Volume 110, Issue 10

Diamonds are crucial in understanding the deep Earth’s carbon cycle, offering essential insights into the composition and processes of the deep mantle. To effectively explore diamond-rich regions, it is essential to understand their unique seismic signatures. In this study, we employed ab initio PBEsol methods to calculate thermodynamic and elastic properties of diamonds under Earth’s mantle conditions. Our findings showed good agreement with experimental data at lower pressures and temperatures. Then we extended the analysis up to 140 GPa and 4000 K, allowing for a comprehensive examination of the entire mantle. The elastic moduli of diamonds exhibit notable nonlinear responses to pressure and temperature variations. Additionally, the elastic anisotropy of ASpo$A_{S}^{po}$of diamonds is significantly pronounced. We calculated density and wave velocity profiles for diamonds along mantle geotherms, revealing that diamond has higher bulk and shear moduli and lower density compared to the reference state (PREM) of the lower mantle. We also calculated the density and seismic wave velocities ( V P , V S ) of a peridotitic mantle mixture as a function of the diamond content. The results indicate that even minimal amounts of diamond integrated with the background mantle can substantially reduce density and enhance seismic velocities in the lower mantle. Consequently, we predict that diamond may accumulate in the upper mantle over time. The diamond-rich regions will exhibit exceptionally high and anisotropic seismic velocities under Earth’s extreme pressure and temperature conditions, as well as within other planetary bodies. These findings provide new approaches for probing diamond-rich zones and investigating the role of carbon in Earth’s geological history.

We report mineralogy, chemical composition, visible near-infrared (VNIR) spectroscopy, and X-ray near-edge spectroscopy of phyllosilicate-sulfate mineral assemblages, including natrojarosite, from the Matanomadh Formation (Palaeocene) of the Kutch region in Gujarat, India. The unit-cell parameters of sulfate minerals and VNIR spectral properties are consistent with natrojarosite and other minerals. The mineralogy and unit-cell parameters indicate supergene or hydrothermal alteration conditions within the Kutch region. The Fe oxidation record (Fe 3+ /total Fe ∼0.83) is quantified using Fe K -edge XANES. Additionally, the Fe-S bond length for natrojarosite is determined to be 3.158 Å, and the experimental result fits well with the modeled iron sulfate data curve. The occurrences of natrojarosite and associated natroalunite and kaolinite in the post-Deccan volcanic province are consistent with an oxidizing environment and periodic shift of humid to arid environments that appear to be similar to the early geologic history of Mars. The oxidative alteration of pyrite further enhances the formation of natrojarosite. Our results, in concert with earlier studies, highlight the formation process of natrojarosite and provide insights into detailed mineralogy at the Kutch region and the magnitude of the redox state. The natrojarosite and the associated minerals (including anhydrite and other sulfates) argue for acidic and water-limited conditions analogous to the Noachian-Hesperian epoch of Mars, with implications for past weathering processes.

Moxuanxueite, ideally NaCa 6 Zr(Si 2 O 7 ) 2 OF 3 , is a new wöhlerite-group mineral from the Gejiu alkaline complex, Yunnan Province, China. This new species occurs as subhedral to euhedral polysynthetic twinned grains, and as elongate crystals up to 1 mm in size, commonly coexisting with tiny baddeleyite at the grain margins and usually associated with calcioancylite-(La), bobtraillite, catapleiite, fluorite, and jadeite. Moxuanxueite is light yellow in color with a metallic luster. It is very brittle with conchoidal fracture and has a calculated density of 3.231 g/cm 3 . The empirical formula calculated on the basis of O+F = 18 apfu is Na 1.38 Ca 5.21 Fe 0.23 Mn 0.22 (Zr 0.88 Ti 0.14 )Σ 1.02 (Si 2.01 O 7 ) 2 (F 3.15 O 0.85 )Σ 4.00 . The ideal formula requires (wt%) Na 2 O 4.06, CaO 44.04, ZrO 2 16.13, SiO 2 31.46, F 7.46, O≡F –3.14, and a total of 100 wt%. The mineral is triclinic, with space group P 1 (#2), a = 11.0235(3) Å, b = 10.9641(4) Å, c = 7.3805(4) Å, α = 109.523(2)°, β = 109.881(2)°, γ = 83.434(2)°, V = 790.63(6) Å 3 , and Z = 2. The seven strongest X-ray powder diffraction lines [ d in Å ( I ) ( hkl )] are: 3.005 (100) ( 1 12), 2.870 (57) (230), 1.849 (32) ( 22 4), 10.42 (30) ( 1 00), 2.496 (26) (1 4 0), 3.280 (23) ( 31 0), and 1.701 (20) (1 6 2). Moxuanxueite is isostructural with hiortdahlite [Na 2 Ca 4 (Zr 0.5 Ca 0.5 )(Si 2 O 7 ) 2 OF 3 ], and the transformation between the two phases is via the substitution mechanism: 2Ca 2+ ⇋Na + + 0.5Zr 4+ +0.5Ca 2+ . The new mineral is named in honor of Xuanxue Mo (b. 1938), a distinguished petrologist from the China University of Geosciences (Beijing), for his outstanding contribution to magmatism and related mineralization in China.

