Partition of Sr,Ba, Ca,Y, Eu2+, Eu3+, and other REE between plagioclase feldspar and magmatic liquid: an experimental study

Plagioclase feldspar/magmatic liquid partition coefficients for Sr, Ba, Ca, Y, Eu²⁺, Eu³⁺ and other REE have been determined experimentally at 1 atm total pressure in the temperature range 1150–1400°C. Natural and synthetic melts representative of basaltic and andesitic bulk compositions were used, crystallizing plagioclase feldspar in the composition range An₃₅–An₈₅. Partition coefficients for Sr are greater than unity at all geologically reasonable temperatures, and for Ba are less than unity above approximately 1060°C. Both are strongly dependent upon temperature. Partition coefficients for the trivalent REE are relatively insensitive to temperature. At fixed temperature they decrease monotonically from La to Lu. The partition of Eu is a strong function of oxygen fugacity. Under extreme reducing conditions D_Eu approaches the value of D_Sr.     

The causes and petrological significance of cathodoluminescence emissions from alkali feldspars

The cathodoluminescence (CL) of a variety of alkali feldspars from South Greenland has been examined in an attempt to understand the causes of the CL and its petrological significance. Analytical methods have included CL spectroscopy, secondary ion mass spectrometry (SIMS) and electron paramagnetic resonance (EPR) to correlate the presence of certain CL emissions to the presence of certain trace element and point defects. Where possible, blue and red luminescent fractions of the same rock samples have been separated and analysed separately. Blue CL appears to relate to the presence of electron holes on bridging oxygens, particularly on the Al-O-Al bridge, as determined from EPR studies. No correlation with other proposed activators for blue CL such as Eu²⁺, Ga³⁺ or Ti⁴⁺ was observed. Some blue luminescent feldspars also have an emission in the infra-red (IR), invisible during normal visible CL petrography. The red and IR CL emissions correspond to features in EPR spectra attributed to Fe³⁺ and support previous suggestions that Fe³⁺ is related to this emission. However, our studies indicate that the visible red CL relates specifically to Fe³⁺ on the T1 site, whereas the equivalent CL from disordered feldspars lies in the IR. The difference between red and IR CL emissions therefore relates to the state of Fe³⁺ order across the tetrahedral sites. These data allow more meaningful interpretations of CL as a petrographic tool in alkali feldspar-bearing rocks. Received: 5 March 1998 / Accepted: 23 November 1998     

The red luminescence emission of feldspar and its wavelength dependence on K, Na, Ca – composition

Summary ?Feldspar specimens covering the whole Or–Ab–An ternary have been investigated by cathodoluminescence (CL), photoluminescence (PL), radioluminescence (RL) and radiophosphorescence (RP) spectrometry. A red luminescence emission, which is commonly explained by Fe³⁺ lattice defects, is a characteristic feature of all the spectra. Different shifts of the peak-wavelength between ∼680–750 nm (1.82–1.65 eV) were observed with varying feldspar composition. Despite the dependence of the peak position on the Ca/Na ratio, initially described for CL in the 1970s, there is also a shift induced by changing NaK composition. The observed effects can be explained by known relations that the peak position of the red luminescence emission in feldspars can be affected both by the structural state of the feldspar and the site occupancy of the trivalent iron. In the case of alkali feldspars another factor may influence the peak-shift. The incorporation of the larger potassium ion causes non-linear variations of the cell dimensions and therefore Fe–O bond distance. The behaviour of the red peak-shift dependent on the feldspar composition is not equal for all types of luminescence investigated. This is most likely caused by the different luminescence excitation mechanism. Received December 3, 2001; revised version accepted March 25, 2002     

Partitioning of rare earth elements between an aqueous chloride fluid phase and melt during the decompression-driven degassing of granite magmas

