building as presented in an additional paper. A Rietveld refinement of this structure model performed on a magadiite mineral sample in space group F 2 dd (No. 43) converged to residual values of R Bragg = 0.031 and R F = 0.026 confirming the structure model. Physico-chemical characterization using solid-state NMR spectroscopy, SEM, TG-DTA, and DRIFT spectroscopy further confirmed the structure. The structure of magadiite contains two enantiomorphic silicate layers of, so far, unknown topology. The dense layers exhibit no porosity or micro-channels and have a thickness of 11.5 Å (disregarding the van der Waals radii of the terminal O atoms) and possess a silicon Q 4 to Q 3 ratio of 2.5. 16 out of 32 terminal silanol groups are protonated, and the remaining groups compensate for the charge of the hydrated sodium cations. Bands of edge-sharing [Na(H 2 O) 6/1.5 ] octahedra are intercalated between the silicate layers extending along (110) and (110). The water molecules are hydrogen bonded to terminal silanol groups with O···O distances of 2.54–2.91 Å. The structure of magadiite is slightly disordered, typical for hydrous layer silicates (HLS), which possess only weak interactions between neighboring layers. In this respect, the result of the structure refinement represents a somewhat idealized structure. Nevertheless, the natural magadiite possesses a higher degree of structural order than any synthetic magadiite sample. The structure analysis also revealed the presence of strong intra-layer hydrogen bonds between the terminal O atoms (silanol/siloxy groups), confirmed by 1 H MAS NMR and DRIFT spectroscopy. The surface zone of the silicate layers, as well as the interlayer region containing the [Na(H 2 O) 6/1.5 ] octahedra, are closely related to the structure of Na-RUB-18.

Tin is a key strategic metal and indispensable in the high-tech industry. Constraining the source of the mineralizing fluids, their pathways, and subsequent ore-forming process is fundamental to optimizing tin exploration and efficient mining operations. Here, we present trace element analysis, LA-ICP-MS mapping, and the first systematic high-precision in situ Sn isotope analysis of cassiterite from several tin deposits (i.e., Weilasituo, Baiyinchagan, Maodeng Sn-polymetallic deposits) in northeast China using UV- fs -LA-ICP-MS. We show that the distribution of trace elements in cassiterite from these localities reflects crystallization under disequilibrium conditions with coexisting fluids or melts, and it suggests intense fluid-rock reactions. Among the three deposits, cassiterite from the Maodeng Sn-Cu deposit has the heaviest weighted mean Sn isotope composition, with δ 124 / 117 Sn values ranging from 0.11 ± 0.04‰ to 0.62 ± 0.08‰. The Baiyinchagan Sn-Ag-Pb-Zn deposit displays the lightest isotope composition with δ 124 / 117 Sn values ranging from –1.43 ± 0.06‰ to –0.50 ± 0.04‰. While the Weilasituo Sn-W-Li-polymetallic deposit shows the largest spread in δ 124 / 117 Sn values, ranging from –0.66 ± 0.05‰ to 0.59 ± 0.03‰. The Sn isotope variability in these natural cassiterites is attributed to Sn isotope fractionation associated with the diversity of Sn mineralization pathways and different physicochemical conditions. Furthermore, the δ 124 / 117 Sn values of cassiterite from the Maodeng and Baiyinchagan deposits gradually decrease from early to late mineralization stages, suggesting that they were generated by Rayleigh fractionation during progressive mineral precipitation from a hydrothermal fluid. In contrast, heavy Sn isotope values in late-stage Weilasituo cassiterites are likely a result of disequilibrium fluid-rock interaction with external, wall-rock-derived fluids. Our results reveal that liquid-vapor partitioning or fluid-rock interaction may have more influence on Sn isotope fractionation between cassiterite and evolving ore-forming fluids than do magmatic differentiation, pH, pressure, and temperature during the formation of tin deposits. According to the tin isotopic data obtained so far from this study and published previously, we observe no relationship between the Sn isotope composition of cassiterite and the age of mineralization or tectonic setting. However, cassiterite displays heavier Sn isotope compositions than coexisting stannite (Cu 2 FeSnS 4 ) regardless of the deposit type and depth of emplacement, suggesting that the redox state may influence Sn isotope fractionation. More importantly, we first recognize a general shift toward light Sn isotope compositions in cassiterite associated with decreasing Ti/Zr ratios, suggesting that Sn isotopes can be a robust tool for identifying the source of the mineralization. Furthermore, based on our Sn isotope data together with previous studies of fluid inclusion, we propose that the dominant Sn(II) species occur in early ore mineralization systems, then shifts to the Sn(IV) species in late stage due to redox change or higher Cl – activity. Tin isotopes may be a robust tool to trace the mineralization center and fluid pathways and to ascertain the mechanisms of metal precipitation.