Chalcopyrite (CuFeS 2 ) and bornite (Cu 5 FeS 4 ) from the Olympic Dam Cu-U-Au-Ag deposit (South Australia) are characterized using electron backscatter diffraction (EBSD) to identify microstructures and their correlations with trace element concentrations measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Natural chalcopyrite is shown to be a rich source of micro-structural and -textural information, preserving <110> and <001> crystallographic preferred orientations (CPO), {110} and {112} twin systems, grain boundary migration, foam textures, and subgrain boundaries. Selected examples of chalcopyrite illustrate different aspects of its behavior and relationship with bornite across the mineralogically zoned deposit. The oriented stress imposed by brecciation and/or fluid fluxes at Olympic Dam, alongside temperature, pressure, and strain rate, is shown to induce various microstructures preserved in chalcopyrite. Microstructures can, therefore, be used to elucidate sequential stages of low- to medium-temperature (<300 °C) ore evolution. Pyrite microstructures are already routinely used to understand higher temperature ore evolution, and the complementary microstructural study of chalcopyrite coexisting with pyrite has the potential to reveal deformational events across a more complete range of temperatures. Chalcopyrite is particularly well suited to unravel episodes of low- to medium-temperature overprinting in ore systems that lack obvious evidence for post-mineralization deformation. EBSD mapping reveals what appears as single grains of chalcopyrite in reflected light are, in fact, aggregates composed of ∼100 individual grains. In contrast, analyzed bornite displays overwhelming crystallographic homogeneity. Rare instances of misorientation in bornite are all associated with replacement and, if correlated with EBSD analysis of coexisting chalcopyrite and its inclusions (e.g., cobaltite), can be used to discern the origin and evolution of different bornite associations. LA-ICP-MS trace element mapping of chalcopyrite aggregates indicates that grain boundaries host Pb, Bi, Ag, and Sb concentrations, with twin boundaries displaying a weaker concentration of the same elements. Bornite grain boundaries are also enriched in Pb. These observations confirm the critical role played by microstructures in Cu-(Fe)-sulfides as traps for Pb, a non-target contaminant in copper concentrates, as well as new evidence for the physical state of Ag and potential value-added critical metals like Bi and Sb. The preferential occurrence of Pb, Bi, Ag, and Sb along permeable grain boundaries may incentivize efforts to remove contaminants and/or recover by-products via leaching.

