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⁺.     

Luminescence properties of Pr3+ and Sm3+ ions in natural apatites

The luminescence spectra of Pr³⁺ and Sm³⁺ ions in apatite Ca₅[F∣(PO₄)₃] crystals from Spain and Russia have been compared with those for phosphate glasses doped with Pr³⁺, Sm³⁺ and Pr³⁺, Sm³⁺ ions. Time-resolved spectra measurements confirm that, in apatites, samarium ions occupy two non-equivalent crystal sites; the same is assumed for praseodymium ions. For the first time in minerals, the Stark splitting energy levels ΔE for ³H₆ and ¹D₂ of Pr³⁺ ion and ⁶H_7/2 of Sm³⁺ ion were determined. Some small differences in ΔE values for the Spanish and Russian apatite are discussed. The decay times of the excited levels of Pr³⁺, Sm³⁺ and Pr³⁺, Sm³⁺ doped in phosphate glass were measured at room temperature and at 77 K. The energy transfer process between samarium and praseodymium ions was observed and the energy transfer rate was calculated.     

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     

Fluid inclusion and cathodoluminescence studies on fluorite from the Kerio valley, Kenya

The Kerio valley lies between the Elgeyo escarpment and the Tugen hills which mark the western margin of the Kenya rift valley. The main fluorite deposits are located in the southern part of the valley at Kimwarer, Choff and Kamnaon.Three types of inclusion fillings were identified: Liquid+Vapour, Liquid+Daughter Minerals and Liquid. The L+V type is dominant. Inclusions occur as clusters, trails along the crystal growth zones and as isolated ones. Low salinities, apparently lower than the 5% wt. NaCl equivalent, were established. Homogenization temperatures suggest that fluorite mineralization took place at different stages and at temperatures between 120 and 180 °C. Isolated readings above 180°C may be referring to the original inclusions in limestone. These measurements and the absence of CO₂ in the inclusions, as well as the occurrence of vugs and crustifications with fluorite, suggest that mineralization took place at relatively shallow depths.Emission spectrum lines representing Eu²⁺, Dy³⁺, Tb³⁺ and Sm³⁺ in fluorite were identified. Sm³⁺ was detected only in the pinkish luminescence of veined fluorite, whereas the pinkish zone in banded fluorite contains Tb³⁺. Eu²⁺ which gives the strongest emission lines in the blue part of the visible spectrum, apparently is responsible for the strong blue cathodoluminescence (CL) in fluorite. The dominance of Eu²⁺ peaks further points to the fact that fluorite mineralization in the Kerio valley took place in an environment that was enriched in Lanthanide Rare Earth Elements (LREE). The presence of rare earths and radioactive elements in fluorite points towards their enrichment in the environment of fluorite mineralization. A juvenile origin of mineral forming solutions is proposed.Two generations of fluorite were established: allotriomorphic fluorite, forming the matrix, and the idiomorphic variety, occurring either in barite or in druzes in early fluorite. Barite in turn forms idiomorphic crystals in allotriomorphic fluorite. Relics of calcite occur in both K-feldspars and in early fluorite. Oxides and hydroxides of Fe, Mn, Ti and Al commonly occur in open spaces in fluorite. Of significance is the presence of gold in fluorite. Fluorite mineralization is of hydrothermal origin in the post-Miocene era and was formed as a result of metasomatic replacement of marble and open space fillings.     

Stabilisation of divalent rare earth elements in natural fluorite

Summary ?The occurrence of divalent rare earth elements (Sm²⁺, Yb²⁺, Tm²⁺, and Ho²⁺) in natural fluorite is evaluated using a suite of 37 samples deriving mainly from Sn–W deposits in the Erzgebirge (Germany), Central Kazakhstan, and the Mongolian Altai. Trace element composition was determined by ICP-AES and ICP-MS. The defect structure of the samples was studied by cathodoluminescence (CL), electron paramagnetic resonance (EPR), and optical absorption spectroscopy. Reduction of cubic Sm³⁺, Yb³⁺, Tm³⁺, and Ho³⁺ under radioactive irradiation produces the corresponding divalent centres. Our data suggest a preferable formation of Sm²⁺ and Yb²⁺ under thorium and of Tm²⁺ and Ho²⁺ under uranium irradiation. Irradiation (indicated by intense brownish (thorium) and deep purple (uranium) coloration of fluorite) gives rise to a population of divalent centres in equilibrium with their decay. However, sporadic radioactive irradiation and stabilisation of the divalent state of the REE by other electron defects were found in most cases. Three models of stabilisation of Sm²⁺, Yb²⁺, Tm²⁺, and Ho²⁺ are discussed. The most effective mechanism for Sm, Yb, Tm, and Ho is coupling with Fe³⁺ centres (REE³⁺+Fe²⁺ → REE²⁺+Fe³⁺). Accordingly, the occurrence of Fe³⁺ centres in natural fluorite is regarded to indicate not an oxidising, but rather a reducing environment during fluorite precipitation. Originally incorporated in the divalent form, Fe²⁺ was converted to Fe³⁺ by radioactive irradiation. Such a conclusion is in agreement with the finding of high contents of interstitial fluorine providing tetragonal local compensation of trivalent REE centres in crystals with high Fe³⁺. If Fe is not present, compensation of divalent Sm, Yb, and Tm is achieved by radiogenic oxidation of Ce(Pr, Tb)³⁺ accompanied by charge transfer (REE³⁺+Ce(Pr, Tb)³⁺ → REE²⁺+ Ce(Pr, Tb)⁴⁺). Ho²⁺ is sometimes stabilised by a hole trapped by an electron localised on a F vacancy (Ho³⁺+e⁻ on □_F → REE²⁺+ self-trapped exciton). Because Sm²⁺ is optically active, the stabilisation by Fe³⁺ (stable up to temperatures above 350 °C) or Ce(Pr, Tb)⁴⁺ (unstable even under visible light) in samples may be determined by careful observations in the field. Institut für Geotechnik, ETH Zürich, ETH-H?nggerberg, Zürich, Switzerland Stanford Linear Accelerator Center, Menlo Park, CA, USA Received January 8, 2002; revised version accepted June 10, 2002     

High-resolution spectrometric analysis of rare earth elements-activated cathodoluminescence in feldspar minerals

Cathodoluminescence (CL) investigations of igneous, metamorphic and sedimentary feldspars indicate that rare earth elements (REE)-activated CL in feldspars is more common than previously assumed. Hot-cathode CL microscopy combined with high-resolution spectrometric analysis of CL emission allow to detect some REE below the detection limits of electron microprobe and proton-induced X-ray emission analysis (PIXE) and reveal variations in the REE distribution within single feldspar crystals. Differently luminescing zones can reflect changes during feldspar crystallization and/or element fluctuations during secondary alteration processes which are not discernible using conventional polarizing microscopy. The results of the study document Eu²⁺, Sm³⁺, Dy³⁺, Tb³⁺, and Nd³⁺-activated CL in feldspars of different origin. The influence of the crystal field on shape and position of REE luminescence spectra significantly differs for divalent and trivalent REE ions. Whereas Eu²⁺ shows a broad band emission (∼420 nm) which is influenced by the local crystal field, trivalent ions of the rare earth show narrow emission lines which reflect the transitions between excited state wave functions lying inside closed electronic shells. The positions of these peaks and the characteristic energies are described for the different REE³⁺.     

Laser excited luminescence of rare earth impurities in natural and synthetic CaF2

This work explores the potential of laser excited luminescence for the study of type and concentration of rare-earth (RE) luminescence centres in CaF₂. A comparison with X-ray excited luminescence is made. The luminescence spectra of several natural and synthetic samples are obtained at low and room temperatures using different Ar⁺ and Kr⁺ laser lines for excitation. Tentative assignment of the luminescent lines to different RE²⁺ and RE³⁺ ions is made. The excitation is shown to take place predominantly through the emission of one phonon. The possibilities for concentration measurements of luminescent centres are discussed.     

Optical properties and crystal chemistry of synthetic rutile implanted with cobalt ions

Implantation of high-energy cobalt ions into plates of synthetic rutile has been studied, and absorption, luminescence, and luminescence excitation spectra have been recorded and interpreted. Long-wave luminescence (820 nm) of Ti _IV ³⁺ ions in rutile has been revealed; its intensity increased after the cobalt implantation. Analysis of luminescence and luminescence excitation spectra has allowed us to specify the scheme of electron energy levels of rutile and to establish the energy levels of impurity Ti³⁺ ions occupying vacant octahedrons with the C _2h symmetry in structure of the mineral.     

Luminescence excitation spectra of Mn2+ in synthetic forsterite

Detailed ligand-field spectra of Mn²⁺ in both microcrystalline and single-crystal synthetic forsterite are obtained using the technique of luminescence excitation spectroscopy. It is shown that Mn²⁺ has an almost exclusive preference for one particular cation site which is most probably the M2 site. Low temperature measurements reveal a no-phonon (purely electronic) transition at 16,260 cm^?1 (615 nm) which is the energy of the lowest split component of the ⁴ T ₁(G) state above the ground state. Phonon replicas of this transition are evident showing that a particular phonon mode (180 cm^?1) is dominantly involved. An analysis of the polarized spectra of Mn²⁺ in single-crystal forsterite shows the choice of C _2v (C ₂, σ _d ) pseudosymmetry for the M2 site yields the best agreement with the polarization dependence of the transitions between the ligand-field states of the Mn²⁺ ion in this site.     

Calculations of crystal-field spectra of Cu2+ ions in some low-symmetry minerals

The expressions of the crystal-field potential energies and perturbation matrix elements corresponding to the point symmetriesC _4v ,D _2h andC _i are given in this paper. The crystal-field transition frequencies of Cu²⁺ ions in metatorbernite, conichalcite and turquoise calculated by using these expressions are also reported. The calculated results are essentially consistent with experimental data.     

Vibrational structure of luminiscence spectrum of Cr3+ in MgAl2O4

The optical absorption and luminescence spectra of MgAl₂O₄:Cr³⁺ natural spinel (from Ural) have been measured at 77 K and 293 K. The luminescent emission from ⁴ T _2g , ² E _g covers wide region of 600–750 nm. The emission spectrum at 77 K shows a very rich vibrational structure which can be mainly explained through the vibrational modes of the oxygen octahedron.     

Factors affecting the Nd3+ (REE3+) luminescence of minerals

In this paper, possibilities and limits of the application of REE³⁺ luminescence (especially the Nd^{3+ 4}F_3/2 → ⁴I_9/2 emission) as structural probe are evaluated. Important factors controlling the Nd³⁺ luminescence signal are discussed, including effects of the crystal-field, crystal orientation, structural state, and temperature. Particular attention was paid to the study of the accessory minerals zircon (ZrSiO₄), xenotime–(Y) (YPO₄), monazite–(Ce) (CePO₄) and their synthetic analogues. Based on these examples we review in short that (1) REE³⁺ luminescence can be used as non-destructive phase identification method, (2) the intensities of certain luminescence bands are strongly influenced by crystal orientation effects, and (3) increased widths of REE³⁺-related emission bands are a strong indicator for structural disorder. We discuss the potential of luminescence spectroscopy, complementary to Raman spectroscopy, for the quantitative estimation of chemical (and potentially also radiation-induced) disorder. For the latter, emissions of Nd³⁺-related centres are found to be promising candidates.     

Electronic absorption spectra of Cr3+ ions in natural clinopyroxenes

The polarized (E‖a′, E‖b and E‖c) electronic absorption spectra of five natural chromium-containing clinopyroxenes with compositions close to chromdiopside, omphacite, ureyite-jadeite (12.8% Cr₂O₃), jadeite, and spodumene (hiddenite) were studied. The polarization dependence of the intensities of the Cr³⁺ bands in the clinopyroxene spectra cannot be explained by the selection rules for the point groups C ₂ or C _2v but can be accounted for satisfactorily with the help of the higher order pseudosymmetry model, i.e. with selection rules for the point symmetry group C _3v. The trigonal axis of the pseudosymmetry crystal field forms an angle of 20.5° with the crystallographic direction c in the (010) plane. D _q increases from diopside (1542 cm^?1) through omphacite (1552 cm^?1), jadeite (1574 cm^?1) to spodumene (1592 cm^?1). The parameter B which is a measure of covalency for Cr³⁺-O bonds at M1 sites in clinopyroxene depends on the Cr³⁺ concentration and the cations at M2 sites.     

Physicomechanical properties of cryptocrystalline fluorite from the Suran deposit, southern Urals

Microhardness (H) and fracture toughness (K _1C) have been studied for the main varieties of shock-resistant cryptocrystalline fluorite, a natural ceramic widespread at the Suran deposit. Suran cryptocrystalline fluorite (SCF) is characterized by high fracture toughness (K _1C), which is 2–5 times higher than K _1C of common fluorite monocrystals. The relationship between K _1C and microhardness H is complex and nonlinear. The SCF varieties from the sellaite-fluorite orebody are distinguished by the highest K _1C = 1.9–2.3 MPa m^1/2, which exceeds K _1C = 0.84 MPa m^1/2 of porcelain-like fluorite from the main fluorite orebody. Qualitative and quantitative variations of structural point defects in the studied samples exert a much stronger effect on microhardness than on fracture toughness, which mainly depends on the size of crystallites, their mutual crystallographic orientation, and the structure of intergranular boundaries, i.e., on the parameters seemingly related to recrystallization and/or twinning of fluorite. In general, the nature of the Suran deposit of fluorite ceramic with unusual physicomechanical properties remains a geological puzzle in many respects.     

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.     

Defects in sodalite-group minerals determined from X-ray-induced luminescence

The luminescence spectra of a suite of natural sodium framework silicates including four different sodalite variants and tugtupite have been collected during X-ray irradiation as a function of temperature between 20 and 673 K. The origin of the emission bands observed in these samples is attributed to F-centres (360 nm), paramagnetic oxygen defects (400 and 450 nm), S₂ ^? ions (620 nm) and tetrahedral Fe³⁺ (730 nm). Luminescence in the yellow (550 nm) is tentatively attributed to Mn²⁺, and red luminescence in Cr-rich pink sodalite is possibly from Cr³⁺ activation. Sudden reduction in luminescence intensities of emission centres was observed for all minerals in the 60–120 K range. Since it is common to all the sodalite-group minerals, we infer it is a feature of the aluminosilicate framework. Sodalite luminescence has responses from substitutions on the framework (e.g. paramagnetic oxygen defects, Fe³⁺) which give sodalite properties akin to other framework silicates such as feldspar and quartz. However, the presence of the sodalite cage containing anions (such as F-centres, S₂ ^? ions) imparts additional properties akin to alkali halides. The possibility of coupling between Fe³⁺ and S₂ ^? is discussed. The overall luminescence behaviour of sodalite group can be understood in terms of competition between these centre types.     

Laser-induced time-resolved luminescence spectroscopy of minerals: a powerful tool for studying the nature of emission centres

The paper summarises new data and results referring to the characterization of the nature of luminescence centres in minerals that were published during the last 8 years. Besides well-established luminescence centres, such as Mn²⁺, Fe³⁺, Cr³⁺, divalent and trivalent rare-earth elements, S₂ ^?, and Pb²⁺, several other centres were proposed and substantiated, such as Mn³⁺, Mn⁴⁺, V²⁺, Ni²⁺, Pb⁺, Mn³⁺, Sb³⁺, Tl⁺, and radiation-induced centres. Also, a relatively new type of luminescence excitation mechanism is discussed briefly, namely plasma-induced luminescence. Here, the emission takes place when the matrix, where the formation of plasma is caused by irradiation with a beam of laser light, is capable to luminescence and contains luminescence centres.     

EXAFS study of rare-earth element coordination in calcite

Extended X-ray absorption fine-structure (EXAFS) spectroscopy is used to characterize the local coordination of selected rare-earth elements (Nd³⁺, Sm³⁺, Dy³⁺, Yb³⁺) coprecipitated with calcite in minor concentrations from room-temperature aqueous solutions. Fitting results confirm substitution in the Ca site, but first-shell Nd-O and Sm-O distances are longer than the Ca-O distance in calcite and longer than what is consistent with ionic radii sums for sixfold coordination in the octahedral Ca site. In contrast, first-shell Dy-O and Yb-O distances are shorter than the Ca-O distance and are consistent with ionic radii sums for sixfold coordination. Comparison of Nd-O and Sm-O bond lengths with those in lanthanide sesquioxides and with ionic radii trends across the lanthanide series suggests that Nd³⁺ and Sm³⁺ have sevenfold coordination in a modified Ca site in calcite. This would require some disruption of the local structure, with an expected decrease in stability, and possibly a different charge compensation mechanism between Nd and Sm vs. Yb and Dy. A possible explanation for the increased coordination for the larger rare-earth elements involves bidentate ligation from a CO₃ group. Because trivalent actinides such as Am³⁺ and Cm³⁺ have ionic radii similar to Nd³⁺, their incorporation in calcite may result in a similar defect structure.     

XAFS characterization of the structural site of Yb in synthetic pyrope and grossular garnets

The incorporation and site preference of minor amounts (about 1 wt%) of Yb³⁺ in synthetic pyrope (Mg₃Al₂Si₃O₁₂) and grossular (Ca₃Al₂Si₃O₁₂) garnet were studied by X-ray Absorption Fine-Structure (XAFS) Spectroscopy. The measurements, performed in the temperature range 77–343 K at both Yb L_I- and L_III-edges, demonstrate that Yb³⁺ enters the garnet structure and is located in the dodecahedral site in both samples. The coordination environment of Yb³⁺ in the two samples was compared to that of the X-site cation in end-member synthetic pyrope and grossular and in Yb₃Al₅O₁₂ as determined by single-crystal X-ray diffraction. The local geometry around Yb³⁺ is different from that of Mg and Ca in the bulk of the garnet, and also from that of Yb³⁺ in Yb₃Al₅O₁₂. Τhe XAFS results indicate that, (1) structural relaxation occurs around Yb³⁺ in the garnet structure; (2) the host garnet matrix exerts a major structural control on the incorporation of Yb³⁺, and (3) minor amounts of Yb³⁺ in garnet are located in structural sites and not in ill-defined defects. Received: 15 January 1998/ Revised, accepted: 21 July 1998     

Single-crystal X-ray diffraction and spectroscopic studies on humboldtine and lindbergite: weak Jahn–Teller effect of Fe<Superscript>2+</Superscript> ion

The single-crystal of humboldtine [Fe²⁺(C₂O₄) · 2H₂O] was first synthesized and the crystal structure has been refined. Single-crystal X-ray diffraction data were collected using an imaging-plate diffractometer system and graphite-monochromatized MoKα radiation. The crystal structure of humboldtine was refined to an agreement index (R1) of 3.22% calculated for 595 unique observed reflections. The mineral crystallizes in the monoclinic system, space group C2/c, with unit cell dimensions of a = 12.011 (11), b = 5.557 (5), c = 9.920 (9) Å, β = 128.53 (3)?, V = 518.0 (8) Å³, and Z = 4. In this crystal structure, the alternation of oxalate anions [(C₂O₄)^2?] and Fe²⁺ ions forms one-dimensional chain structure parallel to [010]; water molecules (H₂O)⁰ create hydrogen bonds to link the chains, where (H₂O)⁰ is essentially part of the crystal structure. The water molecules with the two lone electron pairs (LEPs) on their oxygen atom are tied obliquely to the chains, because the one lone electron pair is considered to participate in the chemical bonds with Fe²⁺ ions. Humboldtine including hydrogen bonds is isotypic with lindbergite [Mn²⁺(C₂O₄) · 2H₂O]. The donor–acceptor separations of the hydrogen bonds in humboldtine are slightly shorter than those in lindbergite, which suggests that the hydrogen bonds in the former are stronger than those in the latter. The infrared and Raman spectra of single-crystals of humboldtine and lindbergite confirmed the differences in hydrogen-bond geometry. In addition, Fe²⁺–O stretching band of humboldtine was split and broadened in the observed Raman spectrum, owing to the Jahn–Teller effect of Fe²⁺ ion. These interpretations were also discussed in terms of bond-valence theory.