Crystal Zone Research Articles

Nanoscale constraints on the in situ transformation of Ru–Os–Ir sulfides to alloys at low temperature

We report new results of a combined focused ion beam high-resolution transmission electron microscopy (FIB/HRTEM) investigation of two grains of platinum-group minerals (PGM) corresponding to the laurite (RuS2)-erlichmanite (OsS2) solid solution series. The grains are included within unaltered chromite and the serpentine-dominant matrix of the mantle-hosted chromitite body of Monte Bueno in eastern Cuba. At the micron-scale, the grain hosted in unaltered chromite preserves magmatic features including a complex oscillatory zoning defined by a core of erlichmanite surrounded by alternating bands of Os-rich/Os-poor laurite, revealed by conventional field emission scanning electron microscopy (FE-SEM) and X-ray mapping by electron microprobe analyzer (EPMA). Combining high-magnification (HMEM), high angle-annular dark field (HAADF) images and the corresponding Energy Dispersive Spectra (EDS), the elemental mappings acquired using HRTEM showed that the oscillatory zoning occurs at both the micron and nano scales. A specific feature highlighted by HRTEM observations is the rather complex structure of the erlichmanite core, consisting of an aggregation of much smaller cores (<1 µm) characterized by an oscillatory zoning linked to bands thinner than 10 nm that are not detectable when the whole grain is imaged using conventional FE–SEM and X–ray EPMA elemental mapping. The nanotextural features identified in this study suggest: (1) formation of the PGM grains characterized by the coalescence of laurite/erlichmanite nuclei that originally formed independently and coalesced in the silicate magma, (2) growth and crystal zoning of compositionally different bands as a result of sudden changes in T, fS2, fO2 and composition of the melt. These thermodynamic and chemical changes also allowed the segregation of nano-sized Fe–Ni–S solid particles/droplets from the silicate magma, which are now found within the PGM grain as nanoparticles of pentlandite of <100 nm in size and had potential implications for laurite nucleation. On the other hand, backscattered FESEM images and X-ray EPMA mapping of the altered PGM grain associated with the silicate matrix of the chromitite revealed a magmatic sector zoned core, which is surrounded by a rim of S-deficient laurite (i.e., desulfurized laurite) that hosts Ru-Os-Ir alloys. The FIB/HRTEM observations on the rim of desulfurized laurite ± Os–Ir–Ru alloys revealed an intergrowth of nano-sized (<5 nm) laurite with ad Ru-Os-Ir-(S-As) and Ru-Os-Ir alloys (50–100 nm) within a Fe-Mn oxide/hydroxide matrix. This microstructure and the coexistence of these nanoparticles within the Fe-Mn oxide/hydroxide matrix illustrates an alteration sequence whereby magmatic laurite was desulfurized and decomposed into smaller laurite nanoparticles, which in turn were progressively transformed to Ru-Os-Ir-(S-As) particles and, finally, to Ru-Os-Ir nanoalloys. Self-reorganization (coarsening) of the desulfurizing laurite within a matrix transforming to Fe-Mn oxide/hydroxide matrix resulted in the formation of nanoscale intergrowths of Ru-Os-Ir-(S-As) particles and Ru-Os-Ir alloys with the Fe-Mn oxide/hydroxide . This took place concomitant to incorporation/oxidation of Fe and Mn by relatively more oxidizing fluids. These alteration fluids were generated during to serpentinization/ocean-floor metamorphism that affected the chromitite body. The observations provided here are relevant to the current debate about the platinum-group element (PGE) mobilization and the possible existence of PGE oxides and hydroxides minerals

Petrological and noble gas features of Lascar and Lastarria volcanoes (Chile): Inferences on plumbing systems and mantle characteristics