The sulfate-bearing sublimate samples from Somma-Vesuvius, collected in the Royal Mineralogical Museum of Naples University Federico II (Italy), record historical exhalative events of this volcano, whose products are no longer available in outcrops. The complex associations have been thoroughly investigated using various analytical methods, including SEM-EDS, PXRD, SC-XRD, FTIR, and Raman spectroscopy. Sulfates (Cu-, alkali-Cu-, and Pb-bearing sulfates) are the most widespread group, closely followed by halides and then oxides, phosphates, and arsenates, often occurring as rare mineral associations. Among sulfates, kaliochalcite, KCu 2 (SO 4 ) 2 [(OH)(H 2 O)], is the most abundant phase, followed by chalcanthite Cu(SO 4 )·5H 2 O. In some cases, both chalcocyanite, CuSO 4 , and chalcanthite are found in the same sample, indicating incomplete alteration of the anhydrous phase. Cyanochroite, K 2 Cu(SO 4 ) 2 ·6H 2 O, is also common, in association with other alkali-Cu sulfates, Pb sulfates, and atacamite, Cu 2 (OH) 3 Cl, in sky-blue encrustations. Steklite, KAl(SO 4 ) 2 , fedotovite, K 2 Cu 3 (SO 4 ) 3 O, and eleomelanite, (K 2 Pb) Cu 4 O 2 (SO 4 ) 4 , are recorded for the first time at the Vesuvius volcano, while eleomelanite represents the second reported worldwide occurrence besides the type locality. Unusual phases containing As, Cd, and/or Tl (i.e., Tl-bearing leucite), as well as N, have also been identified. The presence of both HT and LT sublimates and alteration minerals indicates a progressive cooling of fumaroles, or the variation of genetic conditions at small scales. The presented FTIR and Raman data can be especially relevant in consideration of the potential occurrence of sulfates in planetary bodies.

Snowball quartz is commonly found in highly fractionated peraluminous granites that may be associated with mineralization of Ta, Nb, Li, W, Sn, and Rb. The genesis of snowball quartz has been a subject of controversy, and its genetic relationship with potential rare metal mineralization remains poorly understood. This work presents a study of the mineralogy and chemistry of quartz and plagioclase from the Yashan pluton in southern China. The Yashan pluton hosts the Yichun Ta-Nb-Li deposit and is primarily composed of two-mica granite, Li-mica granite, and topaz-lepidolite granite, which were intruded by granite dikes. The texture known as “snowball” is observed in quartz from Li-mica granite, but it is more frequently and prominently developed in quartz from topaz-lepidolite granite and granite dikes. This texture is characterized by the zonal arrangement of prismatic albite (referred to as snowball albite) within quartz phenocrysts, with the {010} face of snowball albite preferentially adhering to the crystal faces of quartz. It typically develops in specific domains of the mantle and/or rim of quartz phenocrysts, and occasionally in their cores. The snowball texture is preferentially developed on the prism faces ( m ) of quartz, as indicated by a higher abundance of snowball albites on these faces compared to others. The snowball-textured quartz domain (SBTQD) is texturally similar to other domains from the same growth zone and chemically overlaps with magmatic quartz cores or rims. The snowball albites are chemically indistinguishable from magmatic albite inclusions entrapped within quartz cores. These pieces of evidence suggest that the SBTQDs have a magmatic origin. Given the specific morphologies of snowball quartz (well development of prism faces) and albite (prismatic with a high aspect ratio of ∼5–10), along with the development of sector zoning in quartz, it is proposed that the snowball quartz formed as a result of rapid growth of quartz and albite under moderate under-cooling (Δ T = 50–100 °C) at near-solidus conditions, which was associated with multiple episodes of magma degassing. From the perspective of Ta-Nb mineralization, this degree of undercooling would significantly reduce the solubility of tantalite by more than 70–90%, thereby facilitating its crystallization. This process also promotes the further crystallization of already saturated columbite. Therefore, snowball quartz could serve as an exploration indicator for Ta-Nb deposits associated with highly fractionated peraluminous granites.

