Advancing LIBS for Quantitative and Spatial Analysis of Rare Earth Elements in Coal

With the increasing global demand for rare earth elements (REEs), there is a critical need for rapid, field-deployable technologies that are capable of detecting and quantifying REEs in both conventional and unconventional resources. This work evaluates the use of Laser-Induced Breakdown Spectroscopy (LIBS) for the detection of lanthanum (La) and neodymium (Nd) in synthetic and natural rock and coal matrices at extraction-relevant concentrations. Multiple LIBS systems—including commercial and custom benchtop instruments (in single- and double-pulse modes), as well as a developing in situ LIBS probe—were employed. Detection limits as low as 10 ppm for La and 15 ppm for Nd were achieved, with double-pulse mode yielding signal enhancements of 3.5 to 6 times greater than single-pulse mode. Additionally, LIBS was compared with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) across coal samples. Multivariate calibration and principal component analysis (PCA) enabled accurate classification and prediction of REE content, while 2D elemental mapping provided insights into the spatial distribution of REEs. The results demonstrate LIBS to be a promising tool for real-time REE analysis and resource assessment in both laboratory and field environments. LIBS is an emission spectroscopy-based analytical technique. A high-power laser pulse is used as an energy source to cause ablation of the test materials and achieve high-temperature plasma formation, which upon cooling emits light, giving characteristic information of the species that are present in the materials in terms of atomic and molecular spectra.

As the global demand for high-grade iron ore continues to rise, it is estimated that current reserves may be depleted within the next 60–70 years. Among the most prominent and widely distributed geological sources of iron are Banded Iron Formations (BIFs), which are characteristic rock types that formed extensively during the Precambrian era across many of the world’s shield regions. India ranks fourth in global iron ore production, contributing around 246 million tons, or roughly 8% of global output. However, with growing demand and declining reserves of high-grade ore, it has become increasingly important to exploit low- to medium-grade deposits (45–62% Fe). Upgrading these lower-grade ores depends heavily on comminution, which plays a key role in making mineral beneficiation economically viable. Advanced in situ characterization techniques—such as Light Microscopy (LM), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and automated mineralogical methods—enable rapid, accurate, and reliable data acquisition. By integrating these analytical approaches, effective beneficiation strategies have been developed and tested at both laboratory and pilot scales, successfully achieving the intended results. Beyond conventional iron ore beneficiation, there is also significant potential to recover Rare Earth Elements (REEs) and other critical minerals from weathered iron ores, mine tailings, and waste dumps, particularly from hematitic and goethitic ores. This exploration is analogous to China’s world-class Bayan Obo deposit. Recently, the present study in the Ubrani area of Karnataka revealed the presence of lithium-bearing Petalite within titaniferous magnetite seams. Here, lithium is hosted in mineral phases associated with hematitized V-Ti-Mg seams. Petalite appears to represent a late-stage hydrothermal mineralization phase, likely synchronous with the hematitization of these seams, and is possibly linked to the major orogenic gold mineralization events (2.45–2.60 Ga) in the Dharwar Craton. Furthermore, the occurrence of spodumene- and tourmaline-bearing quartz reefs and pegmatites may also be tied to this cratonic evolutionary history.

Brazilian blue apatite is highly valued in gemology for its distinctive neon-blue coloration, yet its chromogenic mechanism remains incompletely understood. This study systematically investigated the color origin through conventional gemological testing, Fourier-transform infrared spectroscopy (FTIR), laser Raman spectroscopy, UV-Vis-NIR absorption spectroscopy, laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS), and heat treatment experiments. The samples were identified as fluorapatite with [CO₃]²⁻ substituting for [PO₄]³⁻. LA-ICP-MS revealed light rare-earth element (LREE) enrichment, heavy REE (HREE) depletion, and negative Eu anomalies, indicating formation under reduced oxygen fugacity conditions. A positive correlation (R² > 0.8) was observed between LREE concentration and color saturation.Transition metals: Mn³⁺ (absorption at 580-650 nm) serves as the primary chromophore, with Mn content positively correlating with color intensity. Fe³⁺ enhances brightness by suppressing red-light absorption. Rare earth elements, such as Nd³⁺, contribute to red-region absorption (745/801 nm), while Ce³⁺-SiO₃⁻ radicals and SO₃⁻ electron centers dominate UV/blue/green absorption. Excessive Ce³⁺ was found to inhibit blue coloration. Th content indicates the presence of SO₃⁻/SiO₃⁻ radicals. Post-heating color fading (threshold at 400°C) results from U decay-induced color center destruction. Blue-purple fluorescence originates from Ce³⁺ (400 nm emission via 5d→4f transitions) and Eu²⁺ (585 nm), with sharp peaks near 600 nm attributed to Sm³⁺/Pr³⁺ transitions. This study elucidates the synergistic effects of Mn³⁺/Fe³⁺, REE electronic transitions, and color centers in generating the characteristic blue coloration, providing fundamental insights for gemological identification and enhancement protocols.

