Aaron J. Celestian

Phosphor imaging as a tool for in situ mapping of ppm levels of uranium and thorium in rocks and minerals

Abstract Phosphor imaging is a type of digital autoradiography that has been widely used in biochemistry to examine radioactively tagged proteins. We used phosphor imaging to map in situ U and Th in polished slabs of geological materials including carbonates, phosphates, and silicate-rich rocks. We examined samples containing between 2 and >500 ppm U and ∼700 ppm Th to evaluate the applicability of the technique to geological samples. Resolution of 1 mm or better was obtained even for low concentration (∼10 ppm) samples. These analyses are routine and only require a light box, phosphor screen, and access to a phosphor imager. The technique is nondestructive, relatively inexpensive, and requires very little processing time. We used this technique to identify U- and Th-enriched carbonates and phosphates, and to find “hot spots” of U- and Th-rich minerals in a granodiorite. These high-resolution maps of U and Th allow us to effectively sample for geochronology and identify potentially interesting samples for synchrotron X-radiation studies. The maps produced by phosphor imaging also have great potential for investigating the details of adsorption of radionuclides to rocks and minerals in contaminated areas.

A new polymorph of eucryptite (LiAlSiO4), ε-eucryptite, and thermal expansion of α- and ε-eucryptite at high pressure

Abstract X-ray diffraction experiments have been carried out on b-eucryptite (LiAlSiO4) at pressures up to 2.5 GPa and temperatures up to 1073 K in a large-volume apparatus. With room-temperature compression, we observed a phase transition to a new polymorph between 0.83 and 1.12 GPa. This transition is reversible in character. The new phase, referred to here as e-eucryptite, can be indexed according to an orthorhombic unit cell with a = 10.217(4) Å, b = 8.487(4), Å, c = 5.751(3) Å, and V = 498.7(4) Å3 for XRD data at 2.2 GPa and 298 K. On heating at 2.2 GPa, e-eucryptite and beucryptite were metastable over the temperature interval 298-873 K; at higher temperatures they underwent an irreversible phase transition to a-eucryptite. Both hexagonal α-eucryptite and ε-eucryptite show anisotropic thermal expansion. For α-eucryptite, we obtained αa = 6.71(±0.25) × 10-6 K-1, αc = 1.07(±0.05) × 10-5 K-1, and αv = 2.42(±0.1) x 10-5 K-1 at 1.94(2) GPa over the temperature range 298-1073 K. For ε-eucryptite at 2.32(8) GPa, we find larger thermal expansion in a smaller temperature range 298-773 K, with αa = 1.47(±0.15) × 10-5 K-1, αb = 6.65(±1.33) x 10-6 K-1, αc = 7.83(±0.88) × 10-6 K-1, and αv = 2.99(±0.15) × 10-5 K-1. In combination with a previous determination of thermal expansion at ambient pressure, the pressure effect on volume thermal expansion of a-eucryptite is determined to be -2.68 × 10-6 GPa-1 K-1, and the temperature derivative of the bulk modulus is estimated to be -0.015 GPa/K.

Effects of Hydration during Strontium Exchange into Nanoporous Hydrogen Niobium Titanium Silicate

A Nb-substituted titanium silicate with the sitinakite (NbTS) topology was exchanged with Sr(2+) to determine the mechanisms and pathways of ion diffusion through this mixed polyhedral nanoporous framework. The refined structural models yield unit cell parameters and atomic positions of Sr(2+) and suggest that there was a two-step process during cation diffusion. The starting material of the exchange experiment was the H(+)-exchanged material, H(1.4)Nb(0.6)Ti(1.4)SiO(7)·1.9H(2)O, with space group P4(2)/mcm. In the beginning of the exchange process, Sr filled the 8-membered-ring channel near the 4(2) axis in the center. Once the Sr(2+) fractional occupancy reached approximate 0.11, Sr positions and extra-framework H(2)O molecules shifted away from the central 8-membered-ring toward the framework, and an increase in Sr hydration and framework bonding was observed. The new H(2)O positions resulted in a lowering of symmetry to the P ̅42m space group, and it is thought that the Sr migration served to enhance Sr(2+) ion diffusion capacity into the channels of NbTS since the exchange rate briefly accelerated after the 0.11 fractional occupancy level was passed. Exchange of Sr(2+) into the nanoporous material reached maximum fractional site occupancy of approximately 0.20 using a 10.0 mM SrCl(2) solution.