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Abstract: A critical radiation damage assessment of the materials that will be present in a Geological Disposal Facility (GDF) for radioactive waste is a priority for building a safety case. Detailed analysis of the effects of high-energy a -particle damage in phyllosilicates such as mica is a necessity, as these are model structures for both the clay-based backfill material and the highly sorbent components of a crystalline host rock. The a -radiation stability of biotite mica [general formula: K(Mg,Fe)3(Al,Si3O10)(F,OH)2 ] has been investigated using the 5 MV tandem pelletron at the University of Manchester’s Dalton Cumbrian Facility (DCF) and both the microfocus spectroscopy (I18) and core X -ray absorption spectroscopy (B18) beamlines at Diamond Light Source (U.K.). Microfocus X-ray diffraction mapping has demonstrated extensive structural aberrations in the mica resulting from controlled exposure to the focused 4He2+ ion (a-particle) beam. Delivered doses were comparable to a-particle fluences expected in the highly active, near-field of a GDF. At doses up to 6.77 displacements per atom (dpa) in the region of highest particle fluence, biotite mica displays a heterogeneous structural response to irradiation on a micrometer scale, with sequential dilation and contraction of regions of the structure perpendicular to the sheets, as well as a general overall contraction of the phyllosilicate layer spacing. At the peak of ion fluence, the structure collapses under a high point defect density and amorphous areas are pervasive among altered domains of the original lattice. Such structural alterations are likely to affect the material’s capacity to sorb and retain escaped radionuclides over long timescales; increased edge site availability may favor increased sorption while interlayer uptake will likely be reduced due to collapse. Radiation-induced reduction of structural iron at the region of highest structural damage across an a-particle’s track has been demonstrated by Fe K-edge X-ray absorption near edge spectroscopy (XANES) and local structural disorder has been confirmed by analysis of both potassium K-edge XANES and Fe K-edge extended X-ray absorption fine structure analysis. An infrared absorption study of deformations in the OH– stretching region, along with electron probe microanalysis complements the synchrotron data presented here

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Abstract: The 10 Å phase is a high-pressure hydrous magnesium silicate whose composition appears to depend on synthesis conditions. We have measured the compressibility to 10.5 GPa and thermal expansivity to 400 °C of samples of 10 Å phase synthesized in long experiments (400 and 169 h, respectively) designed to maximize compositional equilibrium. The structure was refined using a metrically trigonal unit cell. Compression is highly anisotropic, especially over the first 2 GPa of compression, indicating weak bonding across the interlayer. There is an inflection in the compression curve of c at 8 GPa, suggesting a change in compression mechanism or the onset of non-hydrostaticity in the pressure medium. Fitting the compression data collected below 8 GPa to a Murnaghan equation-of-state gives V0 = 734.8(7) Å3, K0 = 25(1) GPa, K' = 18(1). Thermal expansion is also strongly anisotropic: coefficients for data up to 200 °C are {alpha}a = 0.15(5) x 10^-5 K^-1, {alpha}c = 3.1(2) x 10^-5 K^-1, {alpha}V = 3.4(2) x 10^-5 K^-1. Above 200 °C, the expansivity of c decreased, and all parameters showed a contraction after the experiment, suggesting partial dehydration at high temperatures. Comparison of our compressibility data with those of previous studies suggests that 10 Å phase synthesized in short experiments does not retain all of its interlayer H2O during quenching and decompression. In contrast, samples annealed for many hours at high pressure and temperature are stabilized by small amounts of hydrogarnet-type substitution and consequent hydrogen bond strengthening.

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Abstract: We recorded angle-dependent XANES (AXANES) spectra at the potassium K-edge for three compositionally intermediate polylithionite-siderophyllite trioctahedral 1M-micas using polarized synchrotron radiation. We evaluated the experimental spectra for both their in-plane and out-of-plane component fractions of the electric dipole contribution using the analytical formulae of Brouder (1990), referring to theory to extract the origin of their multiple-scattering pathways of Natoli et al. (2003). This analysis was extended to a fourth lithian mica studied previously and allowed characterization of the local environment and ordering around the potassium atoms in the interlayer of the entire set of micas. The AXANES in-plane components are notably similar to the XANES spectra recorded on randomly oriented powders, provided these are oriented at the "magic angle" (Pettifer et al. 1992). Most observed contributions arise from multiple-scattering interactions of the photoelectron ejected from the potassium absorber colliding with atoms located in the interlayer itself. Note that this includes not only interactions with other coplanar potassium and/or alkali atoms distributed along the interlayer plane, but also with their near- and next-nearest neighboring oxygen atoms which lie on the basal planes of the tetrahedral sheets facing the interlayer. By contrast, the AXANES out-of-plane component suggests that several multiple-scattering pathways cross the energetic and structural-barrier represented by the tetrahedral sheets. They reach not only the X anions that are located on the upper level of the octahedral sheets, at the center of the open cavity in the tetrahedral sheet, but also the metal cations centering the octahedral sheet itself. Therefore, the out-of-plane components provide indirect information on the number of independent octahedral sites, the cation oxidation state, and the trans- vs. cis-orientation of the anionic sites.