This paper reports the results of numerical simulation for the behavior of rare earth elements (REE) during decompression degassing of H₂O- and Cl-bearing granite melts at pressures decreasing from 3 to 0.5–0.3 kbar under near isothermal conditions (800 ± 25°C). Fluid phase in equilibrium with the melt contains mainly chloride REE complexes, and their behavior during magma degassing is, therefore, intimately related to the behavior of chlorine. It was shown that the contents and distribution patterns of REE in the aqueous chloride fluid phase formed during decompression vary considerably depending on (1) the contents of volatiles (Cl and H₂O) in the initial melt, (2) the redox state of the magma, and (3) the dynamics of fluid phase separation from magmas during their ascent toward the Earth’s surface. During decompressiondriven degassing, the contents of both Cl and REE in the fluid decrease, especially dramatically under opensystem conditions. The REE patterns of the fluid phase compared with those of the melt are characterized by a higher degree of light to heavy REE fractionation. A weak negative Eu anomaly may be present in the REE patterns of Cl-rich fluids formed during the early stages of degassing at relatively high pressures. At a further decrease in pressure and Cl content in the fluid, it is transformed into a positive Eu anomaly increasing during decompression degassing. Such an anomalous behavior of Eu during degassing is related to its occurrence in magmatic melts in two valence states, Eu³⁺ and Eu²⁺, whereas the other REE occur in melts mainly as (REE)³⁺. The Eu³⁺/Eu²⁺ ratio of melt is controlled by the redox state of the magmatic system. The higher the degree of melt reduction, the more pronounced the anomalous behavior of Eu during decompression degassing. The amount of REE extracted by fluid from melt during various stages of degassing does not significantly influence the content and distribution patterns of REE in the melt.     

The luminescence properties of rare-earth ions in natural fluorite

For the first time, the luminescence properties of Pr³⁺, Nd³⁺ and Tm³⁺ and Yb³⁺ ions in fluorite crystal have been obtained by steady-state measurements. In addition, the luminescence spectra of Ce³⁺, Sm²⁺, Sm³⁺, Dy³⁺, Er³⁺ and Yb³⁺ were measured. It was pointed out that λ_exc.?=?415?nm is most suitable for measuring the Ho³⁺ emission beside the Er³⁺. The emission of trivalent holmium and erbium ions was measured independently using time-resolved measurements and tentative assignment of luminescence lines to C _3v and C _4v symmetry sites was proposed. Besides for natural fluorite crystal, the transitions between Stark energy levels of lanthanide ions were presented.     

Electron paramagnetic resonance study of the site preferences of Gd3+ and Eu2+ in polycrystalline silicate and aluminate minerals

Electron paramagnetic resonance (EPR) measurements were made on Gd³⁺ and Eu²⁺ ions in polycrystalline samples to determine the nature of the sites occupied by those ions in mineral structures. Both Gd³⁺ and Eu²⁺ ions were incorporated at Ca²⁺ structural sites in β-Ca₂SiO₄, pseudo-CaSiO₃, CaMgSiO₄, CaMgSi₂O₆, hex-CaAl₂Si₂O₈, CaAl₂O₄, and Ca₃Al₂O₆. For tri-CaAl₂Si₂O₈, Eu²⁺ was incorporated at a Ca²⁺ site and Gd³⁺ was incorporated at a site where the crystalline electric field was disordered. That difference in behavior may contribute to the anomalous behavior of Eu in plagioclase feldspar. Both Gd³⁺ and Eu²⁺ were incorporated as aggregates or clusters of those ions in Mg₂SiO₄ and clino-MgSiO₃.     

Natural fluorite emitting yellow fluorescence under UV light

Many mineralogists believe that fluorite emits violet fluorescence under UV light, but a special fluorite from Japan emits yellow fluorescence under UV light. The analysis by inductively coupled plasma-mass spectrometry (ICP-MS) shows that this fluorite includes high concentrations of Dy together with various rare-earth (RE) impurities other than Pm and Eu. Photoluminescence (PL) emission and excitation spectra of the fluorite are investigated at 10, 80 and 300 K. The origin of yellow fluorescence is attributed to the electronic transition within Dy³⁺. Profiles of the PL and excitation spectra depend on the excitation wavelength and on the observation wavelength, respectively. The obtained spectra are ascribed to the RE ions Ce³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Sm²⁺ and Yb²⁺ in the fluorite. In natural fluorite, the low concentration of Eu enables us to observe the bright fluorescence characteristic of trivalent RE ions, instead of the bluish violet fluorescence due to Eu²⁺.     