Lascar (5592 m a.s.l.) and Lastarria (5697 m a.s.l.) are Chilean active stratovolcanoes located in the Central Volcanic Zone (CVZ; 16°S to 28°S) that have developed on top of a 71 km thick continental crust. Independently of the similarities in their Plinian/Vulcanian eruptive styles, their complex magmatic feeding structures and the origins of their magmatic fluids still necessitate constraints in order to improve the reliability of geochemical monitoring. Here we investigate the petrography, bulk-rock chemistry, and mineral chemistry in products from the 1986–1993 explosive eruptive cycle at Lascar and from several Holocene eruptive sequences at Lastarria. These data are integrated with measurements of the noble gas isotopes in fluid inclusions (FIs) of minerals from the same products as well as in fumarole gases. The geochemistry of minerals and rocks shows that the studied products belong to high-K–calc-alkaline series typical of subduction-related settings, and provide evidence of differentiation, mixing, and crustal assimilation that are higher at Lastarria. The contribution of slab sediments and fluids to magma genesis in the wedge is limited, suggesting a homogeneous mantle beneath CVZ. The deepest crystallization processes occurred at variable levels of the plumbing systems according to the lithostatic equivalent depths estimated with mineral equilibrium geobarometers at Lascar (15–29 km) and Lastarria (~20–40 km). The 40Ar/36Ar and 4He/20Ne ratios in FIs and fumarole gases indicate the presence of some degree of air contamination in the fluids from both volcanoes. The 3He/4He values at Lascar (6.9–7.3 Ra) are relatively homogeneous and comparable to those of fumaroles, suggesting a main zone of magma crystallization and degassing. In contrast, the 3He/4He values at Lastarria (5.31–8.01 Ra) vary over a wide range, suggesting various magma storage levels and providing evidence of crustal contamination, as indicated by the rock chemistry. We argue that mantle beneath the two volcanoes has a MORB-like signature of 3He/4He, while local crustal contamination explains the lower ratios measured at Lascar.

Atom column detection from simultaneously acquired ABF and ADF STEM images

In electron microscopy, the maximum a posteriori (MAP) probability rule has been introduced as a tool to determine the most probable atomic structure from high-resolution annular dark-field (ADF) scanning transmission electron microscopy (STEM) images exhibiting low contrast-to-noise ratio (CNR). Besides ADF imaging, STEM can also be applied in the annular bright-field (ABF) regime. The ABF STEM mode allows to directly visualize light-element atomic columns in the presence of heavy columns. Typically, light-element nanomaterials are sensitive to the electron beam, limiting the incoming electron dose in order to avoid beam damage and leading to images exhibiting low CNR. Therefore, it is of interest to apply the MAP probability rule not only to ADF STEM images, but to ABF STEM images as well. In this work, the methodology of the MAP rule, which combines statistical parameter estimation theory and model-order selection, is extended to be applied to simultaneously acquired ABF and ADF STEM images. For this, an extension of the commonly used parametric models in STEM is proposed. Hereby, the effect of specimen tilt has been taken into account, since small tilts from the crystal zone axis affect, especially, ABF STEM intensities. Using simulations as well as experimental data, it is shown that the proposed methodology can be successfully used to detect light elements in the presence of heavy elements.

A New Cooling Strategy in Curved Continuous Casting Process of Vanadium Micro-alloyed YQ450NQR1 Steel Bloom Combining Experimental and Modeling Approach

In the continuous casting process of steel, the bloom surfaces would experience intensive cooling from the water-cooled copper mold to secondary cooling water spray. If the cooling process is not controlled properly, hot ductility of the bloom surface microstructures would deteriorate, and bloom surface cracks would form easily under straightening deformation in a curved caster. Considering the above facts, the cooling scheme for the continuous casting of YQ450NQR1 steel bloom, a kind of vanadium-containing micro-alloyed steel, is studied with both experimental investigation and mathematical modeling in this work. The authors first investigate the hot ductility of bloom surface microstructures at various cooling rates using a Gleeble thermal simulator. Then, the precipitation of V(C, N) particles and its influence on ferrite formation during continuous cooling are studied and characterized using High-Temperature Laser Scanning Confocal Microscopy. Based on these, the preferred cooling rate for surface microstructures at the straightening position in the caster is obtained. To further reduce the solute macro-segregation through enlargement of the equiaxed crystal zone, a cellular automaton-finite element model is used to calculate heat transfer and solidification structure evolution during the continuous casting process. After calibration with industrial trials, the model is utilized to determine the critical position for columnar to equiaxed transition and to adjust the pouring temperature for the melt. Combining the above research, a new cooling strategy for YQ450NQR1 steel bloom is obtained, which can improve crack resistance of bloom surface microstructures and reduce solute macro-segregation at the same time.

