Fluorite Crystals Research Articles

Room-temperature high-power laser emission of erbium-doped fluorite crystals at 2.8 µm.

We report on a high-power continuous-wave (CW) laser at 2.8 µm employing erbium (Er)-doped fluorite crystals as gain materials. With an optimized Er3+ ion concentration, thin "slab" geometry of the sample matching with the tailored pump beam profile and compensated negative thermal lens using a pair of concave mirrors cavity configuration, a highest power of 14.5 W is achieved from a dual-end-pumped Er:CaF2 laser, which, to the best of our knowledge, presents the record power from the room-temperature Er-bulk lasers in the 3-µm spectral range. In addition, 8.05 W output power is obtained from the Er:SrF2 laser with an RMS power stability of 0.35%. This work indicates that Er-doped fluorite crystals with large-scale available fabrication are promising candidates for high-power laser emission at ∼3 µm.

High-temperature thermal stability modification of (La0.2Y0.2Sm0.2Eu0.2Nd0.2)2Zr2O7 ceramic aerogel for enhanced thermal insulation performance

In this study, tetraethylorthosilicate and dimethyldiethoxysilane were employed to modify high-entropy (La0.2Y0.2Sm0.2Eu0.2Nd0.2)2Zr2O7 (5RE2Zr2O7) ceramic aerogels to address the challenges of particle agglomeration and pore structure collapse that occur during the calcination step in the preparation of high-entropy ceramic aerogels. Following the modification, the specific surface area of the ceramic aerogel increased significantly from 32.22 m2/g to 148.65 m2/g to 201.61 m2/g. Simultaneously, the formation of defective fluorite or pyrochlore crystal structures led to reduced thermal conductivity in the three types of high-entropy ceramic aerogels, namely 5RE2Zr2O7 (from 0.212 W m−1•K−1 to 0.139 W m−1•K−1), 5RE2ZSA (from 0.065 W m−1•K−1 to 0.049 W m−1•K−1), and DDS/5RE2ZSA (from 0.046 W m−1•K−1 to 0.039 W m−1•K−1), compared to their respective non-high-entropy counterparts. Balancing the porous structure of high-entropy ceramic aerogels with an adequate number of defective fluorite or pyrochlore crystals is challenging. Addressing this challenge is a key direction for advancing high-entropy aerogels in the future.

LITHIUM IN THE SUBSOIL OF UKRAINE Part 7. Genesis of lithium-bearing objects: rare-metal granites of the Ukrainian Shield

In the genetic sense, the following lithium-bearing objects have been characterized: vein Li-F granites and areal Liznykivsky granites of the Korosten pluton, rare-metal granites of the Kamianomohylsky complex, and ongonites of the Azov region. Most researchers consider Li-F granites as the final phase of magmatism of the Korostensky complex. Our conclusion differs from the mainstream view: vein Li-F granites of the pluton are products of crystallization of deep fluid flows that penetrated into the fissure cavities of the pluton rocks after the formation of magmatic massifs. In our opinion, the Liznykivsky granites are metasomatic formations that have developed on different-age rapakivi-like biotite-amphibole granites under the influence of deep fluid flows. The rare-metal granites of the Kamianomohylsky complex form three massifs in the Azov region: Kamiana Mohyla, Katerynivsky and Starodubivsky. Initially, all granite massifs were formed in the process of crystallization of magmatic melt, which later changed to metasomatic alteration of granites. Post-magmatic changes in granites that contributed to the development of rare metal mineralization include: 1) biotitization; 2) microclinization; 3) early albitization; 4) greisenization; 5) late albitization. According to the data of thermobarogeochemical studies, the stage-by-stage development of the post-magmatic alteration of the Kamiana Mohyla granitoids was substantiated and physicochemically characterized: microclinization (500-600 °С), albitization (430 °С), topazization (430 °С), silicification (430-440 °С), sulfidization (320 °С), late albitization (290-420 °С), crystallization of xenotime (200-210 °С), vein fluoritization and the growth of fluorite crystals (125-155 °С). The crystallization of ongonites took place at high temperature, which was 1140 °C at the final stage of quartz phenocrysts growth.