The unconformity-related uranium (URU) deposits in the Proterozoic Athabasca Basin (Canada) represent the richest, and one of the most important, uranium endowments in the world. Most of the URU deposits are associated with pre-existing graphitic basement faults that were reactivated after the formation of the basin. These graphite-rich structures have been widely used as a vector for exploration, but the nature of the association of the URU deposits with graphitic basement faults has been debated for over four decades. Proposed roles of graphite include: (1) as a direct reducing agent to reduce U 6+ to U 4+ and precipitate uraninite; (2) as a precursor of hydrocarbons (mainly CH 4 ) produced in situ or nearby and then used as a reducing agent for uraninite precipitation; (3) as a precursor of hydrocarbons produced at depth that were remobilized to the site of mineralization and acted as a reducing agent for uraninite precipitation; and (4) as a lubricant facilitating faulting and fluid flow that led to uranium mineralization. This paper uses the Phoenix uranium deposit in the southeastern Athabasca Basin as a case study to address these uncertainties. Petrographic studies indicate that there is no direct contact between graphite and uraninite at microscopic scales, and the content of graphite in the graphitic metapelite along the ore-controlling WS Shear Zone does not show a systematic change with the distance from the unconformity surface. Raman spectroscopic studies of graphite suggest that the degree of structural disorder of graphite, expressed by various parameters related to the D bands and G band ratios, does not change systematically with the distance from the unconformity surface either. The minor irregularities in these parameters near the unconformity are better explained by paleo-weathering related to the unconformity and/or diagenetic processes than by hydrothermal activity related to uranium mineralization. Based on these observations and interpretations, the role of graphite as an in situ reducing agent, either directly or as a provider of hydrocarbons, is discounted. It is proposed that hydrocarbons derived from graphite at depth, tapped by episodic reactivation or seismicity of the basement faults that was facilitated by graphite as a lubricant, were responsible for URU mineralization.

Pomite (IMA2021-063), ideally Ca3[ V54+V105+O37(CO3) ]⋅37H2O,  $\text{(IMA2021-063), ideally }\!\!~\!\!\text{ C}{{\text{a}}_{3}}\left[ \text{V}_{5}^{4+}\text{V}_{10}^{5+}{{\text{O}}_{37}}\left( \text{C}{{\text{O}}_{3}} \right) \right]\cdot 37{{\text{H}}_{2}}\text{O, }\!\!~\!\!\text{ }$and pseudopomite (IMA2021-064), ideally Ca3.5[ V64+V95+O37(CO3) ]⋅32H2O,  $\text{C}{{\text{a}}_{3.5}}\left[ \text{V}_{6}^{4+}\text{V}_{9}^{5+}{{\text{O}}_{37}}\left( \text{C}{{\text{O}}_{3}} \right) \right]\cdot 32{{\text{H}}_{2}}\text{O, }\!\!~\!\!\text{ }$are two new polyoxometalate minerals from the Blue Streak mine, Bull Canyon, Montrose County, Colorado, U.S.A. Pomite properties: striated blades up to ~1 mm long; very dark green-blue color; green-blue streak; vitreous luster; brittle; Mohs hardness ≈2; irregular, splintery fracture; good cleavages on {010} and {001}; 2.19(2) g/cm −3 density; refractive indices in the vicinity of 1.70; weakly birefringent with little or no pleochroism. Pseudopomite properties: striated prisms and blades up to ~1 mm; very dark blue-green color; blue-green streak; vitreous luster; brittle; Mohs hardness ≈2; curved, irregular fracture; probably two fair cleavages, {100} and {001}; 2.40(2) g/cm −3 density; refractive indices in the vicinity of 1.72; no discernable birefringence or pleochroism. Electron microprobe analyses provided the empirical formulas Ca3.11 [ V5.234+V9.775+O37 $\text{C}{{\text{a}}_{3.11}}\left[ \text{V}_{5.23}^{4+}\text{V}_{9.77}^{5+}{{\text{O}}_{37}} \right.$(CO 3 )]·37H 2 O and Ca3.49[ V5.984+V9.025+O37(CO3) ]⋅29H2O $\text{C}{{\text{a}}_{3.49}}\left[ \text{V}_{5.98}^{4+}\text{V}_{9.02}^{5+}{{\text{O}}_{37}}\left( \text{C}{{\text{O}}_{3}} \right) \right]\cdot 29{{\text{H}}_{2}}\text{O}$for pomite and pseudopomite, respectively. Pomite is triclinic, P 1, with a = 12.3668(10), b = 12.9692(12), c = 22.068(2) Å, α = 99.038(7), β = 95.689(7), γ = 103.249(7)°, V = 3368.7(5) Å 3 , and Z = 2. Pseudopomite is triclinic, P 1, with a = 12.2910(18), b = 12.6205(15), c = 20.917(3) Å, α = 77.381(6), β = 85.965(5), γ = 64.367(7)°, V = 2853.6(7) Å 3 , and Z = 2. The crystal structures of both minerals (pomite, R 1 = 0.103; pseudopomite, R 1 = 0.116) contain a novel [ Vx4+V15−x5+O37(CO3) ](1+x)− ${{\left[ \text{V}_{x}^{4+}\text{V}_{15-x}^{5+}{{\text{O}}_{37}}\left( \text{C}{{\text{O}}_{3}} \right) \right]}^{(1+x)-}}$heteropolyanion, which is unique in natural and synthetic materials but has similarities to the [ V84+V75+O36(CO3) ]7− and [ H8V154+O36(CO3) ]6− ${{\left[ \text{V}_{8}^{4+}\text{V}_{7}^{5+}{{\text{O}}_{36}}\left( \text{C}{{\text{O}}_{3}} \right) \right]}^{7-}}\text{ }\!\!~\!\!\text{ and }\!\!~\!\!\text{ }{{\left[ {{\text{H}}_{8}}\text{V}_{15}^{4+}{{\text{O}}_{36}}\left( \text{C}{{\text{O}}_{3}} \right) \right]}^{6-}}$heteropolyanions reported in synthetic phases.