Linking fluid dynamics and olivine crystal scale zoning during simulated magma intrusion

The compositional zoning styles of natural crystals produced during magma intrusion can be used to investigate the structure of magmatic plumbing systems and its relation to expressions of volcanic unrest (seismic, deformation, volatiles). However, magma intrusion is a progressive, dynamic process and yields non-monotonic heterogeneities in physio-chemical variables such as complex spatial variations in temperature and liquid composition with time. Such changes in variables are difficult to incorporate in models of crystal zoning in natural systems. Here we take another approach by integrating the results of a numerical multiphase simulation of melt arrival in an olivine-rich reservoir with models of chemical re-equilibration of olivine. We evaluate the diversity of chemical zoning styles and the inferred time scales using Fe–Mg diffusion in olivine for a limited range of system geometries and time-composition-temperature values. Although our models are still a large simplification of the processes that may occur in natural systems we find several time-dependent and systematic relations between variables that can be used to better interpret natural data. The proportions of zoned and unzoned crystals, the zoning length scales, and the calculated diffusion times from the crystals, vary with time and the initial position of the crystal in the reservoir. These relationships can be used, for example, to better constrain the plumbing structure and dynamics of mafic eruptions from monogenetic volcanoes by detailed studies of changes in the zoning of the crystal cargo with eruptive sequence. Moreover, we also find that the time scales obtained from modeling of crystals at a single temperature and boundary condition tend to be shorter (> about 25%) than the residence time, which could also be tested in natural studies by combining crystal time scale records with monitoring datasets.

Research of non-metallic inclusions of E90KhAF rail steel

This article is devoted to the research of the non-metallic inclusions of the E90KhAF rail steel in different zones of continuous cast billets. It is established that in the rim zone the main non-metallic inclusions are silicates of iron and alumosilicates. Alumosilicates and silicates of iron are identified in the zone of columnar crystals. Sulfides of manganese (MnS), silicates of manganese, aluminum and iron are found in the central zone of the billet.

Modification of a rare-earth element deposit by low-temperature partial melting during metamorphic overprinting: Norra Kärr alkaline complex, southern Sweden

Rare-earth elements play a crucial role in modern technologies and are necessary for a transition to a green economy. Potentially economic deposits of these elements are typically hosted in minerals such as monazite, bastnäsite, and eudialyte (a complex Na-Ca-Fe-Zr silicate mineral with Cl), making these prime targets for geological research. Globally, rare-earth mineral deposits commonly show evidence of polyphase development and mineralisation processes, which need to be better understood to improve exploration strategies. The Norra Kärr alkaline complex (Sweden) contains a globally significant deposit of rare-earth elements, hosted in the mineral eudialyte. In this study, we focussed on eudialyte crystals in undeformed, cross-cutting pegmatoid veins from Norra Kärr. In order to determine their age, we refined an established micromilling method to enable sampling of minerals rich in rare-earth elements for precise analysis of major and trace elements, Nd isotope ratios, and Sm-Nd geochronology down to a scale of <200 μm. Mineral samples were subjected to detailed textural and chemical characterisation by backscattered electron imaging and laser ablation inductively coupled plasma mass spectrometry, by which precise and accurate Sm/Nd ratios were determined to steer subsequent micromill sampling for small-aliquot Sm-Nd isotope analysis by isotope dilution thermal ionisation mass spectrometry. Given enough internal spread in Sm/Nd ratios, reliable Sm-Nd isochrons can be derived from discrete textural domains within a single crystal. This provided an age of 1.144 ± 0.053 Ga (95% confidence); approximately 350 million years younger than the magmatic intrusion of the alkaline complex (ca. 1.49 Ga). Primary compositional sector and oscillatory zoning in these eudialyte crystals shows core-to-rim enrichment in rare-earth elements and significant fractionation of K/Rb, Y/Ho, Zr/Hf, and Nb/Ta, which we attribute to crystallisation under influence of complexing ligands in a confined volume. We argue that these mineralised pegmatoid veins formed by low-temperature (<550 °C) partial melting of the agpaitic host rock during an early Sveconorwegian (Grenvillian) metamorphic overprinting event. Given the challenge of directly dating rare-earth ore minerals by conventional methods, modification of rare-earth mineral deposits may be more widespread than already assumed, which shows the importance of investigations that date the rare-earth minerals themselves.