LITHIUM IN THE SUBSOIL OF UKRAINE Part 7. Genesis of lithium-bearing objects: rare-metal granites of the Ukrainian Shield

In the genetic sense, the following lithium-bearing objects have been characterized: vein Li-F granites and areal Liznykivsky granites of the Korosten pluton, rare-metal granites of the Kamianomohylsky complex, and ongonites of the Azov region. Most researchers consider Li-F granites as the final phase of magmatism of the Korostensky complex. Our conclusion differs from the mainstream view: vein Li-F granites of the pluton are products of crystallization of deep fluid flows that penetrated into the fissure cavities of the pluton rocks after the formation of magmatic massifs. In our opinion, the Liznykivsky granites are metasomatic formations that have developed on different-age rapakivi-like biotite-amphibole granites under the influence of deep fluid flows. The rare-metal granites of the Kamianomohylsky complex form three massifs in the Azov region: Kamiana Mohyla, Katerynivsky and Starodubivsky. Initially, all granite massifs were formed in the process of crystallization of magmatic melt, which later changed to metasomatic alteration of granites. Post-magmatic changes in granites that contributed to the development of rare metal mineralization include: 1) biotitization; 2) microclinization; 3) early albitization; 4) greisenization; 5) late albitization. According to the data of thermobarogeochemical studies, the stage-by-stage development of the post-magmatic alteration of the Kamiana Mohyla granitoids was substantiated and physicochemically characterized: microclinization (500-600 °С), albitization (430 °С), topazization (430 °С), silicification (430-440 °С), sulfidization (320 °С), late albitization (290-420 °С), crystallization of xenotime (200-210 °С), vein fluoritization and the growth of fluorite crystals (125-155 °С). The crystallization of ongonites took place at high temperature, which was 1140 °C at the final stage of quartz phenocrysts growth.

Effects of γ-radiation on fluorite crystals with a trivalent rare-earth ions

The paper considers phenomena of formation of color centers due to intrinsic and impurity defects in fluorite crystals activated by rare-earth ions Pr3+, Nd3+, Yb3+ under γ- and UV-radiation. Using CaF2 activated with Yb3+ and Pr3+ ions as an example, the work shows the effect of the photodynamic process on the photochemical resistance to formation of color centers of a Yb-containing crystal. It also demonstrates the effect of heating on the restoration of the original color of a Pr-containing crystal after γ-radiation as a result of change in the electronic state of praseodymium ions. The features of the formation of ions in the R2+ state in CaF2:R3+ (R3+ = Pr, Nd, Yb) crystals after γ-irradiation are investigated and explained, and quantitative estimates of the R3+→R2+ conversion processes are given.

Hydrothermal fluorite solubility experiments and mobility of REE in acidic to alkaline solutions from 100 to 250 °C

Fluorite is a common vein mineral in critical mineral deposits that are overprinted by hydrothermal processes. Its chemistry potentially reflects the mobilization conditions of rare earth elements (REE) in these hydrothermal fluids. However, the controls of temperature and fluid chemistry (i.e., pH and salinity) on fluorite solubility and coupled REE mobilization is still uncertain. In this study, batch-type experiments were conducted to investigate REE mobility in aqueous fluids controlled by the solubility of fluorite at temperatures between 100 and 250 °C. Natural fluorite crystals from Cooke’s Peak (New Mexico) were equilibrated with aqueous solutions of variable starting pH (2–10), salinities (0 and 15 wt.% NaCl), and initial REE concentrations (0 and 0.5 ppm). Reacted fluorite crystals display dissolution-controlled reaction textures (i.e., etch pits and dissolution channels along fluorite cleavage planes), which are most pronounced in experiments conducted in acidic fluids and at elevated temperature. Addition of NaCl considerably enhances fluorite dissolution, in both acidic and alkaline solutions, which is also reflected by the compositions of the reacted solutions displaying an increase in Ca, F, and REE concentrations. The overall fluorite solubility is controlled by the association behavior of HF0 at acidic conditions, and by the formation of Ca chloride, Ca hydroxyl, and Na fluoride complexes, which become enhanced by addition of NaCl, including in mildly acidic and alkaline solutions. Mass balance calculations were used to compare the REE stoichiometry of unreacted fluorite crystals with measured REE concentrations in the reacted solutions, which reveal several possible controls on REE mobility: i) stoichiometric release of REE upon fluorite dissolution; ii) precipitation of secondary REE-depleted fluorite; and/or iii) precipitation of secondary light REE fluorides, which were observed to form on fluorite crystals surfaces in the REE-doped experiments. Fluorite solubility constants (Ksp) were regressed with available low temperature solubility data using the equation logKsp = A + BT + C/T + Dlog(T) (T, temperature in K) valid between 25–250 °C. Fitted coefficients yield: A = 171.180, B = 0.01477, C = −7794.10, D = −64.6634. The experimental results have important implications for geochemical modeling of natural systems, where fluorine controls the mobility of REE, and can be enhanced by addition of NaCl in both acidic and alkaline solutions. Therefore, the salinity of magmatic-hydrothermal fluids plays a crucial role in mobilizing REE during the formation of hydrothermal fluorite veins.