He+ ion implantation and electron irradiation effects on cathodoluminescence of plagioclase

Cathodoluminescence (CL) spectra of unirradiated, He⁺ ion-implanted and electron-irradiated plagioclase minerals contain the following emission bands: (1) below 300 nm due to Pb²⁺, (2) at ~320 and ~350 nm to Ce³⁺, (3) at 380–420 nm to Eu²⁺, Ti⁴⁺ and/or Al–O^?–Al/Ti defects, (4) at 560–580 nm to Mn²⁺ and (5) at 720–760 nm to Fe³⁺. During the implantation of He⁺ ion, much of their energy may be dissipated by partial destruction and strain of the feldspar framework, resulting in quenching of CL. Deconvolution of CL spectra acquired from albite and oligoclase reveals an emission component at 1.86 eV (666 nm) assigned to a radiation-induced defect center associated with Na⁺ atoms. As its intensity increases with radiation dose, this emission component has potential for geodosimetry and geochronometry. Electron irradiation causes Na⁺ migration in plagioclase, and then a considerable reduction in intensity of emissions assigned to impurity centers, which is responsible for an alteration in the energy state or a decrease in luminescence efficiency following the change of activation energy. Emission intensity at 1.86 eV positively correlates with electron irradiation time for unimplanted and He⁺ ion-implanted albite and oligoclase, but negatively for the implanted albite above 1.07 × 10^?4 C/cm². It implies that radiation halo produced by α-particles should not be measured using CL spectroscopy to estimate β radiation dose on albite in the high radiation level.     

Quantitative cathodoluminescence (CL) spectroscopy of minerals: possibilities and limitations

Summary ?The luminescence spectrum of a mineral contains complex information related to the intrinsic crystal and the defect structure. For quantitative analysis of cathodoluminescence (CL) the spectra have to be deconvoluted by fitting and filtering procedures to identify and measure individual peaks. Peak-width, peak-position and transition probability of the luminescence centres are influenced by effects such as interactions within the defects themselves, and interaction between defects and the surrounding crystal lattice. For calcite and feldspar a linear correlation between the defect concentration of manganese and the Mn²⁺-activated CL-intensity is documented. Combined Micro-Particle Induced X-ray Emission (μ-PIXE) and CL-spectroscopy analyses of REE-doped synthetic calcite suggest a linear correlation between REE-activated CL intensity and REE-concentration at REE-concentration levels below approximately 500 ppm. Sensitising and quenching by other REE are dominant effects yielding strong variations in the correlation between the REE-activated CL-intensity and the REE-content. Received December 6, 2001; revised version accepted May 10, 2002     

Geochemical behavior of rare earth elements in hydrothermally altered rocks of the Kuroko mining area,Japan

In this study we analyzed the chemical composition of hydrothermally altered dacite and basalt from the Kuroko mining area, northeastern Honshu, Japan, by REE (rare earth element). Features of rare earth element analyses include: (1) altered footwall dacite exhibits a negative Eu anomaly compared with fresh dacite, suggesting preferential removal of Eu²⁺ from the altered dacite via hydrothermal solutions, (2) altered hangingwall dacite and basalt and dacite and basalt adjacent to ore deposits exhibit positive Eu anomalies compared with fresh dacite and basalt, suggesting addition of Eu²⁺ from hydrothermal solutions, (3) LREE ratio (∑LREE/∑REE) from altered dacite of chlorite–sericite zone and K-feldspar zone show a negative relationship with δ¹⁸O, and La/Sm ratios show a positive correlation with the K₂O index. These trends indicate the addition of light rare earth elements such as La to the altered dacite from hydrothermal solution and/or leaching of heavy rare earth elements such as Sm and Yb, (4) Principal component analysis (PCA) indicates that light rare earth elements enrichment is related to the formation of sericite zone near the Kuroko deposits but not to the formations of chlorite and K-feldspar zones, and (5) The correlations among REE features (LREE ratio, MREE ratio, HREE ratio, Eu/Eu?), δ¹⁸O and K₂O index are not found for montmorillonite zone, mixed layer clay mineral zone and mordenite zone. Therefore, it is inferred that sericite, chlorite and K-feldspar alterations are related to the Kuroko and vein-type mineralization, but montmorillonite and mordenite alterations are not related to the mineralizations, and probably they formed at the post-mineralization stage.     