Hubble Space Telescope Search for Activity in High-perihelion Objects

Abstract Solar system objects with perihelia beyond the orbit of Jupiter (q &gt; 5 au) are too cold for water ice to generate an appreciable coma via sublimation. Despite this, numerous high-perihelion objects (HPOs) including many comets and recently escaped Kuiper Belt objects (Centaurs) are observed to be active out at least to the orbit of Saturn (q ∼ 10 au). Peak equilibrium temperatures at 10 au (∼125 K), while far too low to sublimate water ice, are sufficient to sublimate supervolatiles such as CO and CO2 ice. Temperatures at 10 au are also high enough to trigger the rapid crystallization of exposed amorphous ice, thus constituting another possible driver of distant activity. While supervolatile ices can sublimate strongly (as ) to at least Kuiper Belt (30 au) distances, crystallization is an exponential function of temperature that cannot be sustained much beyond ∼10 au. The heliocentric dependence of the activity thus suggests an observational test. If activity in high-perihelion objects is triggered by crystallization, then no examples of activity should be found with perihelia of q ≫ 10 au. If, on the other hand, activity is due to free sublimation of exposed supervolatile ices, or another cause, then distant activity might be detected. We obtained sensitive, high-resolution Hubble Space Telescope observations of HPOs to search for activity beyond the crystallization zone. No examples of activity were detected in 53 objects with q &gt; 15 au, consistent with the crystallization trigger hypothesis. However, sensitivity limits are such that we cannot reject the alternative hypothesis that mass loss is driven by the sublimation of supervolatile ices. We also searched for binary companions in our sample, finding none and setting an empirical 3σ limit to the binary fraction of &lt;8%.

Gradient transition zone structure in “steel–copper” sample produced by double wire-feed electron beam additive manufacturing

This paper describes the results of an investigation into a microstructure formation on a wire-feed electron beam additive manufactured “steel–copper” bimetallic sample. The peculiarities of a gradient zone structure with a smooth change of components’ concentration are revealed. The heterogeneity of copper and steel distribution in the gradient zone is provided by copper solidification and precipitation mechanisms. Both solidification of coarse copper inclusions in the interdendrite areas or along the dendrite boundaries and precipitation of fine Cu-based particles at the cooling stage from the solid solution of Cu in γ-Fe are the main factors of structure formation during the double wire gradient zone deposition. The presence of such fine copper precipitates from the supersaturated solid solution was revealed by means of transmission electron microscopy. The shape of copper particles in the gradient zone varies from spherical to oblong and irregular. The shape of steel particles and/or grains is mainly determined by the peculiarities of the crystallization zone and is characterized by the primary crystallization of γ-iron dendrites from the liquid melt. A physical scheme describing a variation in phase composition and microstructure in gradient zone of the bimetallic specimen was proposed.

Evaluation of structure transformation and biocompatibility of chitosan in alkali/urea dissolution system for its large-scale application

Alkali/urea, a green dissolution system for chitosan, have been widely used in preparation of chitosan-derived materials. However, there are no related reports detail the structure stability and biocompatibility of chitosan in alkali/urea, which are important for its large-scale application. In this work, chitosan was dissolved in KOH/urea solution and stored at different temperature for different time. The structure, viscosity, molecule weight (Mw), degree of deacetylation (DD), and biocompatibility of chitosan were determined. The Mw of chitosan decreased from 1011 KDa to 827–834 KDa, and DD increased from 76.9% to 85.7–93.5% after been stored at 25 °C and 4 °C for 5 weeks. Incomplete dissolution of chitosan and increase of DD enhanced its thermal stability by forming new crystallization zone. In contrast, chitosan stored in −20 °C for 5 weeks was stable without obvious change of Mw and DD. Moreover, the processed chitosans were no-toxic and safety for the biomedicine applications.

Solid−liquid phase equilibria of the quinary system containing lithium, sodium, calcium, strontium and chloride ions at 273.15 K and 101.325 kPa

The solid–liquid phase equilibria of systems LiCl–NaCl–CaCl2–SrCl2–H2O and CaCl2–SrCl2–H2O at 273.15 K and 0.1 MPa were investigated with the isothermal equilibrium method. The solid–liquid equilibrium phase diagrams were constructed with the experimental solubilities. The phase diagram in the ternary system consists of one univariant solubility curve saturated with solid solution (Ca,Sr)Cl2·6H2O, and one crystallization field of the solid solution. There are three invariant points, seven isothermal univariant curves, and five crystallization regions saturated with NaCl corresponding to (Ca,Sr)Cl2·6H2O, SrCl2·2H2O, LiCl·2H2O, CaCl2·4H2O and LiCl·CaCl2·5H2O in the dry-salt diagram of the quinary system. The areas of crystallization zones decrease according to the above order. The J(NaCl), J(H2O), refractive index and density of the solution gradually change regularly with the increasing of J(CaCl2) in the solution. These results on phase diagrams of the quinary system can supply theoretical reference for lithium salts and strontium salts separation from calcium chloride type brine.