In-situ cracking of oil into gas in reservoirs identified by fluid inclusion analysis: Theoretical model and case study

Natural gas formed by cracking of crude oil occurs in many basins globally. One of the key questions about such gas is whether it was produced in-situ—by cracking of crude oil in the reservoir itself—or whether it was generated elsewhere and accumulated in the reservoir subsequently. Fluid inclusions trapped in minerals can provide direct evidence regarding the genesis of natural gas, because these inclusions represent samples of the fluid present at the time of trapping, and/or resulting from subsequent modification of the trapped fluid. Here, we outline some theoretical aspects of the processes of cracking, trapping of fluid inclusions, and potential post-entrapment modifications, that lead to a model for identifying evidence for in-situ cracking of crude oil in reservoirs based on fluid inclusion assemblages (FIAs). This model divides the crude oil cracking process into three main stages, and considers the predicted evolution of fluids and possibility of forming FIAs. In the initial step, before cracking, only liquid oil and aqueous fluid are present in a reservoir, and two endmember types of inclusions may be formed. If cracking of crude oil occurs in the reservoir after these inclusions were trapped, then in-situ cracking of oil within the inclusions gives rise to systematic changes that can be interpreted through the lens of progressive thermal maturation, and hence provide direct evidence for cracking in the reservoir. In the second step, during cracking, the reservoir may host up to four fluid phases (residual oil, aqueous fluid, natural gas, and solid bitumen), and in theory as many as fourteen types fluid inclusions can be trapped; hence, complex FIAs comprising multiple fluid inclusion types can be considered evidence for trapping during in-situ cracking of crude oil in the reservoir. In the third and final step, after complete cracking, liquid oil has been exhausted and only natural gas, solid bitumen and aqueous fluid remain in the reservoir; at this stage, up to six fluid inclusion types can be trapped, which can be indicative of cracking. We applied this model to study the genesis of natural gas in the Sinian Dengying Formation reservoir of the Anyue Gas Field in the Sichuan Basin, southwest China. Six types of hydrocarbon-bearing primary fluid inclusions were observed in fluorite crystals formed during hydrocarbon accumulation. Notably, one type of inclusion showed consistent volumetric phase ratios of solid bitumen plus gas, suggesting that these inclusions originally captured crude oil, which subsequently underwent in-situ cracking in a closed system (within the inclusions) to generate fixed proportions of gas and solid bitumen. These FIAs are consistent with our model and suggest that the fluid inclusions trapped in fluorite can record the in-situ cracking of crude oil in the reservoir, and can therefore be used as direct evidence for the genesis of natural gas.

Probing luminescence of rare earth ions in natural pink fluorites using Raman microscopes

AbstractIt is well‐known that many natural fluorite crystals contain rare earth ions. In this work, a series of pink fluorites is studied to demonstrate that the use of Raman microscopes allows to obtain high resolution emission spectra of trivalent rare earth ions. These emission spectra allow in principle to distinguish differences of the local environment related to the charge compensation necessary when the divalent Ca ion is replaced by a trivalent rare earth ion. The pink fluorites from the Alps have grown under hydrothermal conditions, which results in the incorporation of oxygen into the fluorite. In one sample from Juchlistock (Bern, Switzerland), a trigonal Eu3+‐oxygen center is clearly identified. Interestingly, a pink fluorite from China (Huanggang Mine, Inner Mongolia, China) shows a quite different emission spectrum from those of the Alps. In this sample, the emission bands have much lower relative intensity compared with the intensity of the CaF2 Raman peak, indicating a much lower content of Er3+ and Ho3+ which emit around 520–560 nm. Literature examples show that Raman measurements of differently colored fluorite crystals can also present sharp rare earth spectra. By extension, it is thus possible to observe and identify emission spectra of rare earth ions in other mineral crystals using Raman microspectroscopy and avoid misassignments of the corresponding observed Raman spectra.