Mineralogy inspires the search for functional magnetic materials. Fumarolic copper oxysalts, with the Cu²⁺ spin-1/2 state, are prime candidates for quantum magnetism studies. Recent focus has been on minerals featuring a square kagome lattice of copper ions, such as nabokoite, KCu₇TeO₄(SO₄)₅Cl. Here we report a new compound structurally related to nabokoite.

Green plate crystals of KCu₇BiO₄(SO₄)₅ were synthesized via chemical vapor transport. A stoichiometric mixture of K₂SO₄, CuSO₄, CuO, and Bi₂O₃ was sealed in a fused silica ampoule with I₂, evacuated, and heated in a 580–640 °C gradient for 30 days. Resulting crystals were characterized by scanning electron microscopy and single-crystal X-ray diffraction. The crystal structure of KCu₇BiO₄(SO₄)₅ was found to be tetragonal with the space group P4/ncc and a = 9.7731(1) Å, and c = 20.4094(6) Å, and Z = 4. KCu₇BiO₄(SO₄)₅ crystallizes in the same tetragonal space group as KCu₇TeO₄(SO₄)₅Cl nabokoite. However, these compounds are not isostructural. The substitution of Te⁴⁺ by Bi³⁺ is accompanied by the absence of chloride ions, as is evident from a comparison of their formulas. The key structural difference lies in the arrangement of Cu-centered polyhedra within the kagome-like layers. In KCu₇BiO₄(SO₄)₅, the Cu–O layer is formed by Cu1O₆ octahedra and Cu2O₅ square pyramids sharing trans-edges, creating four-membered windows. In contrast, analogous layers in nabokoite phases are built from CuO₆ octahedra and CuO₅Cl distorted octahedra, where the latter share common chlorine vertices. Furthermore, as seen in projection along the layers, the decorating Cu atoms in nabokoite-like phase are in mixed coordination and form CuO₄Cl pyramids that share a chlorine vertex with the polyhedra of the square kagome lattice. In our compound, the decorating Cu atoms are in CuO₄ square coordination and connect to the SKL exclusively via sulfate tetrahedra. KCu₇BiO₄(SO₄)₅ is promising for studyingmagnetic behavior.

The unique conditions of fumaroles give rise to an exceptional diversity of mineral species due to a combination of key factors: high temperatures, atmospheric pressure, gas transport of chemical components, and the unusual geochemical setting of active volcano exhalations. Using high-temperature synthesis, we obtained single crystals of a new copper phosphate K₂CaCu₆O₂(PO₄)₄, which represents a new morphotropic modification of the fumarolic arsenate shchurovskyite, K₂CaCu₆O₂(AsO₄)₄. The K₂CaCu₆O₂(PO₄)₄ crystal structure and chemical composition were studied by single-crystal X-ray diffraction and EDS spectroscopy.

In this work, six structurally related oxysalts were grouped into the shchurovskyite family with the general formula A₂B[M₆O₂](TO₄)₄, (A is K⁺ or Rb⁺; B is Ca²⁺, Cu^2+, or K⁺; T is P⁵⁺ or As⁵⁺; and M is Cu²⁺, Cu^2+, or Al³⁺). In their structures, two-periodic blocks composed of TO₄ tetrahedra and Cu²⁺-centered polyhedra are interconnected into a heteropolyhedral framework with channels occupied by extra-framework cations. The geometric topology of the Cu-ion substructure is similar across the family. However, the anionic environment of the copper polyhedra systematically varies with symmetry changes, exhibiting a large diversity of Cu²⁺ coordination geometry. This structural flexibility arises from the Jahn–Teller effect of Cu²⁺ cations, resulting in structural rearrangement driven by temperature and compositional variations.