Emergent dual topology in the three-dimensional Kane-Mele Pt2HgSe3

Recently, the very first large-gap Kane-Mele quantum spin Hall insulator was predicted to be monolayer jacutingaite (Pt$_2$HgSe$_3$), a naturally-occurring exfoliable mineral discovered in Brazil in 2008. The stacking of quantum spin Hall monolayers into a van-der-Waals layered crystal typically leads to a (0;001) weak topological phase, which does not protect the existence of surface states on the (001) surface. Unexpectedly, recent angle-resolved photoemission spectroscopy experiments revealed the presence of surface states dispersing over large areas of the 001-surface Brillouin zone of jacutingaite single crystals. The 001-surface states have been shown to be topologically protected by a mirror Chern number $C_M=-2$, associated with a nodal line gapped by spin-orbit interactions. Here, we extend the two-dimensional Kane-Mele model to bulk jacutingaite and unveil the microscopic origin of the gapped nodal line and the emerging crystalline topological order. By using maximally-localized Wannier functions, we identify a large non-trivial second nearest-layer hopping term that breaks the standard paradigm of weak topological insulators. Complemented by this term, the predictions of the Kane-Mele model are in remarkable agreement with recent experiments and first-principles simulations, providing an appealing conceptual framework also relevant for other layered materials made of stacked honeycomb lattices.

Strain hardening behaviors and mechanisms of polyurethane under various strain rate loading

AbstractThe uniaxial tension experiments are performed on thermoplastic polyurethane to investigate its mechanical behaviors and related potential mechanisms, and the loading strain rate is designing to be wide ranging from 0.0001 to 1 s−1. It is found that the polyurethane presents an obvious rate‐dependence, and the stress strain curves share distinct strain hardening characteristics under the investigated strain rates. Furthermore, the strain hardening ratios are sharing nearly same trends and appear to be influenced by both strain rate and the induced adiabatic heating. Besides, the ratio is also strain‐dependent on previous loading history. Then, a two‐dimension unit cell model is built to investigate potential equivalent mechanisms, of which the hard phase as inclusion is equivalent with crystallization zone, crosslinking sites, and so forth. The simulation results facilitate to explain the distinct strain hardening ratios, even for the matrix from the extrapolated curves under super‐low strain rate loading. Finally, the analogic mechanisms of equivalent hard inclusions are proposed, which can reasonably explain the strain rate‐ and strain‐dependence characteristics of polyurethane mechanical behaviors.

Band unfolding made simple

We present a simple view on band unfolding of the energy bands obtained from supercell calculations. It relies on the relationship between the local density of states in reciprocal space and the fully unfolded band structure. This provides an intuitive and valid approach not only for periodic, but also for systems with no translational symmetry. By refolding into the primitive Brillouin zone of the pristine crystal we recover the conventional unfolded bands. We implement our algorithm in the Siesta package. As an application, we study a set of benchmark examples, ranging from simple defects on crystals to systems with increasing complexity and of current interest, as the effect of external pressure on rotated graphene bilayers.

Effect of Electromagnetic Stirring on the Solidification Behavior of High-Magnetic-Induction Grain-Oriented Silicon Steel Continuous Casting Slab

The solidification behaviors of high-magnetic-induction grain-oriented silicon steel slabs have been investigated during experiments in industrial strand electromagnetic stirring (S-EMS). The current intensities of S-EMS were 0, 120, 200, and 350 A, and the frequency was 5 Hz. The ratio of the equiaxed crystal was 14.95%, 15.64%, 45.22%, and 66.96%, respectively. Central porosity cannot be eliminated by increasing the current intensity. The number, size, and ratio of segregation spots were markedly reduced by minimizing the equiaxed crystal zone. Carbon in the 0 and 120 A slabs exhibited a lower degree of macrosegregation compared with the 200 and 350 A slabs. Controlling the cooling rate and increasing the total reduction are the directions to further improve solidification structures, defects, and carbon segregation.

Determination and Modeling of Solid–Liquid Equilibrium for the Ternary System of l-Pyroglutamic Acid + l-Glutamic Acid + Water at T = 298.15–328.15 K

The solid-liquid phase equilibrium for the ternary system of L-Pyroglutamic acid + L-Glutamic acid + water was determined experimentally by the isothermal solution saturation method at temperatures of 298.15, 308.15, 318.15, and 328.15K under normal atmosphere. According to the obtained solubility data, four ternary phase diagrams and the variable-temperature phase diagram were constructed. Two pure solids, pure L-Pyroglutamic acid and pure L-Glutamic acid, were generated in the L-Pyroglutamic acid + L-Glutamic acid + water system. In the phase diagrams, there were in all three crystallization zones, two cosaturated solubility curves, and one cosaturated point. The variable-temperature phase diagram for the ternary system of L-Pyroglutamic acid + L-Glutamic acid + water could be used for separation and purification L-Pyroglutamic acid and L-Glutamic acid from its mixture. The data of solid-liquid phase equilibrium were correlated and computed with the NRTL model and Wilson model. The RMSD (root mean squar...