Fluorite used in ancient Chinese glassmaking during the 10th to 12th centuries: Evidence from glass products excavated in the capital city site of the Liao dynasty

AbstractThis study conducts a scientific analysis of six pieces of glass artefacts unearthed from Shang‐Jing, the capital city of the Liao dynasty. The test methods include laser ablation inductively coupled plasma atomic emission spectrometry, scanning electron microscopy with energy dispersive spectrometry and X‐ray diffraction. In terms of chemical composition, one sample is K2O‐CaO‐SiO2 glass and the other samples belong to the K2O‐CaO‐PbO‐SiO2 system. Fluorite crystals were discovered in some of the samples. Ancient literature and some analysis of other archaeological materials suggest that using fluorite minerals as opacifiers in both of these kinds of glass is a traditional technique in China. Therefore, fluorite minerals are likely to be one of the raw materials in the studied glass artefacts. This study provides new information about the glass‐making recipe from the 10th to 12th centuries in China and further sheds some light on the development of ancient Chinese glass production.

Rare-earth induced nonlinear structural evolutions in fluorite solid solution crystals

The modification of chemical composition to improve desired material parameters is an effective method in materials science and engineering. In this work, Ca1-xSrxF2 solid solution is chosen as the subject. Nd3+ and Y3+ ions are used as dopants. We have found that spectral properties of Nd3+:Ca1-xSrxF2 and Nd3+,Y3+:Ca1-xSrxF2 crystals vary nonlinearly with the ‘x’. The X-ray diffraction (XRD) patterns and the density functional theory (DFT) calculations on Ca1-xSrxF2 solid solutions have ruled out the influence of matrix crystals on spectral properties. The rare-earth monomer centers of C4v or C3v symmetry, and the high order clusters are modeled. The calculated results show, that thermodynamic stabilities of the centers vary nonlinearly. Temperature-dependent dielectric losses and the results of projected density of states (pDOS) calculations also show nonlinear dependency. The nonlinearly evolved local structures from cubic to square antiprism sublattice cause the nonlinear variation of spectral properties. The methodology of rare-earth induced nonlinear structural evolutions is then proposed, which is useful for exploring new materials.

The host driven local structures modulation towards broadband photoluminescence in neodymium-doped fluorite crystal

The lanthanide-doped materials have been widely used. The well shielded f orbitals make the f-f transitions be sharp lines and non-tunable which are hurdles to the generation of ultrashort lasers. In the work, the spectral properties of neodymium-doped fluorite crystal are modulated to be varied significantly with the compositions of the crystal. The first principles calculation is combined and the results show that there exist two types of centers of cubic and square antiprism sublattice clusters. The concentrations of the centers could be tuned in large scale by adjusting the compositions of the host. Then tunable broadband photoluminescence is obtained which is promising for the generation of ultrashort pulses. The designing of new ultrafast laser materials is highly expected in the rare-earth-doped fluoride crystals.

Structural properties of MeF2 (Me=Ca, Sr, Ba) crystals doped with rare-earth ions RE3+/ RE2+: ab initio route

In this paper, we simulated the structural properties of the cubic impurity centers RE3+/RE2+ (RE = Rare earth) in fluorites crystals MeF2 (Me = Ca, Sr, Ba) by ab initio route. Doping fluorite crystals with the rare-earth ion RE3+/RE2+, this replaces the cation Me2+ and determines distortions of the MeF2 crystal lattice near the impurity center. These distortions characterized by radial and angular displacements of anions were investigated by ab initio method. We have employed the MO LCAO approach, in the frame of DFT method as it is implemented in the CRYSTAL17 software for periodic systems, using the hybrid functional PBE0. Since the main contribution to the crystal field on the rare-earth impurity ions comes from its nearest neighbors, the distance “rare-earth ion – ligand” and the radial and angular coordinates of the RE3+/RE2+ ions at several coordination spheres of anions near the impurity centers are calculated. Also, the changes in the local compressibility of the cationic and anionic sublattice next to the impurity ion are determined. The obtained results were discussed, compared with available experimental data and other theoretical results, and the agreement is quite satisfactory.