Iron redox works well for constraining oxygen fugacity (fO2)$\left( {{f}_{{{\text{O}}_{2}}}} \right)$ in terrestrial igneous materials due to the relatively high fO2${{f}_{{{\text{O}}_{2}}}}$ of the Earth’s atmosphere, crust, and upper mantle [ fO2≥ $\left[ {{f}_{{{\text{O}}_{2}}}}\ge \right.$ quartz-fayalite-magnetite (QFM)], where there are large changes in Fe 2+ /Fe 3+ with relatively small changes in fO2.  At  fO2${{f}_{{{\text{O}}_{2}}}}.\,\,\text{At }{{f}_{{{\text{O}}_{2}}}}$ values <QFM, Fe redox becomes less sensitive, and analytical uncertainties may make it difficult to determine fO2${{f}_{{{\text{O}}_{2}}}}$ differences between samples. The valence change between Cr 2+ and Cr 3+ occurs at lower fO2${{f}_{{{\text{O}}_{2}}}}$ values than for iron, potentially making it a more sensitive oxybarometer for materials equilibrated under reducing conditions. The current approach to measuring fO2${{f}_{{{\text{O}}_{2}}}}$ from X-ray absorption (XAS) measurements derives Cr valence first from the 1 s →4 s transition and then uses that redox couple to estimate fO2${{f}_{{{\text{O}}_{2}}}}$ as a function of temperature and composition. Here, that method is compared to an alternate approach of predicting fO2${{f}_{{{\text{O}}_{2}}}}$ directly from the spectra of experimentally homogenized glasses of geological relevance without an intermediate step of attempting to discern Cr 2+ /Cr 3+ . In this study, partial least-squares (PLS) multivariate (MVA) regression models were trained on the whole XAS energy spectral range, and accuracy was quantified using root mean square error (RMSE). MVA results showed significantly higher accuracy (RMSE-C of ± 0.75 log units) for predicting fO2${{f}_{{{\text{O}}_{2}}}}$ ΔIW relative to known experimental conditions relative to the two-step method, which yielded RMSE-C of ±2.75 to ±7.65 log units for our data set vs. those of Berry and O’Neill (2004) and Berry et al. (2006), respectively. The MVA results calibrate a new Cr oxybarometer for use in low-fO2$\text{low-}{{f}_{{{\text{O}}_{2}}}}$ glasses with a cross-validated (RMSE-CV) accuracy of ±0.84 log units fO2${{f}_{{{\text{O}}_{2}}}}$ relative to a standard oxygen buffer. Finally, the new Cr oxybarometer was applied to lunar glasses, both volcanic and impact metamorphosed, to assess the range in oxidation conditions the materials experienced during formation. Lunar volcanic glasses cluster ∼IW±1, close to that of previous studies while agglutinates and lunar impact melts record a wide range of fO2${{f}_{{{\text{O}}_{2}}}}$ values using Cr oxybarometry.

Two new minerals, nigelcookite, ideally PbFe22+V23+(PO4)3(OH)3,$\text{PbFe}_{2}^{2+}\text{V}_{2}^{3+}{{\left( \text{P}{{\text{O}}_{4}} \right)}_{3}}{{(\text{OH})}_{3}},$and plumbojohntomaite, ideally PbFe22+Fe23+(PO4)3(OH)3,$\text{PbFe}_{2}^{2+}\text{Fe}_{2}^{3+}{{\left( \text{P}{{\text{O}}_{4}} \right)}_{3}}{{(\text{OH})}_{3}},$have been found in the Yushui Cu deposit, South China. They occur in the bedded/massive Cu ore, and often occur in association with chalcopyrite, galena, sphalerite, nolanite, pyrite, anhydrite, apatite, fluorite, xenotime-(Y), and quartz. They are both new members of the bjarebyite group, exhibiting the general formula XM 1 2 M22(PO 4 ) 3 (OH) 3 . Nigelcookite occurs as spearhead-shaped or platy crystals ranging from 5 to 100 μm in size, and plumbojohntomaite occurs as platy crystals ranging from 5 to 80 μm in size. Both minerals are translucent to transparent with vitreous luster, but they differ in color. Nigelcookite ranges from dark brown to greenish brown, while plumbojohntomaite varies from dark green to olive green. On the basis of 8 cations, the empirical formulas are Pb0.98(Fe1.692+Mn0.24Cu0.04Zn0.03)Σ2.00( V1.31Fe0.633+Al0.06)Σ2.00(PO4)3.02(OH)3$\text{P}{{\text{b}}_{0.98}}{{\left( \text{Fe}_{1.69}^{2+}\text{M}{{\text{n}}_{0.24}}\text{C}{{\text{u}}_{0.04}}\text{Z}{{\text{n}}_{0.03}} \right)}_{\Sigma 2.00}}{{\left( ~{{\text{V}}_{1.31}}\text{Fe}_{0.63}^{3+}\text{A}{{\text{l}}_{0.06}} \right)}_{\Sigma 2.00}}{{\left( \text{P}{{\text{O}}_{4}} \right)}_{3.02}}{{(\text{OH})}_{3}}$and Pb1.03(Fe1.912+Mn0.02Cu0.04Zn0.03)Σ2.00(Fe1.953+V0.05)Σ2.00(PO4)2.97(OH)3$\text{P}{{\text{b}}_{1.03}}{{\left( \text{Fe}_{1.91}^{2+}\text{M}{{\text{n}}_{0.02}}\text{C}{{\text{u}}_{0.04}}\text{Z}{{\text{n}}_{0.03}} \right)}_{\Sigma 2.00}}{{\left( \text{Fe}_{1.95}^{3+}{{\text{V}}_{0.05}} \right)}_{\Sigma 2.00}}{{\left( \text{P}{{\text{O}}_{4}} \right)}_{2.97}}{{(\text{OH})}_{3}}$for nigelcookite and plumbojohntomaite, respectively. Both minerals are monoclinic, with space group P 21/ m (#11) and Z = 2. The unit-cell parameters are a = 9.1159(5) Å, b = 12.2328(7) Å, c = 5.0092(3) Å, β = 100.708(6)°, V = 548.86(6) Å 3 , and Z = 2 for nigelcookite, and a = 9.0999(4) Å, b = 12.1911(6) Å, c = 5.0176(2) Å, β = 100.671(4)°, V = 547.02(4) Å 3 , and Z = 2 for plumbojohntomaite. Nigelcookite (IMA 2023-113) and plumbojohntomaite (IMA 2023-119) are approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification. They are isostructural with other minerals of the bjarebyite group, and nigelcookite is also the member of the group with V 3+ as the dominant cation at the M2 site. The negative correlation between V and Fe contents indicates isomorphic substitution between V 3+ and Fe 3+ in nigelcookite and plumbojohntomaite. Given the occurrence of Fe 3+ in plumbojohntomaite, its formation condition is more oxidized than nigelcookite.