An experimental study of hydroxo complex formation in basic and near-neutral solutions of rare-earth elements and yttrium at 25 ºC

We have studied the hydrolytic behavior of Y^3 + and trivalent ions of rare earth elements in aqueous solutions at 25 ºC. The stepwise stability constants of hydroxide complexes were measured by spectrophotometry, using m-cresol purple and 1-(2-pyridylazo)-2-naphthol as pH indicators at an ionic strength no more than 0.0005. The results showed that at pH ranging between 6.0 and 11.0 in freshly prepared solutions of REE trichlorides, lanthanides are presented as Ln3 +, Ln(OH)2 +, Ln(OH)2, and Ln(OH)3. The plots of the formation constants of + 0 monohydroxo complexes of 4f n ions M^3 + versus atomic number Z deviate from smooth ones and consist of four convex curves. This phenomenon is also observed in normalized spectra of REE concentrations in natural objects and is known as the tetrad effect. The obtained data give an insight into the relationship between REE complex formation and REE fractionation in geochemical processes and can be used for physicochemical modeling of geochemical systems.     

Luminescence of Eu<Superscript>2+</Superscript> in feldspars from muscovite pegmatites

The data on photoluminescence (PL) that precisely detects Eu²⁺ centers and X-ray luminescence (XL) were compared for plagioclases and potassium feldspars in 21 samples from muscovite pegmatites of the Mama region. The Eu contents determined in 10 samples vary from 10^?4 to 10^?6 wt %. Europium occurs mainly as bivalent species that replaces Sr²⁺, Ca²⁺, and Ba²⁺. Eu is gained in the products of early crystallization, and its relative amounts decrease by an order of magnitude in the course of pegmatite formation down to complete disappearance in late generations of feldspars. It is shown that Eu²⁺ can be detected in XL spectra, and the Eu²⁺ content can be determined in qualitative terms, for instance, by the intensity of radiation band 400–420 nm in plagioclase.     

Inhomogeneous distribution of REE in scheelite and dynamics of Archaean hydrothermal systems (Mt. Charlotte and Drysdale gold deposits, Western Australia)

Scheelite is a widespread accessory mineral in hydrothermal gold deposits, and its rare earth element (REE) patterns and Nd and Sr isotopic compositions can be used to constrain the path and origin of the mineralising fluids and the age of the hydrothermal activity. Micro-analyses by laser ablation high resolution inductively coupled mass spectroscopy and cathodoluminescence imaging reveal a very inhomogeneous distribution of the REE in single scheelite grains from the Mt. Charlotte and Drysdale Archaean gold deposits in Western Australia. Two end-member REE patterns are distinguished: type I is middle REE (MREE)-enriched, with no or minor positive Eu-anomaly, whereas type II is flat or MREE-depleted with a strong positive Eu-anomaly. The chemical inhomogeneity of these scheelites is related to oscillatory zoning involving type I and type II patterns, with zone widths varying from below 1 to 200 μm. Intra-sectorial growth discontinuities, syn-crystallisation brittle deformation, and variations in the relative growth velocities of crystallographically equivalent faces suggest a complex crystallisation history under dynamic hydraulic conditions. The co-existence of MREE-enriched and MREE-depleted patterns within single scheelite crystals can be explained by the precipitation of a mineral which strongly partitions MREE relative to light and heavy REE. Scheelite itself has such characteristics, as does fluorapatite, which is locally abundant and has REE contents similar to that of scheelite. In this context, the systematic increase of the Eu-anomaly between type I and type II patterns is produced by the difference between the partition coefficients of Eu²⁺ and Eu³⁺, and not by fluid mixing or redox reactions. Consequently, the high positive Eu-anomaly typical of scheelite from gold ores may not necessarily be inherited from the hydrothermal fluid, but may reflect processes occurring during ore deposition. This case study demonstrates that in hydrothermal systems characterised by low REE concentrations in the fluid, and by the precipitation of a REE-rich mineral which strongly fractionates the REE, the REE patterns of such a mineral will be highly sensitive to the dynamics of the hydrothermal system. Received: 1 November 1999 / Accepted: 4 February 2000     