Refinement of the Congruently Melting Composition of Nonstoichiometric Fluorite Crystals Ca1-xYxF2x (x = 0.01–0.14)

The concentration series of nonstoichiometric crystals Ca1–xYxF2+x (x = 0.01–0.14) was obtained from a melt by directional crystallization to refine the composition of the temperature maximum on the melting curves. A precision (±9 × 10−5 Å) determination of lattice parameters of the Ca1–xYxF2+x crystals with the structure of fluorite (sp. gr. Fm-3m) was performed, and a linear equation of their concentration dependence was calculated: a(x) = 5.46385(5) + 0.1999(4) x. The distribution of yttrium along the crystals Ca1–xYxF2+x, the content of which is determined by the precision lattice parameters, is studied. The congruently melting composition x = 0.105(5) of the Ca1–xYxF2+x phase is refined by the method of directional crystallization.

Uranium incorporation in fluorite and exploration of U–Pb dating

Abstract. The age of ore deposits constitutes a decisive element in understanding their formation. Deciphering their precise chronology may be a challenge in the absence of mineral phases that can be dated by conventional geochronometers. Fluorite is very common either as the major or accessory mineral in a wide variety of ores and may provide information regarding the origin and timing of mineralizing fluid flows. In this contribution, we explore U–Pb dating on fluorite crystals from the world-class carbonate strata-bound fluorite ore of Pierre-Perthuis in Burgundy (Morvan massif, France). The uranium distribution within fluorite is mapped using induced fission-track and synchrotron radiation X-ray fluorescence nano-imaging, showing that higher U content is measured in an overgrowth of fluorite (Flog) as a discrete band. Preservation of a micrometer-thick zonation in U, associated with other substituted elements such as Sr, Y, Fe and Zr, implies that neither solid-state diffusion nor dissolution–recrystallization occurred. These U-bearing external fluorite overgrowths contain solid inclusions of about 30 µm globular pyrite crystals with a mean δ34S of −23.6 ± 0.4 ‰V-CDT. We propose that the U incorporation in the fluorite lattice results from the development of a redox front during bacterial sulfate reduction. Flog generation sampled and analyzed by laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) on four different crystals provides identical U–Pb ages within the limits of analytical uncertainty. Considered altogether, these four crystals yield an age estimate of 40.0 ± 1.7 Ma, not corrected for matrix-related elemental fractionation. Our results show that fluorite LA-ICP-MS U–Pb geochronology has potential for dating distinct crystal growth stages, although further research should be conducted to evaluate its accuracy.

Tectonic-controlled sediment-hosted fluorite-barite deposits of the central Alpine-Himalayan segment, Komsheche, NE Isfahan, Central Iran

The Triassic carbonate-hosted Komsheche deposit of Central Iran, in the central segment of the Alpine-Himalayan orogen, is an ideal test site for tectonic-controlled ore formation processes. The mineralization consists of barite, fluorite, minor galena, and subordinate pyrite and chalcopyrite, and gangue minerals of dolomite, quartz, siderite, organic material, and calcite. Radiogenic isotopes point to multiple sources. The Pb composition in galena indicates an origin of Pb from upper crustal material. Barite generations II and III have 87Sr/86Sr = 0.709147 to 0.709595, higher than those of the Triassic host rocks, but more similar to the composition of Paleozoic basement and Miocene seawater. The chondrite-normalized REE patterns of fluorite crystals are subhorizontal with slightly negative or no Eu anomalies and weak enrichment of the MREE. The composition of fluorite, combined with the Pb and Sr isotopic data, reflects a process of fluid-rock interaction for the mineralizing fluids along favorable lithological and structural sites. The anatomy of the Komsheche deposit is dominantly controlled by the fault architecture. The (U-Th)/He thermochronology of fluorite grains from Komsheche yields a range of ages from Early Cretaceous to Pliocene, in concordance with the multistage tectonic evolution of the Zagros orogen. Our proposed model is that the barite-fluorite deposits of Central Iran formed during alternating tectonic episodes, including Early Cretaceous extension, post-Cretaceous - Oligocene compression, Oligo-Miocene extension, and finally Miocene and younger compression. The spatial-temporal correlation of our results with those of other studies along the Alpine-Himalayan and Atlas Mountains domains reveal a tight coupling between tectonism and formation of sediment-hosted fluorine-bearing (±Ba, Pb±Zn) deposits throughout the Mesozoic and Cenozoic. Incipient deposits formed in the Mesozoic along extension-related basement faults and subsequent deposit generations formed during reactivation / inversion of pre-existing structures.