Experimental studies have demonstrated that solid solutions of minerals from the alunite group, with chemical compositions intermediate between the Al and Fe end-members, can be readily synthesized in the laboratory. In contrast, up until about a dozen years ago, there were no confirmed reports of alunite group minerals with intermediate Al-Fe compositions in natural settings, leading some to suggest that minerals with such compositions might not exist in nature. In recent years, however, alunite group minerals with intermediate Al-Fe compositions have been documented in a few isolated locations, which were previously limited to basalt-hosted acid-sulfate fumarole deposits and acid mine drainage pit lakes. These occurrences contrast with nearly all other reports of minerals from this group, whose measured chemical compositions are very close to either the Al or Fe end-members. Here, we report jarosite-alunite solid solutions containing approximately equal amounts of Al and Fe, which are found in mineralized fractures of the Aztec Sandstone in southeast Nevada. Analysis of the minerals by X-ray diffraction, Raman spectroscopy, and visible-near infrared spectroscopy confirms that they are bona fide solid solutions and not intimate mixtures of end-member minerals. This study represents the first documented occurrence of alunite group solid solutions with intermediate Al-Fe compositions in sedimentary rocks. The results further demonstrate that alunite group minerals with a wide range of Al-Fe compositions occur naturally and can persist for millions of years or more in natural systems.