Calculations based on the topology of the framework channels indicate the potential for K⁺-ion migration through the K₂CaCu₆O₂(PO₄)₄ structure. Magnetic susceptibility measurements show that K₂CaCu₆O₂(PO₄)₄ undergoes a transition into a long-range ordered state with a spontaneous magnetic moment at T_C = 10 K. The fit of the χ(T) curve yields a Weiss temperature of -97.8 K, indicating dominant antiferromagnetic interactions at high temperatures. Compared to its Rb-analog (T_C = 25 K), K₂CaCu₆O₂(PO₄)₄ exhibits a lower ordering temperature and signs of magnetic frustration.

The minerals elasmochloite, Na₃Cu₆BiO₄(SO₄)₅, nabokoite, KCu₇TeO₄(SO₄)₅Cl, and atlasovite, KCu₆FeBiO₄Cl(SO₄)₅, are rare fumarolic minerals. In their crystal structures, Cu-centered polyhedra connect via edges and vertices, forming Cu–O/Cl layers with a kagome square topology. Interest in compounds featuring such architectures has grown in recent years due to the potential for realizing a quantum spin liquid state.

Given the scarcity of these minerals, analogs of elasmochloite were synthesized using a chemical vapor transport method, which simulates the fumarol conditions. It was established that, in laboratory experiments, a polymorphic variety crystallizes with the analog of the mineral, Na₃Cu₆BiO₄(SO₄)₅. Interestingly, both polymorphs belong to the same sp. gr., P2₁/n, and have similar cell parameters. Single-crystal XRD analysis revealed that the key structural difference between the polymorphs lies in the degree of distortion and a different relative arrangement of the Cu–O layers alternating with Na–O 2D fragments. In the mineral's structure, the Cu–O layers lie parallel to the monoclinic axis, whereas in the synthetic compound, they are perpendicular to the monoclinic axis. The Na–O sublattice adapts to the copper–oxygen layers, forming unique structural motifs.

In an attempt to substitute sodium ions with larger potassium ions, a new sulfate, KCu₇BiO₄(SO₄)₅, was synthesized. In its crystal structure, the magnetic subsystem of the square kagome type is decorated with additional copper cations compared to the copper subsystem in elasmochloite. The crystal structure of KCu₇BiO₄(SO₄)₅ is related to that of nabokoite, with both crystallizing in the tetragonal space group P4/ncc. The structural difference arises from the substitution of Te⁴⁺ by Bi³⁺, which is compensated by the absence of chlorine atoms in the new phase.

Magnetic susceptibility and heat capacity measurements indicate that both compounds are frustrated antiferromagnets with magnetic ordering transitions at 18 and 12 K for the Na- and K-sulfates, respectively.

Modern dolomite stromatolites were previously described in the hypersaline Petukhovskoe Soda Lake (Southwestern Siberia, Russia). Exopolysaccharides (EPSs) secreted by phototrophic communities have previously been shown to play a crucial role in their formation. In the current study, using transmission electron microscopy (TEM) with energy dispersive X-ray (EDX) spectral microanalysis, we demonstrate that clay minerals also play a significant role in the dolomitization of Ca-Mg carbonates in this lake. The lake is characterized by the development of phototrophic communities, whose huge biomass accumulates near the shoreline and gradually dries out, finally forming cavernous crusts that later become stromatolites. Three samples (S1, S2, and S3) were collected simultaneously in the littoral at different distances from the water and represent successive stages (from S1 to S3) of phototrophic biomass desiccation. TEM revealed ubiquitous clay minerals (consistent with montmorillonite composition) in all samples, which were represented by free clay particles, fine clay particles surrounding cell walls, and a completely interstitial clay-EPS matrix. Carbonate crystals of varying sizes were embedded in this matrix. IR spectroscopy indicates a transition from predominantly high-Mg calcite in S1 to a dolomite composition in S3. Consistently, TEM-EDX analysis of 130 points revealed a statistically significant decrease in the average Mg/Ca (atomic %) ratio in carbonate crystals from 4.8±2.9 (S1) to 2.8±1.2 (S2) and 1.3±0.2 (S3), while the opposite trend was observed in the Mg/Si ratio: 0.7±0.1 (S1), 1.1±0.3 (S2), and 2.0±1.2 (S3). These ratios in clays did not change significantly. The 3-5-fold excess of Mg over Ca at the analyzed points in carbonate crystals in S1 and S2 indicates that Mg may have originally been associated with clay minerals, which act as Mg donors for incorporation into the Ca-Mg carbonate crystal lattice. This research was funded by the Ministry of Science and Higher Education of the Russian Federation.