Layer-dependent band gaps and dielectric constants of ultrathin fluorite crystals

Structure, stability, and thickness-dependent properties of ultrathin crystalline calcium fluoride (fluorite, CaF2) with promising application in two-dimensional (2D) field-effect transistors have been investigated by ab initio calculation. The enthalpy of formation and phonon calculations demonstrate the phase and dynamical stabilities of ultrathin CaF2. The verification of the stability for mono- and few-layer CaF2, which can be referred as idealized reference systems for ultrathin layers of CaF2 grown by CVD/MBE or other growth methods, indicates that 2D materials with thickness varying from monolayer to bulk limit can be obtained for CaF2 with hard-bond interlayer interaction. The strong chemical bonds between the adjacent layers in few-layer CaF2 are displayed, which is completely different from the weak interlayer van der Waals (vdW) interaction in layered MoS2 or graphite, resulting in the increased frequencies of low optical phonon modes due to the stronger interlayer restoring force compared to vdW layered crystals. Through the calculations of electronic structure and dielectric response, the layer-dependent band gaps and dielectric constants of ultrathin CaF2 are investigated, and the material can be designed by adjusting the layer thickness to fit practical applications. The large and tunable band gaps and superior dielectric properties of ultrathin CaF2, demonstrated from DFT-HSE06 and linear response calculations, signify the prospect of ultrathin CaF2 as an emerging 2D dielectric layer.

Efficient 2.76 μm continuous-wave laser in extremely lightly Er-doped CaF2 single-crystal fiber

Rare-earth-doped single-crystal fibers are promising for use in developing high-power lasers due to their improved thermal management and excellent optical properties. In this work, Er-doped CaF2 single-crystal fibers of good optical quality were successfully grown by a modified temperature gradient technique method. For the characteristic clustering effect of trivalent rare earth ions within fluorite crystals, a high-efficiency 2759.1 nm laser was obtained in a 1 at.% Er-doped CaF2 crystal fiber. Moreover, even in an extremely lightly doped crystal fiber with a concentration of 0.5 at.%, a continuous-wave (CW) 2758.2 nm laser with a slope efficiency of 20% was achieved. Without the cascade lasing, a ~2.8 μm CW laser operation pumped by a laser diode was first demonstrated in the case of a Er3+ doping concentration below 1 at.%. The results suggest that Er-doped CaF2 single-crystal fibers are promising for mid-infrared lasers.

Correlation between the Enthalpy of Melting and the Enthalpy of Anti-Frenkel Defect Formation in Homologous MF2 Fluorite Crystals

The homologous series of MF2 (M = Ca2+, Sr2+, Ba2+, Cd2+, Pb2+, Sm2+, Eu2+, Yb2+, Ra2+, and Hg2+) crystals having the fluorite structure (CaF2, space group $$Fm\bar {3}m,$$ Z = 4) holds an important place in fluoride materials science. A correlation between the enthalpy of anti-Frenkel defect formation (Hf) and the enthalpy of melting (Hm) was elucidated for CaF2, SrF2, BaF2, and PbF2 based on the most reliable experimental data set: Hf (kJ/mol) = 8.6Hm (kJ/mol) or Hf (eV) = 0.089Hm (kJ/mol). Experimentally determined Hm values were used to calculate the enthalpies of defect formation Hf = 2.0 and 2.6 eV for CdF2 and EuF2 crystals, respectively. The correlation is prognostic for the whole series of homologous MF2 fluorite crystals and for SrCl2.