Germanium is a classic trace element that rarely forms its own minerals but plays a significant role in geochemical and cosmochemical classification schemes. The geochemical behavior of Ge is very contrasting, depending on the redox environment and sulfur activity in the geosystems. Siderophile germanium, in the form of native germanides, has been reported in several terrestrial and meteoritic occurrences, but the majority of these phases have been poorly characterized. In this study, we report the discovery of Ni 3 Ge, a new germanide mineral, named olgafrankite in honor of Olga Victorovna Frank-Kamenetskaya, Saint Petersburg State University, Russia. The mineral, previously mentioned in the Rumuruti meteorite (the prototype of the R-type chondrites), has been discovered and studied in this work from terrestrial basalts of the Dzhaltul intrusion in the Norilsk ore region, Siberia, Russia. Olgafrankite crystals up to 5 × 7 μm in size were found within magnetite-pentlandite assemblages confined to massive centimeter-sized aggregates of native (telluric) iron. Other associated minerals are graphite, cohenite (Fe 3 C), Ge-bearing taenite (γ-Fe,Ni) (more than 1 wt% Ge), tetrataenite (FeNi), and various sulfides. In reflected light, olgafrankite is bright white with a pink tint. The empirical formula of the mineral calculated on the basis of 4 atoms per formula unit is (Ni 2.72 Fe 0.28 Co 0.01 )Σ 3.01 (Ge 0.97 S 0.02 )Σ 0.99 . Electron backscattered diffraction analysis revealed that olgafrankite is cubic (space group Pm 3 m ) and isostructural with synthetic Ni 3 Ge. Rietveld analysis of the powder X-ray diffraction data has confirmed the space group of olgafrankite and allowed for the refinement of its unit-cell parameter, a = 3.5784(2) Å. The mineral belongs to the perovskite supergroup, B -site-vacant single antiperovskites, auricupride subgroup. In terms of crystal structure and chemical composition, olgafrankite is a direct germanium analog of nisnite, Ni 3 Sn, and carletonmooreite, Ni 3 Si. Here we demonstrate that the new germanide can be an important but still obscure germanium carrier in some reduced assemblages, in particular in iron- and stony-iron meteorites, where the well-known significant correlations between nickel and germanium may be accounted for by the presence of finely dispersed olgafrankite precipitates.

Anew hydrous magnesic-niobic oxide mineral, fuyuanite (IMA 2024-059), ideally Mg 7 Nb 6 O 18 (OH) 8 , has been discovered in the Bayan Obo REE-Nb-Fe deposit, China. The mineral occurs in a Mesoproterozoic dolomite-calcite carbonatite dike, and it is associated with oboniobite, fersmite, pyrochlore, magnetite, dolomite, apatite, and calcite. Individual crystals range in size from 40 to 60 μm, and no twinning is observed. The mineral is reddish-brown to brown and translucent, with a vitreous luster. The calculated density is 4.156 g/cm 3 . Electron microprobe analyses of 18 spots yielded the main components of fuyuanite (total 99.41 wt%): Nb 2 O 5 (average of 67.34 wt%), MgO (21.66 wt%), FeO (2.89 wt%), MnO (1.18 wt%), TiO 2 (0.21 wt%), and H 2 O (6.12 wt%, calculated assuming H = 8 apfu). The empirical formula calculated on the basis of O = 26 apfu is (Mg 6.31 Fe 0.53 Mn 0.20 )Σ 7.03 (Nb 5.96 Ti 0.03 )Σ 5.99 O 18 (OH) 8 , and the simplified formula is (Mg,Fe,Mn) 7 (Nb,Ti) 6 O 18 (OH) 8 . Fuyuanite is trigonal, with space group P 3 (#147), Z = 1, and unit-cell parameters a = 10.7174(5) Å, c = 4.7110(2) Å, and V = 468.62(4) Å 3 . The crystal structure of fuyuanite consists of metal-octahedra (MO 6 ) layers along the c -axis, where Mg and Nb are octahedrally coordinated with oxygen. In the fuyuanite structure, the layer (L1) of edge-sharingMgO 5 (OH) octahedra (Mg2) is stacked with the layer (L2) of discrete rings of six edge-sharing NbO 4 (OH) 2 octahedra enclosing a Mg(OH) 6 octahedron (Mg1), and the cation stacking follows the sequence of –Mg2–(Mg1,Nb)–Mg2–. Fuyuanite shares a similar chemical composition with oboniobite (Mg 4 Nb 2 O 9 ), columbite-(Mg) (MgNb 2 O 6 ) and ternovite (MgNb 4 O 11 ·8–12H 2 O), yet it is notably different in its crystallographic, physical, and optical properties compared to the other three minerals. Therefore, fuyuanite is not a variant of oboniobite, columbite-(Mg), or ternovite. Instead, it is a unique new mineral species.