Geochemical behavior and speciation modeling of rare earth elements in acid drainages at Sarcheshmeh porphyry copper deposit,Kerman Province,Iran

Acid mine drainage is a major source of water pollution in the Sarcheshmeh porphyry copper mine area. The concentrations of heavy metals and rare earth elements (REEs) in the host rocks, natural waters and acid mine drainage (AMD) associated with mining and tailing impoundments are determined. Contrary to the solid samples, AMDs and impacted stream waters are enriched in middle rare earth elements (MREEs) and heavy rare earth elements (HREEs) relative to light rare earth elements (LREEs). This behavior suggests that REE probably fractionate during sulfide oxidation and acid generation and subsequent transport, so that MREE and HREE are preferentially enriched. Speciation modeling predict that the dominant dissolved REE inorganic species are Ln³⁺, Ln(SO₄)₂⁻, LnSO₄⁺, LnHCO₃²⁺, Ln(CO₃)₂⁻ and LnCO₃⁺. Compared to natural waters, Sarcheshmeh AMD is enriched in REEs and SO₄²⁻. High concentrations of SO₄²⁻ lead to the formation of stable LnSO₄⁺, thereby resulting in higher concentrations of REEs in AMD samples. The model indicates that LnSO₄⁺ is the dissolved form of REE in acid waters, while carbonate and dicarbonate complexes are the most abundant dissolved REE species in alkaline waters. The speciation calculations indicate that other factors besides complexation of the REE's, such as release of MREE from dissolution and/or desorption processes in soluble salts and poorly crystalline iron oxyhydroxy sulfates as well as dissolution of host rock MREE-bearing minerals control the dissolved REE concentrations and, hence, the MREE-enriched patterns of acid mine waters.     

Radiation defects in feldspars

Various radiation defects were characterized and analyzed by electron paramagnetic resonance (EPR) in 15 feldspars of different compositions after X-ray irradiation. A hole center on oxygen adjacent to two aluminum ions is formed in most feldspars, except those with very high An content. Since the hole is not localized at room temperature, clusters of more than two Al must be present in all feldspars in amounts of at least 100 ppm. Less frequent radiation defects are trivalent titanium and holes on oxygen ions adjacent to a small divalent ion of a yet unidentified nature on a T site with Si and in some cases also Pb as further neighbors. The directions of the magnetic axes for these centers allowed their assignment to specific sites in the feldspar structure. Characteristic absorption and thermolu-minescence emission bands could also be assigned to these centers. Their properties are remarkably independent of composition and Al, Si disorder of the feldspars.     

Luminescent properties of natural barite: Evidence for its genesis

The study of radiation of intrinsic and impurity excitations in natural barite showed that the patterns of BaSO₄ luminescence were mostly controlled by the presence of the [SO₄] anion complex. Several types of self-radiation were registered including those at the expense of the presence of O^2– ions of the axial and nonaxial configurations of the anionic group (emission bands within the wavelength ranges of 209–213 and 330–350 nm, respectively). Exitons located near the impurity and intrinsic defects largely participate in emission. Impurity defects participating in the luminescent centers of barite from the Ore Altai include Pb²⁺, Gd³⁺, Eu²⁺, Eu³⁺, Cu⁺, and Ag⁺ (under X-ray excitation). Variations in the spectral composition of barite indicate the different conditions of its formation.     