Tourmaline as a Recorder of Ore-Forming Processes in the Xuebaoding W-Sn-Be Deposit, Sichuan Province, China: Evidence from the Chemical Composition of Tourmaline

The Xuebaoding W-Sn-Be deposit located in the Songpan-Ganze Orogenic Belt (Sichuan Province, China) is a hydrothermal deposit with less developed pegmatite stage. The deposit is famous for the coarse-grained crystals of beryl, scheelite, cassiterite, apatite, fluorite, muscovite, and others. The orebody is spatially associated with the Pankou and Pukouling granites hosted in Triassic marbles and schists. The highly fractionated granites are peraluminous, Li-Rb-Cs-rich, and related to W-Sn-Be mineralization. The mineralization can chiefly be classified based on the wallrock and mineral assemblages as muscovite and beryl in granite (Zone I), then beryl, cassiterite and muscovite at the transition from granite to triassic strata (Zone II), and the main mineralized veins composed of an assemblage of beryl, cassiterite, scheelite, fluorite, and apatite hosted in metasedimentary rock units of marble and schist (Zone III). Due to the stability of tourmaline over a wide range of temperature and pressure conditions, its compositional variability can reflect the evolution of the ore-forming fluids. Tourmaline is an important gangue mineral in the Xuebaoding deposit and occurs in the late-magmatic to early-hydrothermal stage, and can thus be used as a proxy for the fluid evolution. Three types of tourmalines can be distinguished: tourmaline disseminations within the granite (type I), tourmaline clusters at the margin of the granite (type II), and tourmalines occurring in the mineralized veins (type III). Based on their chemical composition, both type I and II tourmalines belong to the alkali group and to the dravite-schorl solid solution. Type III tourmaline which is higher in X-site vacancy corresponds to foitite and schorl. It is proposed that the weakly zoned type I tourmalines result from an immiscible boron-rich aqueous fluid in the latest stage of granite crystallization, that the type II tourmalines showing skeletal texture directly formed from the undercooled melts, and that type III tourmalines occurring in the mineralized veins formed directly from the magmatic hydrothermal fluids. Both type I and type II tourmalines show similar compositional variations reflecting the highly fractionated Pankou and Pukouling granites. The higher Ca, Mg, and Fe contents of type III tourmaline are buffered by the composition of the metasedimentary host rocks. The decreasing Na content (<0.8 atoms per formula unit (apfu)) and increasing Fe3+/Fe2+ ratios of all tourmaline samples suggest that they precipitated from oxidized, low-salinity fluids. The decreasing trend of Al content from type I (5.60–6.36 apfu) and type II (6.01–6.43 apfu) to type III (5.58–5.87 apfu) tourmalines, and associated decrease in Na, may be caused by the crystallization of albite and muscovite. The combined petrographic, mineralogical, and chemical characteristics of the three types of tourmalines thus reflect the late-magmatic to early-hydrothermal evolution of the ore-forming fluids, and could be used as a geochemical fingerprint for prospecting W-Sn-Be mineralization in the Xuebaoding district.

Occurrence and Composition of Columbite-(Fe) In the Reduced A-Type Desemborque Pluton, Graciosa Province (S-SE Brazil)

Columbite-(Fe) is a post-magmatic accessory mineral occurring within syenogranites and greisens from the Desemborque Pluton. The petrographic (SEM) and geochemical (EPMA and LA-ICPMS) examination of this mineral shows two distinct textural types within both the rocks, named columbite-1 and columbite-2. The columbite-1 type is characterized by zoned crystals with two stages of crystallization: i) An early Nb-rich cores with low Ta/(Ta + Nb) and Mn/(Mn + Fe) ratios (0.02–0.08 and 0.17 to 0.21 apfu, respectively), and ii) a later Ta-rich rims with higher Ta/(Ta + Nb) ratios (0.11–0.26) and similar Mn/(Mn + Fe) ratios (from 0.14 to 0.22) relative to the former cores. On the other hand, the columbite-2 type is defined by irregular crystals with patchy textures and very low Ta/(Ta + Nb) ratios (0.008–0.038) and moderate Mn/(Mn + Fe) ratios between 0.20 and 0.38. Trace element compositions of all columbite-(Fe) crystals are relatively enriched in HREEs and HFSEs; however, the columbite-2 presents higher abundances of REEs, Y, Th, U, Pb, Sc, and Sn relative to the columbite-1. This study highlights a unique hydrothermal origin for both the columbite types, but the textural relations of the columbite-2 crystals indicated that its formation is related to fluid-induced alterations of post-magmatic fluorite and/or cassiterite crystals at the final stage of the post-magmatic evolution.