In situ Rb-Sr dating of slickenfibres in deep crystalline basement faults

Establishing temporal constraints of faulting is of importance for tectonic and seismicity reconstructions and predictions. Conventional fault dating techniques commonly use bulk samples of syn-kinematic illite and other K-bearing minerals in fault gouges, which results in mixed ages of repeatedly reactivated faults as well as grain-size dependent age variations. Here we present a new approach to resolve fault reactivation histories by applying high-spatial resolution Rb-Sr dating to fine-grained mineral slickenfibres in faults occurring in Paleoproterozoic crystalline rocks. Slickenfibre illite and/or K-feldspar together with co-genetic calcite and/or albite were targeted with 50 µm laser ablation triple quadrupole inductively coupled plasma mass spectrometry analyses (LA-ICP-MS/MS). The ages obtained disclose slickenfibre growth at several occasions spanning over 1 billion years, from at least 1527 Ma to 349 ± 9 Ma. The timing of these growth phases and the associated structural orientation information of the kinematic indicators on the fracture surfaces are linked to far-field tectonic events, including the Caledonian orogeny. Our approach links faulting to individual regional deformation events by minimizing age mixing through micro-scale analysis of individual grains and narrow crystal zones in common fault mineral assemblages.

Investigation of Grey Iron Primary Structure Influence on Hollow Castings Strength

For the hollow cylindrical grey cast iron billets, received by method of the directional solidification on the section of a wall, structural changes are investigated. Micro-hardness in zones of dendritic crystals and the eutectic matrix are determined.

The Significance of Central Segregation of Continuously Cast Billet on Banded Microstructure and Mechanical Properties of Section Steel

The solidification structure and segregation of continuously cast billets produced by different continuous casting processes are investigated to elucidate their effect on segregated bands in hot-rolled section steel. It suggested that segregated spots are mainly observed in the equiaxed crystal zone of a billet. The solidification structure is directly related to superheating and the intensities of secondary cooling. To a certain extent, the ratio of the columnar crystal increases with the increase of superheating and secondary cooling. Moreover, the number of spot segregations decreases with the decrease of the equiaxed crystal ratio. After hot rolling, the segregation spots are deformed to form segregated bands in steels. The severe segregation of Mn in segregated bands corresponds with that in the segregation spots. The elongation ratio and low temperature toughness deteriorate significantly by a high fraction of degenerate pearlite caused by central segregation. With a decrease of central segregation, the total elongation is increased by 10% and the ductile–brittle transition temperature (DBTT) is also reduced from −10 to −40 °C. According to the experimental results, columnar crystal in billets is preferred to effectively reduce the degree of central segregation and further improve low temperature toughness and the elongation ratio.

Active growth of non-hydrothermal subaqueous and subaerial barite (BaSO4) speleothems in Lechuguilla Cave (New Mexico, USA)

Barite (BaSO4) speleothems have been reported from caves around the globe and interpreted to have chiefly formed in phreatic, hypogene, hydrothermal settings. Here we report two contrasting types of barite speleothems (bluish tabular crystals in a shallow pool and actively dripping greenish stalactites), which today form at lower temperatures in the non-hydrothermal and vadose environment of Lechuguilla Cave, New Mexico, USA. Scanning electron microscopy analysis, along with energy- and wavelength-dispersive X-ray spectroscopy (EDS, WDS), as well as X-ray diffraction (XRD), characterize the habit and chemical composition as barite. Fractionation of the minor element calcium is related to growth along different crystal faces whereas variations in strontium concentration are mirrored in blue color zoning of the pool crystals. Two possible modes of non-hydrothermal barite precipitation are discussed: (1) intense evaporation driven by thermal atmospheric convection cells or (2) mixing of barium-rich, sulfate-poor water with water rich in sulfate. Both processes, in isolation or in combination, lead to supersaturation and could explain formation of the investigated barite speleothems. Observations of three types of microbes on the pool barite crystals showing evidence of incrustation raises the question whether there is a potential involvement of microbial activity in the temperate barite precipitation in Lechuguilla Cave.