Character and setting 1 Identified resources 1 Mineral resource potential 3 Introduction 3 Apprai... more Character and setting 1 Identified resources 1 Mineral resource potential 3 Introduction 3 Appraisal of identified resources 3 Methods and scope of investigation 3 History of mining and mineral exploration 5 Identified resources 5 Assessment of mineral resource potential 6 Geology 6 Geochemistry 6 Geophysics 6 Gamma-ray spectroscopy 6 Aeromagnetic survey 7 Gravity survey 7 Mineral resource potential 7 References cited 9 Appendixes Definition of levels of mineral resource potential and certainty of assessment 12 Resource/reserve classification 13 Geologic time chart 14 FIGURES
X-ray absorption spectroscopic studies of cation environments in oxide glasses are selectively re... more X-ray absorption spectroscopic studies of cation environments in oxide glasses are selectively reviewed. New results are presented on K and Yb environments in silicate glasses and on Fe in silicate melts at temperatures up to 1173 " K. INTRODUCTION Recent investigations of oxide glasses and crystalline model compounds by x-ray absorption spectroscopy (XAS and the acronyms EXAFS for extended fine structure and NE)(AFS or XANES for near-edge structure) have provided useful and sometimes unique information on the structural environments of network-forming and network-modifying cations. Synchrotron-based XAS is well suited for studying the local structural environment and bonding of cations in amorphous materials that cannot be probed directly by standard spectroscopic or scattering methods (e.g. Na, Mg, K, Ca, Zr) or which are present in small concentrations (e.g. transition metals, rare earth and actinide elements a t 100-2000 ppm). T o date, several dozen XAS studies of cations in oxide glasses have reported information on local coordination environments of network formers such as Si, Al, Ga, and Ge and of network modifiers such as Na, K, Ca, Ti, V, Fe, Zn, Zr, Pb, and U. This work has provided new insights about oxide glass structure and structure-property-composition relationships which shed light on processes such as homogeneous nucleation, viscous flow, cation diffusion, and corrosion behavior of glasses. This paper presents a brief overview of recent XAS studies of silicate and oxide glasses which is selective because of space limitations. Also reported are new results on the structural environments of K and Yb in silicate glasses under ambient conditions and of Fe in silicate melts at temperatures up t o 1173" K.
The coordination environments of Ge and As atoms in Ge x As y S 1ϪxϪy glasses with x:yϭ1:2, 1:1, ... more The coordination environments of Ge and As atoms in Ge x As y S 1ϪxϪy glasses with x:yϭ1:2, 1:1, and 2.5:1 and with wide-ranging S contents have been studied with Ge and As K-edge x-ray absorption fine structure spectroscopy. The coordination numbers of Ge and As atoms are found to be 4 and 3, respectively, in all glasses. The first coordination shells of Ge and As atoms in the stoichiometric and S-excess glasses consist of S atoms only, implying the preservation of chemical order at least over the length scale of the first coordination shell. As-As homopolar bonds are found to appear at low and intermediate levels of S deficiency, whereas Ge-Ge bonds are formed only in strongly S-deficient glasses indicating clustering of metal atoms and violation of chemical order in S-deficient glasses. The composition-dependent variation in chemical order in chalcogenide glasses has been hypothesized to result in topological changes in the intermediate-range structural units. The role of such topological transitions in controlling the structure-property relationships in chalcogenide glasses is discussed.
Residual stress in an optical fiber impacts several fiber properties, including reliability and g... more Residual stress in an optical fiber impacts several fiber properties, including reliability and geometry. The residual stress profile arises from a thermal expansion mismatch of the constituent materials, the tension applied during fiber forming, and the thermal profile experienced by the fiber during formation. The thermal profile of the fiber is determined by measuring the intensities of the two `defect' bands in the silica cladding of the fiber using Raman spectroscopy. Other studies have shown that the intensities of these bands increase with increasing fictive temperature and that these changes can be used to map changes in fictive temperatures across the fiber diameter. Optical birefringence is used to measure the residual stress profile of the fiber. The relationship between fictive temperature and fiber residual stress is explored for both straight fibers and fibers with a natural radius of curvature (curled fibers).