Laser-induced time-resolved luminescence as a tool for rare-earth element identification in minerals

 We have determined and distinguished a number of rare-earth elements in several minerals by use of laser-induced time-resolved luminescence spectroscopy. Unlike the conventional measurement of steady-state luminescence, the method allows discrimination between ions that emit in the same spectral range but have different decay times. The main new results are the following: decay time data for all REE luminescence centers; Tm³⁺, Pr³⁺, Er³⁺, Ho³⁺ luminescence in apatite, scheelite, zircon, calcite, and fluorite; Eu³⁺ luminescence in apatite, zircon, fluorite, calcite, danburite, and datolite. Received: 17 April 2000 / Accepted: 4 January 2001     

Adsorption of rare earth elements onto Carrizo sand: Experimental investigations and modeling with surface complexation

Rare earth element (REE) adsorption onto sand from a well characterized aquifer, the Carrizo Sand aquifer of Texas, has been investigated in the laboratory using a batch method. The aim was to improve our understanding of REE adsorption behavior across the REE series and to develop a surface complexation model for the REEs, which can be applied to real aquifer-groundwater systems. Our batch experiments show that REE adsorption onto Carrizo sand increases with increasing atomic number across the REE series. For each REE, adsorption increases with increasing pH, such that when pH >6.0, >98% of each REE is adsorbed onto Carrizo sand for all experimental solutions, including when actual groundwaters from the Carrizo Sand aquifer are used in the experiments. Rare earth element adsorption was not sensitive to ionic strength and total initial REE concentrations in our batch experiments. It is possible that the differences in experimental ionic strength conditions (i.e., 0.002-0.01 M NaCl) chosen were insufficient to affect REE adsorption behavior. However, cation competition (e.g., Ca, Mg, and Zn) did affect REE adsorption onto Carrizo sand, especially for light rare earth elements (LREEs) at low pH. Rare earth element adsorption onto Carrizo sand can be successfully modeled using a generalized two-layer surface complexation model. Our model calculations suggest that REE complexation with strong surface sites of Carrizo sand exceeds the stability of the aqueous complexes LnOH²⁺, LnSO₄⁺, and LnCO₃⁺, but not that of Ln(CO₃)₂^- or LnPO₄^o in Carrizo groundwaters. Thus, at low pH (<7.3), where major inorganic ligands did not effectively compete with surface sites for dissolved REEs, free metal ion (Ln³⁺) adsorption was sufficient to describe REE adsorption behavior. However, at higher pH (>7.3) where solution complexation of the dissolved REEs was strong, REEs were adsorbed not only as free metal ion (Ln³⁺) but also as aqueous complexes (e.g., as Ln(CO₃)₂^- in Carrizo groundwaters). Because heavy rare earth elements (HREEs) were preferentially adsorbed onto Carrizo sand compared to LREEs, original HREE-enriched fractionation patterns in Carrizo groundwaters from the recharge area flattened along the groundwater flow path in the Carrizo Sand aquifer due to adsorption of free- and solution-complexed REEs.     

Photoluminescence of synthetic titanite-group pigments: A rare quenching effect

Chromium-doped titanite and malayaite samples, which were synthesised to evaluate their performance as ceramic pigments, show remarkable photoluminescence behaviour. Emissions of centres related to traces of trivalent rare-earth elements (REE) are observed exclusively from chromium-free samples. Their Cr-doped analogues (containing the same REEs on the same concentration levels), in contrast, only show broad-band Cr³⁺ emission whereas all REE emissions are suppressed. This behaviour is assigned to quenching of REE emissions by chromium centres (i.e., REE³⁺ → Cr³⁺ energy transfer).