Introduction
The study of mineral–microbe interactions under extreme conditions has become a cornerstone in understanding both the origins of life and its potential beyond Earth. Minerals play a crucial role as catalytic surfaces, nutrient sources, and preservers of biosignatures. The emerging field of astrobiomineralogy integrates geochemistry, microbiology, and space sciences to explore how such processes operate from Earth’s deepest biosphere to Martian and icy moon environments.

Methods
Following PRISMA guidelines, this systematic review analyzed 180 peer-reviewed studies (2010-2025) retrieved from Scopus and Web of Science. Studies focusing on microbial colonization, biomineralization, and mineral biosignatures in extreme or simulated extraterrestrial conditions were included. Bibliometric and co-word analyses were conducted using VOSviewer.

Results
Results reveal convergent patterns between terrestrial extremophiles (e.g., cyanobacteria, Deinococcus spp.) and their mineral substrates, particularly sulfates, phyllosilicates, and iron oxides, mirroring assemblages detected on Mars and Europa. Laboratory simulations demonstrate that microbial films induce mineralogical transformations that can persist as biosignatures detectable by Raman and X-ray spectroscopy.

Conclusions
Astrobiomineralogy bridges the gap between mineral sciences and space biomedicine, elucidating how life-mineral systems respond to radiation, desiccation, and low gravity. The synthesis highlights new frontiers for planetary exploration, life detection strategies, and potential biotechnological applications of extremophilic mineral interactions in human spaceflight contexts.

Accurate identification of alteration minerals in remote polar regions remains a significant challenge due to limited field accessibility, extreme environmental conditions, and the inherent spectral complexity of exposed lithologies. Satellite-based multispectral remote sensing, particularly using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), provides an efficient means of investigating surface mineralogy. However, ASTER data are often affected by spectral redundancy, sensor noise, and overlapping absorption features that reduce mineral classification accuracy. This study introduces a hybrid dimensionality reduction and spectral classification workflow integrating Minimum Noise Fraction (MNF), Independent Component Analysis (ICA), and Spectral Angle Mapper (SAM) algorithms to enhance lithological mapping in polar environments. The workflow was applied to ASTER imagery from South Victoria Land, Antarctica, where exposure of metamorphic and hydrothermally altered rocks offers an ideal setting for method evaluation. The MNF transform was first used to suppress noise and extract high-variance components, while ICA separated independent spectral sources representing unique mineralogical signatures. Subsequently, SAM classification calibrated with United States Geological Survey (USGS) reference spectra enabled precise identification of alteration minerals. The integrated MNF–ICA–SAM approach effectively discriminated key alteration minerals including alunite, kaolinite, jarosite, chalcedony, opal, and hematite, corresponding to diagnostic Al–OH, Fe–OH, and hydrous silica absorptions across the VNIR–SWIR spectrum. Comparative analyses demonstrate that the hybrid workflow significantly improves spectral separability and classification accuracy compared with single-method techniques. These results highlight the potential of integrated spectral processing as a robust, transferable, and data-driven framework for mineral mapping in polar terrains. The proposed methodology not only enhances the geological interpretability of ASTER imagery but also establishes a foundation for future integration with hyperspectral, UAV, and machine-learning-based systems in remote and data-limited environments.