We present estimates of the refractive-index change in waveguides in silica produced by focused f... more We present estimates of the refractive-index change in waveguides in silica produced by focused femtosecond laser pulses. The estimates are based on the shift of the central frequency of omega4 (TO) band (Si-O stretching mode) in micro-Raman spectra. These data were compared with the relation of this parameter to density and to refractive index changes in seen in glasses modified by high pressure or irradiation. We conclude that the measured refractive-index increase in the waveguides can be explained by densification of glass.
... Article Outline. References. ELSEVIER Journal of NonCrystalline Solids 201 (1996) 8194 ]OUR... more ... Article Outline. References. ELSEVIER Journal of NonCrystalline Solids 201 (1996) 8194 ]OURNAL OF BCI ZM Xray absorption study of the coordination of titanium in sodiumtitaniumsilicate glasses Carl W. Ponader *, Heather Boek, James E. Dickinson Jr. ... Chem. ...
The coordination environments of Ge and As atoms in Ge x As x S 100À2x glasses with 13:3 6 x 6 32... more The coordination environments of Ge and As atoms in Ge x As x S 100À2x glasses with 13:3 6 x 6 32:5 have been studied with Ge and As K-edge EXAFS spectroscopy. Ge and As atoms are fourfold-and threefold-coordinated, respectively, in all glasses. The atomic structures of the stoichiometric and S-excess glasses are found to consist of GeS 4 tetrahedra and AsS 3 trigonal pyramids implying the preservation of chemical order at least over the length scale of the ®rst coordination shell. As±As homopolar bonds are found to appear at low and intermediate levels of S-de®ciency. Ge±Ge bonds are formed in extremely S-de®cient glasses only after all As atoms participate in homopolar As±As bonds, implying clustering of like metal atoms and violation of chemical order in S-de®cient glasses. Intermediate-range structural order induced by such clustering is shown to play a critical role in controlling the compositional variation of physical properties in these glasses.
Potassium K-EXAFS and XANES measurements were made using synchrotron radiation on selected glasse... more Potassium K-EXAFS and XANES measurements were made using synchrotron radiation on selected glasses along the NaAISi,O,-KAlSisOs (albite-orthoclase) binary to characterize the local structural environment of potassium: Comparison of XANES spectra indicates that the potassium environment in the glasses is more similar to that in crystalline KAlSiaOa (leucite) than in crystalline KAlSisO, (orthoclase) or KNa,AI,Si,O,e, (nepheline). Consequently, the potassium environment in these aluminosilicate glasses were modelled in the analysis of the EXAFS spectra using empirical phase and amplitude parameters for potassium in leucite. The derived K-O bond lengths and coordination numbers exhibit maxima near the composition albitesa-orthoclase,,. A model, based on previous theoretical and experimental investigations of crystalline and amorphous aluminosilicates and the mixed-alkali effect, is suggested to explain these variations in the local coordination environment of potassium across the binary alkali feldspar glass join.
Ternary GeAsS glasses have been investigated to determine the compositional dependence of selecte... more Ternary GeAsS glasses have been investigated to determine the compositional dependence of selected physical properties. Although the latter show a linear variation with Ge:As ratio at a constant S content, many exhibit a nonlinear dependence on the S content at a ®xed Ge:As ratio. The trend of optical properties tracks the increasing formation of metal±metal bonds as the S concentration of these glasses is reduced below stoichiometry. Molar volume and thermal expansion coecient display maxima near an average coordination number of 2.6 for which there is no apparent indication in the Raman spectra. The spectra of stoichiometric and S-excess glasses are dominated by a band at 345 cm À1 due to the symmetric stretching of both AsS 3 and GeS 4 groups; those of S-de®cient glasses display bands associated with metal±metal bonds between 210 and 240 cm À1 whose intensity increase with increasing S de®ciency.
ABSTRACT Stoichiometric Ge-poor (< 50% GeS2) GeAsPS glasses, as well as GeAsPS glasses wit... more ABSTRACT Stoichiometric Ge-poor (< 50% GeS2) GeAsPS glasses, as well as GeAsPS glasses with a fixed Ge:As:P ratio of 5:1:1 and S contents ranging from 56%, to 74%, were synthesized. The compositional dependence of the physical properties and nuclear magnetic resonance (NMR) spectra of the stoichiometric glasses indicates that tetrahedral S = PS3/2 groups constitute the dominant P species. On reducing the S content, the absorption edge of variable S GeAsPS glasses remains constant until S deficits of about 25% are attained. Raman spectra of stoichiometric and S-deficient glasses show bands attributed to As2P2S8 and (As1P)(4)S-3 species, respectively, whereas those of variable S glasses with S deficits less than or equal to 15% show a high frequency band indicating the presence of tetrahedral S = PS3 .2 groups. NMR spectra of glasses with S deficits of 7-15'% are dominated by a P-31 resonance at about 117 ppm that is assigned to trigonal PS3/2 groups. NMR spectra of glasses with S deficits greater than or equal to 10% have a P-31 resonance near -100 ppm, indicating that tetrahedral S = PS3/2 and trigonal PS3 .2 groups are replaced by species containing P-P and, ultimately, P-(Ge, As) bonds as the S content of these glasses decreases. Comparison of these results with those for GeAsS glasses demonstrates that P-P bonds form preferentially to metal-metal bonds containing either Ge or As, and that P tends to associate with As instead of Ge as next nearest neighbors.
Origin of the refractive-index increase in laser-written waveguides in glasses. [Journal of Appli... more Origin of the refractive-index increase in laser-written waveguides in glasses. [Journal of Applied Physics 103, 063516 (2008)]. Carl W. Ponader, Joseph F. Schroeder, Alexander M. Streltsov. Abstract. We present firm evidence ...
The structural environments of trace levels (~0.2 wt%) of La3+, Gd'+, and Yb3+ in several silicat... more The structural environments of trace levels (~0.2 wt%) of La3+, Gd'+, and Yb3+ in several silicate glasses were examined as a function of melt composition and ~lyme~~tion using Extended Xray Absorption Fine Structure (EXAFS) spectroscopy. Glass compositions were albite, sodium trisilicate, and a peralkaline composition approximately halfway between the two. Yb is 5-6 coordinated [d(Yb-0) = 2.21 A]. Gd coordination ranges from 6 in albite glass [d(Gd-0) = 2.30 A] through a mixture of 6 to 8 in peralkaline glass [d(Gd-0) = 2.36 A] to 8 in sodium trisilicate glass [d(Gd-0) = 2.43 A]. La is 7coordinated in the peralkaline and sodium trisilicate glasses [&_a-0) = 2.43 A], but has a coordination of 8-9 in albite glass [d(La-0) = 2.59 A]. Regularity of the rare earth site (i.e., the range of REE-0 distances) appears to decrease as ~lyme~zation increases. This is caused, in part, by the decreasing number of non-bridging oxygens in the coordination sphere of the rare earth element with increasing melt polymerization. Regularity of the rare earth site also appears to increase as the size of the rare earth decreases. This is due to the greater ability of the smaller, higher field strength rare earths to compete for non-bridging oxygens. An exception to these trends is the La site in albite glass; it is more regular than in the other glasses because the large La ion is centered in the "cage" formed by interconnected rings of Si-and Al-tetrahedra, which leads to a more regular dist~bution of La-0 bond lengths. As ~lyme~zation decreases, the rare earth's local environment is more strongly influenced by its bonding requirements and less influenced by network topology.
The structural environments of trace to minor levels (=2000 ppm to ~~3.0 wt%) of U in several sil... more The structural environments of trace to minor levels (=2000 ppm to ~~3.0 wt%) of U in several silicate glasses were examined as a function of oxygen fugacity, melt composition, and melt polymerization using X-ray (XANES and EXAFS) and optical absorption spectroscopies. Glass compositions were diopside (CaMgSi20,: DI), anorthite (CaAISizOg: AN), albite (NaAlSi30s: AB), sodium trisilicate (Na2Si 30, : TS) , a peralkaline composition (Nas.3A1Si7017 : PR, approximately halfway between AB and TS), and a talc-alkaline rhyolite composition (RH). A second set of silicate glasses of the same base com~sitions confining-2000 ppm to m3.0 wt% U and-0.6 to 2.5 wt% F or Cl was also synthesized. In the glasses synthesized under oxidizing conditions (in air), U"' occurs as uranyl groups with two axial oxygens at = 1.77-1.85 + 0.02 A and four to five equatorial oxygens at =2.21-2.25 + 0.03 A. In glasses synthesized under more reducing conditions (fo, = IO-'-IO-' atm), U" occurs in moderately distorted B-coordinated polyhedra [ d(U"-0) = 2.19-2.24 + 0.03 %I-], which may co-exist with smaller numbers of Uv' species and/or U'" species. Under the most reducing conditions used (h, = lo-*-IO-'* atm), U'" occurs in less distorted octahedra [d(U'"-0) M 2.26-2.29-t 0.02 A]. No clear evidence for U-F or U-Cl bonds was found for any of the halogen-containing glasses, suggesting that U-halogen "complexes" are not present. In addition, no U-U (second-neighbor) interactions were detected, indicating that no significant clustering of U atoms is present in any of the glasses studied. Bond strength-bond length calculations and constraints placed on loca1 bonding by Pauling's second rule suggest that U'" and U" in 6~oordinated sites in silicate melts will preferentially bond to nonb~d~ng oxygens f NBO's) rather than bridging oxygens (BO's). The unusually low &fold coordination of U" and U" in relatively depoIymerized silicate melts (e.g., peralkaline and halogen-rich melts) results in a high U-O bond strength in the melt that is not observed in crystalline U-bearing minerals. This difference in bond strength is partially responsible for the small crystal-melt partition coefficients of U"'. In addition, the common silicate minerals comprising igneous rocks lack appropriate crystallographic sites which can stably accommodate this large and highly charged cation. These factors help explain the normally incompatible character of U"' during magmatic differentiation. In contrast, the low solubility of U'" and U" in more polymerized silicate melts, such as those produced during the late stages of magmatic differentiation, can be explained by a shortage of NBO's. Increasing amounts of S-fold coordinated U should favor the incorporation of both U" and Uv in accessory minerals like zircon, thorite, titanite, apatite, uranium oxides, etc., thus its more compatible behavior in the latest stages of magmatic differentiation. INTRODUCI'ION URANIUM IS AN IMPORTANT TRACE element in igneous systems, providing information about the age, origin, and crystallization history of magmas (ALL~ZGRE et al., 1977; HEN-DERSON, 1982; CUNEY and FRIEDRICH, 1987; HOFMANN, 1988). It is also an important source of heat in the Earth's crust and mantle (VIGNERESSE and CUNEY, 199 1). Because of its reIativeIy large size (U"' m 0.94 A in g-fold coordination with "'0, SHANNON and PREWITT, 1969), U is considered one of the LILE-type elements (Large Ion Lithophile Efemerit), which are characterized, in general, by very small mineral-melt partition coefficients (
Character and setting 1 Identified resources 1 Mineral resource potential 3 Introduction 3 Apprai... more Character and setting 1 Identified resources 1 Mineral resource potential 3 Introduction 3 Appraisal of identified resources 3 Methods and scope of investigation 3 History of mining and mineral exploration 5 Identified resources 5 Assessment of mineral resource potential 6 Geology 6 Geochemistry 6 Geophysics 6 Gamma-ray spectroscopy 6 Aeromagnetic survey 7 Gravity survey 7 Mineral resource potential 7 References cited 9 Appendixes Definition of levels of mineral resource potential and certainty of assessment 12 Resource/reserve classification 13 Geologic time chart 14 FIGURES
X-ray absorption spectroscopic studies of cation environments in oxide glasses are selectively re... more X-ray absorption spectroscopic studies of cation environments in oxide glasses are selectively reviewed. New results are presented on K and Yb environments in silicate glasses and on Fe in silicate melts at temperatures up to 1173 " K. INTRODUCTION Recent investigations of oxide glasses and crystalline model compounds by x-ray absorption spectroscopy (XAS and the acronyms EXAFS for extended fine structure and NE)(AFS or XANES for near-edge structure) have provided useful and sometimes unique information on the structural environments of network-forming and network-modifying cations. Synchrotron-based XAS is well suited for studying the local structural environment and bonding of cations in amorphous materials that cannot be probed directly by standard spectroscopic or scattering methods (e.g. Na, Mg, K, Ca, Zr) or which are present in small concentrations (e.g. transition metals, rare earth and actinide elements a t 100-2000 ppm). T o date, several dozen XAS studies of cations in oxide glasses have reported information on local coordination environments of network formers such as Si, Al, Ga, and Ge and of network modifiers such as Na, K, Ca, Ti, V, Fe, Zn, Zr, Pb, and U. This work has provided new insights about oxide glass structure and structure-property-composition relationships which shed light on processes such as homogeneous nucleation, viscous flow, cation diffusion, and corrosion behavior of glasses. This paper presents a brief overview of recent XAS studies of silicate and oxide glasses which is selective because of space limitations. Also reported are new results on the structural environments of K and Yb in silicate glasses under ambient conditions and of Fe in silicate melts at temperatures up t o 1173" K.
The coordination environments of Ge and As atoms in Ge x As y S 1ϪxϪy glasses with x:yϭ1:2, 1:1, ... more The coordination environments of Ge and As atoms in Ge x As y S 1ϪxϪy glasses with x:yϭ1:2, 1:1, and 2.5:1 and with wide-ranging S contents have been studied with Ge and As K-edge x-ray absorption fine structure spectroscopy. The coordination numbers of Ge and As atoms are found to be 4 and 3, respectively, in all glasses. The first coordination shells of Ge and As atoms in the stoichiometric and S-excess glasses consist of S atoms only, implying the preservation of chemical order at least over the length scale of the first coordination shell. As-As homopolar bonds are found to appear at low and intermediate levels of S deficiency, whereas Ge-Ge bonds are formed only in strongly S-deficient glasses indicating clustering of metal atoms and violation of chemical order in S-deficient glasses. The composition-dependent variation in chemical order in chalcogenide glasses has been hypothesized to result in topological changes in the intermediate-range structural units. The role of such topological transitions in controlling the structure-property relationships in chalcogenide glasses is discussed.
Residual stress in an optical fiber impacts several fiber properties, including reliability and g... more Residual stress in an optical fiber impacts several fiber properties, including reliability and geometry. The residual stress profile arises from a thermal expansion mismatch of the constituent materials, the tension applied during fiber forming, and the thermal profile experienced by the fiber during formation. The thermal profile of the fiber is determined by measuring the intensities of the two `defect' bands in the silica cladding of the fiber using Raman spectroscopy. Other studies have shown that the intensities of these bands increase with increasing fictive temperature and that these changes can be used to map changes in fictive temperatures across the fiber diameter. Optical birefringence is used to measure the residual stress profile of the fiber. The relationship between fictive temperature and fiber residual stress is explored for both straight fibers and fibers with a natural radius of curvature (curled fibers).
We present estimates of the refractive-index change in waveguides in silica produced by focused f... more We present estimates of the refractive-index change in waveguides in silica produced by focused femtosecond laser pulses. The estimates are based on the shift of the central frequency of omega4 (TO) band (Si-O stretching mode) in micro-Raman spectra. These data were compared with the relation of this parameter to density and to refractive index changes in seen in glasses modified by high pressure or irradiation. We conclude that the measured refractive-index increase in the waveguides can be explained by densification of glass.
... Article Outline. References. ELSEVIER Journal of NonCrystalline Solids 201 (1996) 8194 ]OUR... more ... Article Outline. References. ELSEVIER Journal of NonCrystalline Solids 201 (1996) 8194 ]OURNAL OF BCI ZM Xray absorption study of the coordination of titanium in sodiumtitaniumsilicate glasses Carl W. Ponader *, Heather Boek, James E. Dickinson Jr. ... Chem. ...
The coordination environments of Ge and As atoms in Ge x As x S 100À2x glasses with 13:3 6 x 6 32... more The coordination environments of Ge and As atoms in Ge x As x S 100À2x glasses with 13:3 6 x 6 32:5 have been studied with Ge and As K-edge EXAFS spectroscopy. Ge and As atoms are fourfold-and threefold-coordinated, respectively, in all glasses. The atomic structures of the stoichiometric and S-excess glasses are found to consist of GeS 4 tetrahedra and AsS 3 trigonal pyramids implying the preservation of chemical order at least over the length scale of the ®rst coordination shell. As±As homopolar bonds are found to appear at low and intermediate levels of S-de®ciency. Ge±Ge bonds are formed in extremely S-de®cient glasses only after all As atoms participate in homopolar As±As bonds, implying clustering of like metal atoms and violation of chemical order in S-de®cient glasses. Intermediate-range structural order induced by such clustering is shown to play a critical role in controlling the compositional variation of physical properties in these glasses.
Potassium K-EXAFS and XANES measurements were made using synchrotron radiation on selected glasse... more Potassium K-EXAFS and XANES measurements were made using synchrotron radiation on selected glasses along the NaAISi,O,-KAlSisOs (albite-orthoclase) binary to characterize the local structural environment of potassium: Comparison of XANES spectra indicates that the potassium environment in the glasses is more similar to that in crystalline KAlSiaOa (leucite) than in crystalline KAlSisO, (orthoclase) or KNa,AI,Si,O,e, (nepheline). Consequently, the potassium environment in these aluminosilicate glasses were modelled in the analysis of the EXAFS spectra using empirical phase and amplitude parameters for potassium in leucite. The derived K-O bond lengths and coordination numbers exhibit maxima near the composition albitesa-orthoclase,,. A model, based on previous theoretical and experimental investigations of crystalline and amorphous aluminosilicates and the mixed-alkali effect, is suggested to explain these variations in the local coordination environment of potassium across the binary alkali feldspar glass join.
Ternary GeAsS glasses have been investigated to determine the compositional dependence of selecte... more Ternary GeAsS glasses have been investigated to determine the compositional dependence of selected physical properties. Although the latter show a linear variation with Ge:As ratio at a constant S content, many exhibit a nonlinear dependence on the S content at a ®xed Ge:As ratio. The trend of optical properties tracks the increasing formation of metal±metal bonds as the S concentration of these glasses is reduced below stoichiometry. Molar volume and thermal expansion coecient display maxima near an average coordination number of 2.6 for which there is no apparent indication in the Raman spectra. The spectra of stoichiometric and S-excess glasses are dominated by a band at 345 cm À1 due to the symmetric stretching of both AsS 3 and GeS 4 groups; those of S-de®cient glasses display bands associated with metal±metal bonds between 210 and 240 cm À1 whose intensity increase with increasing S de®ciency.
ABSTRACT Stoichiometric Ge-poor (< 50% GeS2) GeAsPS glasses, as well as GeAsPS glasses wit... more ABSTRACT Stoichiometric Ge-poor (< 50% GeS2) GeAsPS glasses, as well as GeAsPS glasses with a fixed Ge:As:P ratio of 5:1:1 and S contents ranging from 56%, to 74%, were synthesized. The compositional dependence of the physical properties and nuclear magnetic resonance (NMR) spectra of the stoichiometric glasses indicates that tetrahedral S = PS3/2 groups constitute the dominant P species. On reducing the S content, the absorption edge of variable S GeAsPS glasses remains constant until S deficits of about 25% are attained. Raman spectra of stoichiometric and S-deficient glasses show bands attributed to As2P2S8 and (As1P)(4)S-3 species, respectively, whereas those of variable S glasses with S deficits less than or equal to 15% show a high frequency band indicating the presence of tetrahedral S = PS3 .2 groups. NMR spectra of glasses with S deficits of 7-15'% are dominated by a P-31 resonance at about 117 ppm that is assigned to trigonal PS3/2 groups. NMR spectra of glasses with S deficits greater than or equal to 10% have a P-31 resonance near -100 ppm, indicating that tetrahedral S = PS3/2 and trigonal PS3 .2 groups are replaced by species containing P-P and, ultimately, P-(Ge, As) bonds as the S content of these glasses decreases. Comparison of these results with those for GeAsS glasses demonstrates that P-P bonds form preferentially to metal-metal bonds containing either Ge or As, and that P tends to associate with As instead of Ge as next nearest neighbors.
Origin of the refractive-index increase in laser-written waveguides in glasses. [Journal of Appli... more Origin of the refractive-index increase in laser-written waveguides in glasses. [Journal of Applied Physics 103, 063516 (2008)]. Carl W. Ponader, Joseph F. Schroeder, Alexander M. Streltsov. Abstract. We present firm evidence ...
The structural environments of trace levels (~0.2 wt%) of La3+, Gd'+, and Yb3+ in several silicat... more The structural environments of trace levels (~0.2 wt%) of La3+, Gd'+, and Yb3+ in several silicate glasses were examined as a function of melt composition and ~lyme~~tion using Extended Xray Absorption Fine Structure (EXAFS) spectroscopy. Glass compositions were albite, sodium trisilicate, and a peralkaline composition approximately halfway between the two. Yb is 5-6 coordinated [d(Yb-0) = 2.21 A]. Gd coordination ranges from 6 in albite glass [d(Gd-0) = 2.30 A] through a mixture of 6 to 8 in peralkaline glass [d(Gd-0) = 2.36 A] to 8 in sodium trisilicate glass [d(Gd-0) = 2.43 A]. La is 7coordinated in the peralkaline and sodium trisilicate glasses [&_a-0) = 2.43 A], but has a coordination of 8-9 in albite glass [d(La-0) = 2.59 A]. Regularity of the rare earth site (i.e., the range of REE-0 distances) appears to decrease as ~lyme~zation increases. This is caused, in part, by the decreasing number of non-bridging oxygens in the coordination sphere of the rare earth element with increasing melt polymerization. Regularity of the rare earth site also appears to increase as the size of the rare earth decreases. This is due to the greater ability of the smaller, higher field strength rare earths to compete for non-bridging oxygens. An exception to these trends is the La site in albite glass; it is more regular than in the other glasses because the large La ion is centered in the "cage" formed by interconnected rings of Si-and Al-tetrahedra, which leads to a more regular dist~bution of La-0 bond lengths. As ~lyme~zation decreases, the rare earth's local environment is more strongly influenced by its bonding requirements and less influenced by network topology.
The structural environments of trace to minor levels (=2000 ppm to ~~3.0 wt%) of U in several sil... more The structural environments of trace to minor levels (=2000 ppm to ~~3.0 wt%) of U in several silicate glasses were examined as a function of oxygen fugacity, melt composition, and melt polymerization using X-ray (XANES and EXAFS) and optical absorption spectroscopies. Glass compositions were diopside (CaMgSi20,: DI), anorthite (CaAISizOg: AN), albite (NaAlSi30s: AB), sodium trisilicate (Na2Si 30, : TS) , a peralkaline composition (Nas.3A1Si7017 : PR, approximately halfway between AB and TS), and a talc-alkaline rhyolite composition (RH). A second set of silicate glasses of the same base com~sitions confining-2000 ppm to m3.0 wt% U and-0.6 to 2.5 wt% F or Cl was also synthesized. In the glasses synthesized under oxidizing conditions (in air), U"' occurs as uranyl groups with two axial oxygens at = 1.77-1.85 + 0.02 A and four to five equatorial oxygens at =2.21-2.25 + 0.03 A. In glasses synthesized under more reducing conditions (fo, = IO-'-IO-' atm), U" occurs in moderately distorted B-coordinated polyhedra [ d(U"-0) = 2.19-2.24 + 0.03 %I-], which may co-exist with smaller numbers of Uv' species and/or U'" species. Under the most reducing conditions used (h, = lo-*-IO-'* atm), U'" occurs in less distorted octahedra [d(U'"-0) M 2.26-2.29-t 0.02 A]. No clear evidence for U-F or U-Cl bonds was found for any of the halogen-containing glasses, suggesting that U-halogen "complexes" are not present. In addition, no U-U (second-neighbor) interactions were detected, indicating that no significant clustering of U atoms is present in any of the glasses studied. Bond strength-bond length calculations and constraints placed on loca1 bonding by Pauling's second rule suggest that U'" and U" in 6~oordinated sites in silicate melts will preferentially bond to nonb~d~ng oxygens f NBO's) rather than bridging oxygens (BO's). The unusually low &fold coordination of U" and U" in relatively depoIymerized silicate melts (e.g., peralkaline and halogen-rich melts) results in a high U-O bond strength in the melt that is not observed in crystalline U-bearing minerals. This difference in bond strength is partially responsible for the small crystal-melt partition coefficients of U"'. In addition, the common silicate minerals comprising igneous rocks lack appropriate crystallographic sites which can stably accommodate this large and highly charged cation. These factors help explain the normally incompatible character of U"' during magmatic differentiation. In contrast, the low solubility of U'" and U" in more polymerized silicate melts, such as those produced during the late stages of magmatic differentiation, can be explained by a shortage of NBO's. Increasing amounts of S-fold coordinated U should favor the incorporation of both U" and Uv in accessory minerals like zircon, thorite, titanite, apatite, uranium oxides, etc., thus its more compatible behavior in the latest stages of magmatic differentiation. INTRODUCI'ION URANIUM IS AN IMPORTANT TRACE element in igneous systems, providing information about the age, origin, and crystallization history of magmas (ALL~ZGRE et al., 1977; HEN-DERSON, 1982; CUNEY and FRIEDRICH, 1987; HOFMANN, 1988). It is also an important source of heat in the Earth's crust and mantle (VIGNERESSE and CUNEY, 199 1). Because of its reIativeIy large size (U"' m 0.94 A in g-fold coordination with "'0, SHANNON and PREWITT, 1969), U is considered one of the LILE-type elements (Large Ion Lithophile Efemerit), which are characterized, in general, by very small mineral-melt partition coefficients (
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