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Abstract: A detailed description of the interlayer site in trioctahedral true micas is presented based on a statistical appraisal of crystal-chemical, structural, and spectroscopic data determined on two sets of trioctahedral micas extensively studied by both X-ray diffraction refinement on single crystals (SC-XRD) and X-ray absorption fine spectroscopy (XAFS) at the potassium K-edge. Spectroscopy was carried out on both random powders and oriented cleavage flakes, the latter setting taking advantage of the polarized character of synchrotron radiation. Such an approach (AXANES) is shown to be complementary to crystal-chemical investigation based on SC-XRD refinement. However, the results are not definitive as they focus on few samples having extreme features only (e.g., end-members, unusual compositions, and samples with extreme and well-identified substitution mechanisms). The experimental absorption K-edge (XANES) for potassium was decomposed by calculation and extrapolated into a full in-plane absorption component (?||) and a full out-of-plane absorption component (??). These two patterns reflect different structural features: ?|| represents the arrangement of the atoms located in the mica interlayer space and facing tetrahedral sheets; ?? is associated with multiple-scattering interactions entering deep into the mica structure, thus also reflecting interactions with the heavy atoms (essentially Fe) located in the octahedral sheet. The out-of-plane patterns also provide insights into the electronic properties of the octahedral cations, such as their oxidation states (e.g., Fe2+ and Fe3+) and their ordering (e.g., trans- vs. cis-setting). It is also possible to distinguish between F- and OH-rich micas due to peculiar absorption features originating from the F vs. OH occupancy of the O4 octahedral site. Thus, combining crystal-chemical, structural, and spectroscopic information is shown to be a practical method that allows, on one hand, assignment of the observed spectroscopic features to precise structural pathways followed by the photoelectron within the mica structure and, on the other hand, clarification of the amount of electronic interactions and forces acting onto the individual atoms at the various structural sites.

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Abstract: Two types of metamorphic phengites are known: one is linked to high pressure and is 3T; the other is 2M1, and its composition is linked to rock-compositional constraints. This work investigates the octahedral sheet crystal-chemical differences between the two phengite types. Seven dioctahedral micas were studied: (1) one 3T phengite from an ultrahigh-pressure metagranitoid in the Dora Maira massif, Italy (P ~ 4.3 GPa, T ~ 730 °C); (2) five 2M1 phengites from medium-P orthogneisses in the Eastern Alps metamorphic basement, Italy (P ? 0.7 GPa, T ~ 500–600 °C); and (3) one 2M1 ferroan muscovite from pegmatite in Antarctica (P ? 0.2 GPa, T ~500 °C). All micas display significant extents of celadonite substitution. In particular, the 2M1-phengite formulae (calculated on the basis of 11 O) have 0.68 < IVAl < 0.82 atoms per formula unit (apfu); octahedral atoms are dominated by Al (1.6–1.8 apfu), with minor and variable Fe (0.20–0.35 apfu) and Mg (0.05–0.17 apfu), and very minor Ti, Mn, and Cr. Total octahedral occupancies are slightly above 2.00 apfu, i.e., there seems to be partial occupancy of the third M site. For all micas, we recorded XAFS spectra on mosaics of carefully separated flakes oriented flat on a plastic support that could be rotated so as to account for the polarization of the synchrotron radiation beam, and we processed them on the basis of the AXANES theory. Spectra show angle-dependent absorption variations for Al and Fe, which can be deconvoluted and fitted by dichroic effects. Pre-edges consistently show most Fe to be Fe3+ and little angle-dependent intensity variations. The 2M1-ferroan muscovite from Antarctica displays the same AXANES behavior as 2M1-phengites. By contrast, the ultrahigh-pressure 3T-phengite from Dora Maira (having IVAl = 0.42 apfu, and Al and Mg as the dominant octahedral constituents) has XAFS spectra that differ significantly. Not only is the IVAl feature strongly reduced, in agreement with the increased Si content, but also Fe XAFS spectra show one broad feature only, indicating that all Fe is Fe2+ in a fully disordered distribution with no angle-dependent variations. We conclude that this 3T-phengite is actually contaminated by exsolved Fe-bearing pyrope platelets, which cannot be resolved under SEM examination; by contrast, the 2M1-phengites, unrelated to high-pressure, suggest Al/Fe3+ order over the M1 and (M2, M3) sites, as also does the 2M1 pegmatitic muscovite.