Phoxite, (NH 4 ) 2 Mg 2 (C 2 O 4 )(HPO 4 ) 2 ·4H 2 O, was initially discovered in 1977 and then confirmed in 2018. It is the first mineral known to contain both phosphate and oxalate groups. Phoxite has only been found so far, in the Petrogale Cave, Western Australia, and Rowley mine, Arizona, U.S.A. In this work, we synthesized phoxite using oxammite and newberyite as precursors under different relative humidity conditions. Then, a systematic spectral characterization of this mineral was performed using Raman spectroscopy, mid-infrared (MIR) spectroscopy, and visible-near infrared (VNIR) spectroscopy. The obtained spectra are compared with those of other oxalates, hydrated sulfates, and carbonates to determine the diagnostic spectral characteristics of phoxite. In addition, the thermal behavior of phoxite during pyrolysis was investigated using evolved gas analysis (EGA), thermogravimetry (TG), and differential scanning calorimetry (DSC). Our results indicate that phoxite may be a potential biomarker on Earth and an indicator of a warm and humid environment. The results may also serve as the spectral database for the identification of phoxite within the habitable environment on Earth and even other planets.

The vibrational properties of hydroxyl groups in pure and Fe-bearing brucite have been investigated under high-pressure and -temperature conditions relevant to subduction zones. Using externally heated diamond-anvil cell techniques combined with in situ Raman spectroscopy, we observed that the hydroxyl stretching modes in pure brucite change from blueshift to redshift at ∼600 K and 1.2 GPa. This indicates a sublattice proton order-disorder transition associated with decreasing interlayer spacing in brucite. Moreover, the Fe-Mg substitution in brucite leads to a secondary hydroxyl stretching band at 3644.6 cm −1 for Mg 0.9 Fe 0.1 (OH) 2 , while the primary Raman band is changed to 3636.7 cm −1 . This new Raman mode persists with increasing temperature at atmospheric pressure but disappears at about 3.0 GPa. The decomposition temperature of this Fe-bearing brucite sample is about 130 K lower compared to pure brucite under high oxygen fugacity conditions. These results suggest that the sublattice transition and Fe-Mg substitution may influence the stability of brucite, providing insight into the deep-water cycle at shallow subduction zones.

The chemical composition and crystal structure of boleite from the Amelia Mine (Boléo District, Mexico) were investigated by a series of chemical analytical techniques and single-crystal X-ray (data collected at 293 K) and neutron diffraction (at 20 K). The concentrations of more than 60 elements were measured. The empirical formula of boleite, based on the multi-analytical approach, is: (K 0.390 Ca 0.165 Na 0.095 Rb 0.075 Cd 0.040 Cs 0.035 Tl 0.002 )Σ 0.80 Pb 26.05 Ag 8.93 Cu 23.91 Cl 61.64 (OH) 48.39 , of which the simplified formula should be given as (K,Ca,Na,Rb,Cd,Cs)Pb 26 Ag 9 Cu 24 Cl 62 (OH) 48 . However, as Cd can also be considered as a potential substituent for Ag or Cu, the simplified formula transforms to (K,Ca,Na,Rb,Cs)Pb 26 Ag 9 Cu 24 Cl 62 (OH) 48 . This finding indicates a more complex scenario with respect to the previous formula reported in the literature, i.e., KPb 26 Ag 9 Cu 24 Cl 62 (OH) 48 . Chemical data obtained in this study show no significant evidence of potential substituents of Pb, Ag, and Cu; the concentrations of REE, PGE, and other industrially relevant elements are insignificant. Despite a lack of crystallographic evidence, chemical data appear to suggest that partial Cl − vs. OH − substitution can occur. Other potential substituents of Cl − , such as F − , have not been detected at a significant level. X-ray and neutron diffraction data confirm the previously reported general structural model of boleite but consistently show that a substitutional disorder affects the K site, manifested by a large and unusual displacement parameter. The magnitude of the displacement parameter reflects static disorder, in the form of substitutional disorder, due to differences in the local bonding topology among K, Ca, Na, Rb, (Cd), and Cs statistically populating the same site. The H-bonding network in the structure of boleite is now unambiguously described on the basis of the neutron structural model, with two energetically favorable bonds, both having an O donor -H···Cl acceptor configuration. The structure of boleite does not contain H 2 O molecules but, instead, only two crystallographically independent hydroxyl groups.