In attempting to evaluate the possible health consequences of chronic ionizing radiation exposure during extended space travel (e.g., Mars Mission), ground-based experimental studies of the clinical and pathological responses of canines under low daily doses of 60Co gamma irradiation (0.3–26.3 cGy d−1) have been examined. Specific reference was given to responses of the blood forming system. Results suggest that the daily dose rate of 7.5 cGy d−1 represents a threshold below which the hematopoietic system can retain either partial or full trilineal cell-producing capacity (erythropoiesis, myelopoiesis, and megakaryopoiesis) for extended periods of exposure (> 1yr). Trilineal capacity was fully retained for several years of exposure at the lowest dose-rate tested (0.3 cGy d−1) but was completely lost within several hundred days at the highest dose-rate (26.3 cGy d−1). Retention of hematopoietic capacity under chronic exposure has been demonstrated to be mediated by hematopoietic progenitors with acquired radioresistance and repair functions, altered cytogenetics, and cell-cycle characteristics. Radiological, biological, and temporal parameters responsible for these vital acquisitions by hematopoietic progenitors have been partially characterized. These parameters, along with threshold responses, are described and discussed in relation to potential health risks of the space traveler under chronic stress of low-dose irradiation.

The combined effects of injury from exposure to ionizing radiation and the potential biological warfare agent Venezuelan equine encephalitis (VEE) virus remain largely unknown. To study these effects, 4- to 5-week-old B6D2F1/J female mice were given a sublethal whole-body 7 Gy dose of 60Co gamma-photon radiation followed 48 hours later by aerosol or intraperitoneal challenge with enzootic VEE IIIA virus. Survival was observed for 30 days. A single sublethal 7 Gy dose of gamma radiation reduced the LD50/30 of VEE IIIA virus, in intraperitoneal challenged mice by a factor of 10(4) from 1.1 x 10(6) plaque-forming units (pfu) to 1 x 10(2) pfu, and in aerosol challenged mice, by a factor of 5 from 70 pfu to 14 pfu. These findings further confirm there is a combined effect of exposure to ionizing radiation and biological warfare agents, which could be devastating to unprotected populations and thus should be investigated further.

X-ray absorption spectroscopy (XAS) of Fe(2+) in Fe(2)SiO(4) liquid at 1575 kelvin and 10(-4) gigapascal (1 bar) shows that the Fe(2+) -O bond length is 1.98 +/- 0.02 angstroms compared with approximately 2.22 angstroms in crystalline Fe(2)SiO(4) (fayalite) at the melting point (1478 kelvin), which indicates a decrease in average Fe(2+) coordination number from six in fayalite to four in the liquid. Anharmonicity in the liquid was accounted for using a data analysis procedure. This reduction in coordination number is similar to that observed on the melting of certain ionic salts. These results are used to develop a model of the medium-range structural environment of Fe(2+) in olivine-composition melts, which helps explain some of the properties of Fe(2)SiO(4) liquid, including density, viscosity, and the partitioning of iron and nickel between silicate melts and crystalline olivines. Some of the implications of this model for silicate melts in the Earth's crust and mantle are discussed.

The oxidation states and coordination environment of Fe in 4 aquamarine samples with different color shades were studied. The C axis along with plane A and plane C of the samples were identified and the measurements were made with plane A oriented facing incoming beam and plane C oriented facing incoming beam. NIR and Raman spectroscopy were employed to confirm the structure of the samples. XRD was used to confirm the polished faces. UV–Vis and X-ray absorption near edge structure (XANES) were used to identify the presence of both Fe2+ and Fe3+ in the samples. The results indicated that Fe resided predominantly in 6-fold coordinated sites and the deep blue shade depended on the amount of Fe2+ in 6-fold coordinated sites. The effect of dichroism was noticeable in NIR, