David A. McKeown

Characterization of manganese oxide mineralogy in rock varnish and dendrites using X-ray absorption spectroscopy

Abstract X-ray absorption data were collected for a series of varnish and dendrite Mn oxide coatings on rock substrates containing a wide variety of mineralogies exposed to a variety of environments. Near-edge spectra of the coatings indicate that the Mn-oxide phases present have Mn valences between 3+ and 4+, with average Mn valences for the varnishes closer to 4+ than those for the dendrites. Mn EXAFS data and analyses indicate that Mn-oxide structure types for the varnishes range, perhaps continuously, from large tunnel phases, similar to todorokite and romanechite, to layer phases, i.e., birnessite-family. Similar results were found for the dendrite samples, except that the variety of Mn-oxide phases is somewhat larger than those found for the varnishes. No correlations were found between Mn-oxide structure-type within these coatings and the corresponding substrate petrology.

VIBRATIONAL ANALYSIS OF PALYGORSKITE AND SEPIOLITE

Lattice dynamic calculations for the sepiolite and palygorskite structures using polarized Raman and FTIR spectra provide a fundamental basis for interpreting spectral features by assigning vibrational modes. The Si-O stretch and O-Si-O bond bending force constants determined for palygorskite are similar to equivalent values calculated previously for other phyllosilicates. The Mg-O bond stretch values, on the other hand, are about half of those determined for the equivalent Al-O and Mg-O bond stretch environments in other phyllosilicates, suggesting that the bonding within the octahedral ribbons in palygorskite and sepiolite is weaker than that in the continuous octahedral sheets in micas. The weaker bonding allows more flexible octahedral environments in palygorskite and sepiolite, giving rise to higher probabilities for cation substitutions and vacancies relative to the micas. Above ∼700 cm−1 in the IR and 750 cm−1 in the Raman spectra, the eigenmodes are dominated by atomic displacements within the silicate sheets. Below 700 cm−1 the eigenmodes become mixed with motions among the Mg octahedra and the silicate sheets; the eigenmodes assigned to the most prominent peaks in the Raman spectra (near 700 cm−1) belong to this group. As mode frequencies decrease, the corresponding eigenmodes evolve from more localized Mg-O stretch, O-Mg-O bend and O-Si-O bend motions to longer-range motions such as silicate sheet deformations caused by silicate tetrahedra rotation and silicate sheet shearing around the Mg-octahedral sheets.

Raman spectroscopy and vibrational analyses of albite: From 25 °C through the melting temperature

Abstract Raman spectra were collected for crystalline albite from 25 °C to above the 1118 °C melting temperature, where vibrational assignments for the crystal spectra were determined by lattice dynamics (LD). The Raman spectra and associated vibrational assignments are reported for triclinic albite (NaAlSi3O8) at 25 °C and monoclinic albite at 1060 °C. The 25 °C calculations determined that localized T-O stretch and O-T-O bend modes are above 900 cm-1 (where T = Si,Al), while motions from the aluminosilicate tetrahedral cage mixed with Na displacements occur in modes as high as 814 cm-1. Vibrational modes for the most prominent peaks in the spectrum, between 350 and 550 cm-1, are dominated by four-membered tetrahedral ring deformations. For completeness, calculated infrared mode frequencies and their atomic displacements are reported for the 25 °C structure and compared with normal mode calculation results and observed infrared mode frequencies presented by von Stengel (1977). At higher temperatures, modes above 550 cm-1 broaden and shift to lower frequencies by 15 to 27 cm-1; modes below 550 cm-1 broaden, but experience little, if any frequency shifts. Albite melted sluggishly, was completely liquid at 1320 °C, and remained amorphous upon cooling to room temperature. At frequencies above 550 cm-1, the crystalline albite peaks, and possibly their vibrational assignments, can be correlated to Raman bands for albite glass. Spectral differences below 550 cm-1 between crystal and glass correspond to changes of average tetrahedral ring type upon melting, as shown by Taylor and Brown (1979).

Raman studies of sulfur in borosilicate waste glasses: sulfate environments

Abstract Raman spectroscopy has been used to characterize sulfur environments in a variety of borosilicate glass formulations developed for long-term radioactive waste storage. The spectra of these glasses all have S–O symmetrical stretch modes (ν1) near 1000 cm −1 from tetrahedral SO4 (sulfate) environments. The Raman data indicate that the sulfate environments are independent of the borosilicate network; in particular, isolated SO4 tetrahedra in the glass are surrounded by network modifying cations, such as Na. By changing the type of network modifying cations in the borosilicate glass, the ν1 peak shifts, in such a way that larger cation charge densities correlate to higher ν1 frequencies. The ν1 peak for Li, Ca, Na, K, and Cs borosilicate glasses is broadened and shifted to lower frequencies with respect to ν1 for Li, Ca, Na, K, and Cs sulfate crystals, respectively; this indicates that sulfate tetrahedra in borosilicate glass are more disordered and more weakly bonded to their surrounding environments than sulfate tetrahedra in the corresponding sulfate crystals.

Vibrational Analysis of the Dioctahedral Mica: 2M1 Muscovite

Abstract Raman spectra and lattice dynamics calculations are presented for the dioctahedral mica, muscovite. Calculated fundamental mode frequencies for the Raman-active and Ag and Bg species were fit to observed fundamentals assigned to features in the two polarized Raman spectra collected, so that unambiguous vibrational assignments could be made to most peaks in the Raman data. Calculated frequencies for the IR-active Au and Bu modes generally fall within the frequency ranges of bands in the IR spectra for muscovite presented earlier. Factor group analysis indicates that motion from all atom types in the muscovite structure can be found in modes for all four vibrational species. Force constant values determined for muscovite are similar to equivalent values calculated for the trioctahedral mica, phlogopite. Raman and IR-active modes calculated at frequencies greater than 800 cm-1 are dominated by internal sheet T-O stretch and T-O-T bend motions, where T is a tetrahedral site. Modes between 800 and 360 cm-1 have internal tetrahedral sheet motions mixed with K and octahedral Al displacements. Modes at frequencies less than 360 cm-1 have lattice and OH motions. Inter-sheet bonding in the muscovite structure is strong enough to affect modes at frequencies as high as 824 cm-1.

Raman spectra and vibrational analysis of the trioctahedral mica phlogopite

Abstract The use of a direct crystallographic technique is reported for locating Cr atomic sites in a mullite containing 11.5 wt% Cr2O3 by monitoring variations in characteristic X-ray emission rates as a function of fast electron beam orientation. Systematic examination of two dimensional incoherent channeling patterns (ICP), formed from characteristic X-ray emissions from Al, Si, and Cr, and recorded near low index zone axis orientations, has enabled the preferred lattice position of Cr in mullite to be identified as the interstitial site 0, 0.25, 0. Although the method of atom location by channeling enhanced microanalysis (or ALCHEMI) generally has been applied in situations where introduced minority atom species are accommodated in substitutional atomic positions, this study illustrates the identification of an interstitial site of an introduced dopant species. This result does not coincide with that derived from X-ray Rietveld refinement. The ICP method is analytically robust and, unlike Rietveld refinement, does not require a highly accurate model of the host lattice framework and composition. ICP analysis therefore may be more appropriate for this particular application.

X-ray absorption studies of manganese valence and local environment in borosilicate waste glasses

Abstract X-ray absorption data were collected and analyzed to characterize the manganese environments in borosilicate glass formulations to be used for immobilization of nuclear wastes. Mn can become a significant constituent in some radioactive wastes, because of the use of Mn-compounds in waste pretreatment processes. Sixteen borosilicate glasses were investigated, which were synthesized to simulate the Mn environments in the anticipated waste glasses, where MnO concentrations range from 0.4 to 13.6 wt%. The X-ray absorption near edge structure (XANES) for all glasses investigated indicate that most of the manganese within these samples is divalent. The extended X-ray absorption fine structure (EXAFS) analysis results for the glasses show average Mn–O distances near 2.07 A, coordination numbers between 4.3 and 5.2, and large first-shell Debye–Waller factors. The EXAFS findings indicate that Mn2+ in borosilicate glass is most likely within a distribution of environments that include 4- and 5-coordinated sites. EXAFS data and fitting results also show that the average manganese environments in these glasses are statistically invariant with respect to composition as well as to synthesis conditions.

Structural characterization of high-zirconia borosilicate glasses using Raman spectroscopy

Polarized Raman spectra were obtained for a collection of borosilicate glasses that have been developed as candidate compositions for the immobilization of wastes generated from the reprocessing of Zircaloy-clad spent nuclear fuel. Raman spectra were obtained for borosilicate glasses with zirconia compositions as high as 21 wt%, as well as for crystalline ZrO2 (baddeleyite) and crystalline ZrSiO4 (zircon). As zirconia content in the glass is increased, two trends in the spectra indicate that the partially polymerized silicate tetrahedral network becomes more depolymerized: one, the polarized mid frequency envelope near 450 cm ˇ1 , assigned to Si‐O‐Si symmetrical bend modes, decreases in area; and two, the higher frequency band assigned to Si‐O stretch in Q 2 units (silicate chains) increases in area, while band areas decrease for modes assigned to Si‐O stretch in more polymerized Q 3 and Q 4 units (silicate sheets and cages). These trends take place whether the glass composition is relatively simple or considerably more complex. As zirconia concentrations in the glass increase beyond 15 wt%, a series of sharp lines are observed in the spectra from baddeleyite crystals, and to a minor extent Zn‐Cr spinel phases, superimposed on broad features from the glass matrix. A low intensity, broad band near 1400 cm ˇ1 in the glass spectra is probably due to B‐O stretch modes within BO3 units. ” 2000 Elsevier Science B.V. All rights reserved.

Theoretical determination of the Raman spectra of single-crystal forsterite (Mg2SiO4)

Abstract Density functional perturbation theory is used to calculate the Raman spectrum of forsterite (Mg2SiO1). In addition to the fundamental mode frequencies and atomic displacements, the scattering intensities are computed from first principles for the first time. Six independent single-crystal Raman spectra are measured for synthetic forsterite, and good agreement is found between calculation and experiment over a range of nearly six orders of magnitude of scattered intensity. Calculated atomic displacements of these fundamental Raman modes generally agree closely with the results of previous lattice dynamics studies. Modes with frequencies above 500 cm-1 consist primarily of motions internal to the SiO4 tetrahedra, while those below 500 cm-1 are dominated by Mg2 displacements mixed with SiO4 translations and rotations. The considerably larger Raman amplitudes for modes above 500 cm-1 appear to be due to displacements within the highly polarizable oxygen environments surrounded by covalently bonded Si4+ and ionically bonded Mg2+. With regard to calculated frequencies, the theory underestimates frequencies by as much as 8 cm-1 for modes over 500 cm-1, while it generally overestimates frequencies by as much as 17 cm-1 for modes below 500 cm-1. An equivalent set of Raman spectra were measured for the Fe end-member of the olivine solid-solution series, fayalite (Fe2SiO4), and compared to the results for forsterite

X-ray absorption studies of vanadium valence and local environment in borosilicate waste glasses using vanadium sulfide, silicate, and oxide standards

X-ray absorption spectroscopic data were collected and analyzed to characterize vanadium in borosilicate glasses used for immobilization of sulfur-containing nuclear wastes. Data are presented for borosilicate glasses, some with and some without sulfur, that have V2O5 concentrations as high as 12 wt%, and for the sulfides: sulvanite and patronite, the silicates: cavansite, hadaraite, and roscoelite, and the oxide: vanadinite. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data for the glasses have no sulfur dependent features, but do show changes that parallel various redox conditions for the corresponding melts. EXAFS data for the glasses indicate V–O distances near 1.70 A that are considerably shorter than typical V–S distances found in the sulfides. Both XANES and EXAFS indicate that most or all vanadium in these glasses is in the form of V5+O4 tetrahedra; glasses synthesized under reducing conditions can have penta-coordinated V4+ populations up to approximately 20–25% of all vanadium present.