Radiation-induced defects in euclase: formation of O<Superscript>−</Superscript> hole and Ti<Superscript>3+</Superscript> electron centers

Irradiation techniques are often applied to gem minerals for color enhancement purposes. Natural green, blue and colorless specimens of rare gemological quality euclase, BeAlSiO₄(OH), from Brazil were irradiated with gamma rays in the dose range from 10 to 500 kGy. Although the colors of the different specimens were not strongly influenced, two different irradiation-induced paramagnetic defect centers were found by electron paramagnetic resonance (EPR). The first one is an O⁻ hole center interacting with one Al neighbor and the second is a Ti³⁺ electron center. The EPR angular rotation patterns of both irradiation-induced defects were measured and analyzed. The results suggest that O⁻ hole centers are formed by dissociation of the hydroxyl ions, similar as in topaz crystals. In euclase the OH⁻ ions interconnect distorted Al octahedra and Be tetrahedra in O₅ positions. During irradiation, the electrons are captured by titanium ions (Ti⁴⁺ + e⁻), leading to the formation of paramagnetic Ti³⁺ ions. From the EPR rotation patterns it is clear that these ions substitute for Al ions. The spin Hamiltonian parameters of the irradiation-induced defects are analyzed and compared to similar defect centers in other mineral specimens. Thermal annealing experiments show that the O⁻ hole centers and Ti³⁺ electron centers are directly connected through the radiation process.     

Factors influencing the high fluoride concentration in groundwater of Vellore District,South India

Most of the arid and semi-arid zones of the Indian subcontinent experience serious health problems due to high concentration of fluoride in drinking water. The Vellore District of Tamil Nadu suffers from high concentration of fluoride in water. However, most of the past studies in this region focused on tannery-related pollution and not on fluoride contamination. The present study attempts to identify the factors influencing the origin and spatial distribution of fluoride in the district. From the observed hydrochemical results of 68 well samples in the context of water level, well depth and hydrochemical parameters, F^? concentration showed increasing trend in the presence of Na⁺ and HCO₃ ^?. This is due to the alkaline nature of groundwater that favors the dissolution of F^?-rich minerals. The occurrence of high fluoride in Na–HCO₃ type of water confirmed this hypothesis. However, Ca²⁺ showed an insignificant correlation with F^?. The high Na/Ca ratio (>1) in 73 % of the samples and the result of Na/Na+Cl plot suggest the occurrence of cation exchange in the study area. The major source of F^? was identified as products of the weathering and the dissolution of fluorites, amphiboles and micas present in the geological formations in the study area. The positive relationship between NO₃ ^? and F^? in few wells located in agricultural fields suggest possible source of F^? from the application of fertilizers. More than 25 % of the samples had higher values of fluoride than the permissible limit of drinking water according to Indian standards. Spatial distribution of fluoride showed a higher concentration in the southwest part of the study area, namely, Thirupathur and Vaniyambadi. This study shows that contamination was high in certain parts of Vellore District and the quality of water must be maintained by resorting to appropriate treatment and management strategies.     

The use of vein fluorite as probe for paleofluid REE and Sr-Nd isotope geochemistry: The Santa Catarina Fluorite District, Southern Brazil

Fluorite can be used as a probe for the source of Sr and REE, as well as for the Sr and Nd isotope systematics of mineralizing solutions, allowing characterization of the composition, oxidation state and sources of the fluids. The ⁸⁷Sr / ⁸⁶Sr ratios in vein fluorite from the Santa Catarina Fluorite District, southern Brazil, are low (0.720 to 0.745) relative to those of the majority of host granites at the time of mineralization (90 Ma), but are similar to those of less abundant and less evolved Sr- and Ca-rich granites and plagioclases of the heterogeneous Pedras Grandes granite association. Major contributions of Sr from the unradiogenic Parana Basin rocks (⁸⁷Sr / ⁸⁶Sr_{90 Ma} = 0.705 to 0.718) are unlikely, considering the radiogenic character of the lower ⁸⁷Sr / ⁸⁶Sr end-member in fluorite mixing lines. Estimated fluorite fluid partition coefficients (K_d^Sr-Ca = 0.019 and D^Sr ≈ 600) indicate a Sr / Ca ratio in the fluorite-forming solution of 0.012, and Sr contents of 0.05 to 0.25 ppm, which are similar to those of present-day granitic geothermal waters. Initial Nd isotopic compositions of the vein fluorites (0.5120 to 0.512) are similar to those of the Pedras Grandes granites. The ¹⁴³Nd / ¹⁴⁴Nd_{90 Ma} of the evolved granites of the Tabuleiro granite association, their accessory fluorites and the Parana Basin rocks are considerably more radiogenic (0.5120 to 0.5127) and these are thus considered to be unlikely sources of the fluids. The REE patterns of vein fluorites, normalized to upper continental crust, show a range of LREE-depleted patterns, with highly variable positive and negative Eu anomalies. The host Pedras Grandes granites show flat to slightly depleted UCC normalized LREE patterns with strong negative Eu anomalies. Depletion of the LREE in fluorites resulted from the mobility of HREE fluoride complexes during fluid migration. A REE fractionation model based on ionic potential ratios indicates that Eu³⁺ was stable during fluid migration and fluorite precipitation. The coexistence of pyrite and Eu³⁺ in the mineralizing fluids is consistent with low pH and oxygen fugacities near the hematite-magnetite buffer.     

EPR study of coordination of Ag and Pb cations in BaB<Subscript>2</Subscript>O<Subscript>4</Subscript> crystals and barium borate glasses

It is shown the possibility to determine the coordination of paramagnetic ions in disordered solid structures, e.g., in barium borate glasses. For this purpose the electron paramagnetic resonance (EPR) method was used to study α-and β-BaB₂O₄ crystals and glasses of 45·BaO × 55·B₂O₃ and 40·BaO × 60·B₂O₃ (mol%) composition activated by Ag⁺ and Pb²⁺ ions. After the samples were exposed to X-rays at 77 K, different EPR centers were observed in them. In α-and β-BaB₂O₄ crystals and glasses the EPR centers Ag²⁺, Ag⁰, Pb⁺, Pb³⁺, and hole centers of O⁻ type were studied. The EPR parameters of these centers and their arrangement in crystal structure were determined. It is shown that Pb³⁺ ions in β-BaB₂O₄ crystals occupy Ba²⁺ position in an irregular polyhedron from the eight oxygen, whereas in α-BaB₂O₄ crystals they occupy Bа₂ position in a sixfold coordination. Pb⁺ ions in α-BaB₂O₄ crystals occupy Bа₁ position in a ninefold coordination from oxygen. In barium borate glasses, Pb³⁺ ions were studied in coordination polyhedron from six oxygen atoms and in a polyhedron from nine to ten oxygen atoms. It is assumed that the established difference in the structural position of Pb³⁺ ions in glasses is due to their previous incorporation in associative cation–anion complexes (AC) and “free” structure-forming cations (FC). Computer simulations have been performed to analyze the stability of specific associative complexes and to compare their bond lengths with experimental data.     

Optical spectroscopy study of variously colored gem-quality topazes from Ouro Preto,Minas Gerais,Brazil

Variously colored gem-quality topazes from Ouro Preto, Minas Gerais, Brazil, were studied by optical absorption spectroscopy and photoluminescence methods. In the near infrared range (750–2500 nm) the absorption spectra display an identical pattern of narrow intense absorption lines caused by overtones and combination vibrations of OH groups, which do not relate to the coloration of the topazes studied. Their colors were found to be caused by combination of three sets of absorption features, (1), (2), and (3) in the visible and near-UV range, which are due to different color center. (1) denotes a pair of broad split bands with maxima 18 000 and 25 000 cm^–1 caused by electronic spin-allowed dd transitions of Cr³⁺ ions. They cause a light rose to deep violet color and characteristic pleochroism of Cr³⁺-containing topazes. Photoluminescence evidences of at least three different types of Cr³⁺ complexes which, most probably, differ by ligand surroundings, O₄F₂, O₄F(OH) and O₄(OH) (2) Corresponds to the intense weakly polarized UV absorption edge. Two different parts, the thermally stable one, caused by ligand-to-metal charge transfer, and the thermally unstable one, caused by some defect center(s), contribute to the edge. (3) denotes a system of two broad unstructured bands with maxima around 19 000 cm^–1 (X>Y Z) and 24 000 cm^–1 (Y Z X). They cause the unique orange color and characteristic pleochroism of Brazilian Imperial topazes. Combinations of (1) and (3) absorption features cause various yellow-rose colors of the samples. Investigations of natural irradiated and thermally treated topazes show that the color centers (1) and (3) transform to each other at annealing and X- or gamma irradiation. The color of natural orange-red Imperial topazes is assumed to be caused by Cr⁴⁺, stabilized by other impurity ions and/or defect irradiation EPR centers. At T=300 °C Cr⁴⁺ reduces to Cr³⁺, the color of Imperial topazes changes to pale rose, caused by spin-allowed bands of Cr³⁺. In artificially irradiated crystals the (3)-center, Cr⁴⁺, may be induced according to the reaction 2Cr³⁺ Cr⁴⁺ + Cr²⁺, which involves chromium pairs in adjacent Al sites of the structure. Such artificially induced color is unstable at room temperature and in daylight. The process of the decay of (3)-centers may be described as a recombination Cr⁴⁺+Cr²⁺ 2Cr³⁺ that results in vanishing of the (3)-bands accompanied by the appearance or increase in Cr³⁺ dd bands, the original orange color turning to a pale rose.     

The Mobility of Rare—Earth Elements During Hydrothermal Activity：A Review

The mobility of the rare-earth elements(REE)during hydrothermal activities is increasingly documented.Geological and experimental evidence suggests that REE may be mobile in solutions rich in F^-,Cl^-,HCO3^-,CO^2- 3,HPO4^2-,PO4^3-,or in combinations of the above ligands,even though little has been known about which ligand or which combination is most effective in mobilizing REE. The fractionation of REE resulting from hydrothermal activities is inconsistent.One set of field data implies the prererential mobility of the light rare-earth elements(LREE).whereas another set of field observations indicates the dominant mobilization of the heavy rare earth elements(HREE),and some theoretical prediction is comtradictory to the field evidence.The Eu anomalies due to hydrothermal activities are complex and plausible explanation is not available.The existing experimental approaches dealing with REE are not adequate for explanation ofREE behaviour in aqueous solutions.Systematic experimental approaches are suggested.     

The Laal-Kan fluorite deposit,Zanjan Province,NW Iran: constraints on REE geochemistry and fluid inclusions

The Laal-Kan fluorite deposit (west of Zanjan city, NW Iran) mainly occurred as some open-space filling and vein/veinlet in the schist of the Paleozoic age. Mineralogically, calcite, fluorite types (white, smoky, and violet), and quartz are the principal constituents accompanied by a number of minor accessory minerals such as hemimorphite, hematite, barite, and clays. Based on chemical analyses, fluorites of various colors were found to have low rare earth element (REE) concentrations (4.16–25.67 ppm). The chondrite-normalized REE patterns indicated that early fluorites were enriched in LREE, relative to HREE, whereas late fluorites were enriched in HREE relative to LREE. This study, therefore, indicated that fugacity of oxygen likely played a significant role in the occurrence of positive Ce and negative anomaly in the late fluorite. Furthermore, the Gd behavior of the fluorite samples could be attributed to the Gd-F complex in ore-forming fluids. On the other hand, low pH hydrothermal fluids under alkaline conditions were probably the main mechanism responsible for the deposition of the early fluorites in this district. Fluorite-hosted fluid inclusion analyses also indicated that fluorite-forming fluids consisted of NaCl, MgCl₂, CaCl₂, and LiCl with a narrow T_H (118–151 °C) and high salinities (18.96–23.47 wt.% NaCl equiv.). Further, the diagram of Tb/La-Tb/Ca ratios revealed that fluorites were predominantly deposited in the hydrothermal environment and the late stage fluorites could be considered as the product of the secondary mineralization of the early fluorites due to the interaction of the fluid with the early fluorites.     

Geochemical processes controlling fluoride-bearing groundwater in the granitic aquifer of a semi-arid region

The aim of the present study is to identify the geochemical processes responsible for higher fluoride (F^–) content in the groundwater of the Yellareddigudem watershed located in Nalgonda district, Andhra Pradesh. The basement rocks in the study area comprise mainly of granites (pink and grey varieties), which contain F-bearing minerals (fluorite, biotite and hornblende). The results of the study area suggest that the groundwater is characterized by Na⁺: HCO facies. The F^– content varies from 0.42 to 7.50 mg/L. In about 68% of the collected groundwater samples, the concentration of F^– exceeds the national drinking water quality limit of 1.5 mg/L. The weathering of the granitic rocks causes the release of Na⁺ and HCO ions, which increase the solubility of ions. Ion exchange between Na+ and Ca²⁺, and precipitation of CaCO₃ reduce the activity of Ca²⁺. This favours dissolution of CaF₂ from the F-bearing minerals present in the host rocks, leading to a higher concentration of F^– in the groundwater. The study further suggests that the spatial variation in the F^– content appears to be caused by difference in the relative occurrence of F-bearing minerals, the degree of rockweathering and fracturing, the residence time of water in the aquifer materials and the associated geochemical processes. The study emphasizes the need for appropriate management measures to mitigate the effect of higher F groundwater on human health.     

Uptake of fluoride by aragonite

The uptake of F by aragonite is attributed to the ion-exchange process, in which one CO₃^2? ion in the structure is replaced by two F^? ions. Under the equilibrium condition at 15° C and 1 atm., the partition of F between aragonite and aqueous solution is described by:

$\text{log}\text{(}\text{[}\text{F}\text{]}\text{a}{}_{\mathrm{<mtext>F</mtext>}}\text{)=1.95 + 0.54}\text{log}\text{a}{}_{\mathrm{<mtext>Ca</mtext>}}$

were [F] denotes the F content of aragonite in mol/g, and a_F and a_Ca are the aqueous activities of F^? and Ca²⁺, respectively. The equation was successfully applied to estimating the F content of marine aragonite.     

Mössbauer spectra of kyanite,aquamarine, and cordierite showing intervalence charge transfer

The blue colors of several minerals and gems, including aquamarine (beryl, Be₃Al₂Si₆O₁₈) and cordierite (Al₃(Mg, Fe)₂Si₅AlO₁₈), have been attributed to charge transfer (CT) between adjacent Fe²⁺ and Fe³⁺ cations, while Fe²⁺→Ti⁴⁺ CT has been proposed for blue kyanites (Al₂SiO₅). Such assignments were based on chemical analyses and on polarization-dependent absorption bands measured in visible-region spectra. We have attempted to characterize the Fe cations in each of these minerals by Mössbauer spectroscopy (MS). In blue kyanites, significant amounts of both Fe²⁺ and Fe³⁺ were detected with MS, indicating that Fe²⁺→Fe³⁺ CT, Fe²⁺→Ti⁴⁺ CT, and Fe²⁺ and Fe³⁺ crystal field transitions each could contribute to the electronic spectra. In aquamarines, coexisting Fe²⁺ and Fe³⁺ ions were resolved by MS, supporting our assignment of the broad, relatively weak band at 16,100 cm^?1 in E∥c spectra to Fe²⁺→Fe³⁺ CT between Fe cations replacing Al³⁺ ions 4.6Å apart along c. A band at 17,500 cm^?1 in E⊥c spectra of cordierite is generally assigned to Fe²⁺ (oct)→Fe³⁺ (tet) CT between cations only 2.74 Å apart. However, no Fe³⁺ ions were detected in the MS at 293K of several blue cordierites showing the 17,500 cm^?1 band and reported to contain Fe³⁺. A quadrupole doublet with parameters consistent with tetrahedral Fe³⁺ appears in 77K MS, but the Fe³⁺/Fe²⁺ ratios from MS are much smaller than values from chemical analysis. These results sound a cautionary note when correlating Mössbauer and chemically determined Fe³⁺/Fe²⁺ ratios for minerals exhibiting Fe²⁺→Fe³⁺ CT.     

Crystal field spectra and jahn teller effect of Mn3+ in clinopyroxene and clinoamphiboles from India

Blanfordite (I), winchite (II), and juddite (III), all showing vivid colors and pleochroism, from highly oxidized parageneses of Indian gondites were studied by microprobe, Mössbauer, and microscope-spectrophotometric techniques and by X-ray structure refinements. The compositions of the Mn-bearing minerals were close to diopsideacmite (I) and magnesio-arfvedsonite to magnesio-riebeckite (II and III). Transition metal ions are located inM(1)-octahedra (I) or predominantlyM(2)-octahedra (II, III). Mössbauer spectra of⁵⁷Fe(IS, ΔE _Q) are typical of octahedral Fe³⁺ only. Polarized absorption spectra in the UV/VIS/NIR ranges explain color and pleochroism of the minerals. The position of the UV-“edge” is correlated with Fe³⁺-contents of the minerals, except for judditeE ∥Z, where the edge shows an unusual low energy position. This is most likely due to Mie-scattering of submicroscopic inclusions of braunite with nearly uniform dimensions. In the VIS range, the spectra are dominated by a complex band system between 15,000 and 20,000 cm^?1. Energies and ?-values of component bands are compatible with those of Mn³⁺ d-d transitions in other Mn³⁺-bearing silicates. The polarization behavior of component bands can best be explained by aC ₂(C2″) symmetry of the crystal field. The Jahn-Teller splitting (<9,000 cm^?1) of the⁵ E _g ground state of Mn³⁺ inO _h crystal fields is appreciably smaller than in other Mn³⁺-silicates. Crystal field parameters 10Dq, (I) 13,650, (II) ca. 11,640, and (III) 11,925 cm^?1, are near to that in piemontite. The crystal field stabilization energy of Mn³⁺, (I) 146, (II) ca. 140, (III) 142 \({{{\text{kJ}}} \mathord{\left/ {\vphantom {{{\text{kJ}}} {\text{g}}}} \right. \kern-0em} {\text{g}}}{\text{ - atom}}_{{\text{Mn}}^{{\text{3 + }}} } \) , is appreciably smaller than that found in other Mn³⁺-silicates (piemontites and manganian andalusites, viridines and kanonaite).     

Natural background levels for some ions in groundwater of the Campania region (southern Italy)

The aim of the study was the determination of the natural background levels (NBLs) for the ions Na⁺, Cl^?, SO₄ ^2?, As³⁺, F^?, Fe²⁺, and Mn²⁺, in some groundwater bodies of the Campania region (southern Italy). The ??Protocol to evaluate the natural background concentrations?? proposed in 2009 by ISPRA (Italian Institute for Environmental Protection and Research) has been applied to the chemical data set of groundwater of the examined groundwater bodies. These analyses have also been examined following the guidelines of the BRIDGE project (Background cRiteria for the IDentification of Groundwater thrEshold). These approaches to evaluate the Threshold Values (TVs) and the NBLs, based on probability distribution functions, have been applied in many countries by various authors during the last 5?years. Changes applied to ISPRA Protocol in this study concern mainly the preselection criteria, in particular threshold values of specific ions, deriving from the aquifers geochemical features. The preselection criteria of the ISPRA Protocol have been merged with those of the BRIDGE Project in order to define a procedure suitable for the definition of the NBLs in the examined aquifers. The NBL of fluoride for the ??Phlegrean Fields?? and the ??eastern Plain of Naples?? groundwater bodies shows values deeply exceeding the reference value (REF) of 1,500???g/L, ranging between 3,600 and 15,000???g/L. The cause of this very high fluoride content is in the natural features of the aquifers constituted by volcanic and pyroclastic rocks. The volcanic origin of the aquifers is also the reason for the high arsenic content in ??Phlegrean Fields?? groundwater. Here the NBL calculated was about 47???g/L against the drinking water standard value of 10???g/L. The widespread high content of manganese and iron for the groundwater body of the ??eastern Plain of Naples?? is due to the reducing conditions related to the extensive marshlands present in the past. The very high NBL of all the examined ions for the groundwater body of ??Ischia Island?? depends on the existence of a geothermal system.     

Hydrochemistry and isotopic studies to identify Ganges River and riverbank groundwater interaction, southern Bangladesh

The Ganges River water and riverbank shallow groundwater were studied during a single wet season using the hydrochemical and isotopic composition of its dissolved load. The dissolved concentrations of major ions (Cl^?, SO₄ ^2?, NO₃ ^?, HCO₃ ^?, Ca²⁺, Na⁺, Mg²⁺, and K⁺), trace elements (barium (Ba) and strontium (Sr)) and stable isotopes (O and D) were determined on samples collected from the Ganges River and its riverbank shallow aquifers. In the present study, the shallow groundwater differs significantly from the Ganges River water; it shows distinct high concentrations of Ca²⁺, Mg²⁺, HCO₃ ^?, Ba, and Sr due to water–rock interaction and this in particular suggests that the Ganges River may not contribute significantly to the riverbank shallow aquifers during wet season. Besides, the sum of the total cationic charge (∑⁺, in milliequivalents per liter) in the groundwater shows high values (2.48 to 13.91 meq/L, average 9.12 meq/L), which is much higher than the sum of the cations observed in the Ganges water (1.36 to 3.10 meq/L, average 1.94 meq/L). Finally, the more depleted stable isotopic (δ ¹⁸O and δ ²H) compositions of the Ganges River water are in contrast to those of the riverbank aquifer having enriched stable isotopic values during the wet season and the riverbank groundwater thus has a purely local origin from precipitation.     

Crystal chemistry of selenates with mineral-like structures. IV. Crystal structure of Zn(SeO<Subscript>4</Subscript>)(H<Subscript>2</Subscript>O)<Subscript>2</Subscript>, a new compound with a mixed framework of the variscite type

The crystal structure of a new compound Zn(SeO₄)(H₂O)₂ (orthorhombic, Pbca, a = 9.0411(13), b = 10.246(2), c = 10.3318(15) Å, V = 957.1(3) ų) has been solved by direct methods and refined to R ₁ = 0.033 on the basis of 1076 observed reflections with |F _hkl | ≥ 4σ|F _hkl |. The structure contains one independent Zn²⁺ cation coordinated by two water molecules and four oxygen atoms of selenate group. The only independent (SeO₄)^2? tetrahedral oxoanion is tetradentate, sharing its corners with four adjacent [Zn²⁺O₂(H₂O₄)]²⁺ octahedrons. The structure can be described as consisting of heteropolyhedral sheets parallel to the (001) plane and linked together into a three-dimensional network. The compound belongs to the variscite structure type and is the first structurally characterized selenate of this group.     

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.     

Single-crystal EPR study of three radiation-induced defects (Al–O2 3?, Ti3+ and W5+) in stishovite

Single-crystal electron paramagnetic resonance spectra of electron-irradiated stishovite, measured at temperatures from 3.5 to 294?K, reveal three S?=?1/2 radiation-induced defects: an aluminum-associated oxygen hole center and two nd ¹ centers (Ti³⁺ and W⁵⁺). The aluminum-associated oxygen hole center, characterized by an orthorhombic site symmetry, coaxial matrices of the electronic Zeeman g, nuclear hyperfine A(²⁷Al) and nuclear quadrupole P(²⁷Al), and the orientation of the g-minimum axis along an O–O direction and those of the unique A(²⁷Al) and P(²⁷Al) axes perpendicular to the O–O direction, is an Al–O₂ ^3? center, with the unpaired electron equally distributed on two equatorial oxygen atoms of a substitutional Al³⁺ ion at the octahedral Si site. Fully optimized Al-doped structure, theoretical ²⁷Al nuclear hyperfine and quadrupole coupling constants and directions, and 3D spin densities from periodic hybrid density functional theory calculations provide further support for this structural model. Spin Hamiltonian parameters of the Ti³⁺ and W⁵⁺ centers, which are confirmed by their diagnostic ⁴⁷Ti, ⁴⁹Ti and ¹⁸³W hyperfine structures, arise from electron trapping on substitutional Ti⁴⁺ and W⁶⁺ ions at the octahedral Si site.     

Scolecite, Part I: Refinement of high-order data, separation of internal and external vibrational amplitudes from displacement parameters

A single crystal of scolecite, CaAl₂Si₃O₁₀· 3H₂O, was studied by X-ray diffraction methods at room temperature. The intensities were measured with MoKα radiation (λ=0.71069?Å) in a complete sphere of reflection up to sinθ/λ=0.9?Å^?1. The structure was refined in the pseudo-orthorhombic setting of space group F1d1 instead of the conventional setting Cc for better comparison with natrolite (Fdd2). The cell parameters are: a=18.502(1)?Å, b=18.974(2)?Å, c=6.525(1)?Å, β=90.615(7)°, V=2290.6(3)?ų, Z=8. A refinement of high-order diffraction data yielded residuals of R(F)=0.9%, R _w (F)=0.9%, GoF=1.73 for 1831 high-angle reflections (0.7≤sinθ/λ≤0.9?Å^?1) and R(F)=1.2%, R _w (F)=1.4%, GoF=3.22 for all 3478 independent reflections. In comparison with natrolite, the replacement of 2 Na⁺ by 1 Ca²⁺ and 1 H₂O leads to a reduction of symmetry from Fdd2 to F1d1. Each general atomic position in natrolite (except of Na) splits into two crystallographically independent positions in scolecite. The T?O distances and T?O?T angles of these two sites differ distinctly from each other due to the influence of the calcium ions on the framework. An unexpected result of our detailed analysis of the data is that the additional water molecule (O7) disturbs the symmetry of the framework to a greater extent than the replacement of Na⁺ by Ca²⁺. As a comparison of the displacement parameters indicates, the bonds within the tetrahedral framework and to the extraframework cations are stronger in scolecite than in natrolite. The isotropic U(equ) values of the framework atoms and extraframework cations are about 10% smaller in scolecite compared to natrolite. The same tendency is shown by the analysis of the internal vibrational amplitudes ΔU. The corresponding force constants are in the range of F=358 to 3367?Nm^?1 for the T?O bonds in scolecite (in natrolite: F=354 to 824?Nm^?1). The values of the force constants which determine the vibrations of the Ca ions and water molecules against the framework oxygen atoms lie in the range of F=33 to 1757?Nm^?1 (in natrolite: F=57 to 293?Nm^?1).     

Partitioning of fluorine and chlorine between biotite and granitic melt: experimental calibration at 200 MPa H2O

Experiments from 640 to 680?°C, 200 MPa H₂O at?f _O2?≈?NNO, employing a natural?F-rich?vitrophyric rhyolite from Spor Mountain, Utah, assessed the effect of variable Mg′ [100Mg/(Mg?+?Mn?+?Fe)] on the partitioning of fluorine and chlorine between biotite (Bt) and melt. Over this temperature interval, Bt (?±?fluorite, ?±?quartz) is the sole liquidus phase. Partition coefficients for fluorine between biotite and glass (D_F ^Bt/melt) show a strong dependence on the Mg′ of Bt.?D_F ^Bt/melt varies from???1.5 to 7.2 over the range of Mg′ from 21 to 76. A strong linear correlation between?D_F ^Bt/melt?and Mg′ has a slope of 9.4 and extrapolates through the origin (i.e., D_F ^Bt/melt?≈?0 at Mg′?=?0, an annite-siderophyllite solid solution in these experiments). D_Cl ^Bt/melt values (???1 to 6) in the same experiments vary inversely with Mg′. The Al-content of biotite does not vary with the aluminum saturation index (ASI?=?molar Al₂O₃/Σ alkali and alkaline earth oxides) of melt, but two exchange mechanisms involving Al appear to operate in these micas: (1) Al^vi?+?Al^iv?? Si^iv?+?Mg^iv, and Mg^iv?+?2Al^iv? 2Si^iv?+?□^iv. The effects of other components such as Li or other intensive parameters including f _O2 have yet to be evaluated?systematically. At comparable Mg′ of Bt, however, the Spor Mountain rhyolite yields higher D_F ^Bt/melt values than an Li-rich, strongly peraluminous melt previously investigated. The results indicate that the Mg′ of Bt exerts the principal control on halogen partitioning, with ASI and T as second-order variables. The experimental partition coefficients compare well with other experimental results but not with most volcanic rocks. Magmatic Bt from most rhyolites records higher D_F ^Bt/melt due to reequilibration with degassed (H₂O-depleted) magma and perhaps with F₂O_?1 exchange that may accompany oxidation ([Fe³⁺O] [Fe²⁺OH]_?1). This behavior is evident in magmatic biotite from a zoned peraluminous rhyolite complex near Morococala, Bolivia: Bt is sharply zoned with F-rich rims, but Bt(core)-melt inclusion pairs fall on our experimental curve for D_F ^Bt/melt. These experimental data can be used in part to assess the preservation of magmatic volatile contents in plutonic or volcanic silicic rocks. For plutonic rocks, the actual F-content of melt, not a relative activity ratio involving HF species, can be reasonably estimated if the mica has not undergone subsolidus reequilibration. This information is potentially useful for some shallow-level Ca-poor magmas that are thought to be rich in F (e.g., A- and S-type granites) but do not conserve F well as rocks.     

The investigation of defects in shock-metamorphosed quartz

The shock-metamorphosed quartz exhibits thermal luminescence (TL) with maxima at 365 nm, 470 nm and 610–680 nm. By electron paramagnetic resonance (EPR) analysis E₁ type electron centers and hole centers have been found which originate from vacancies including those from the substitution of Al³⁺ and/or Fe³⁺, for Si⁴⁺. The EPR and TL spectra may be interpreted mainly in terms of vacancy type defects associated with dislocations in the crystal structure of quartz.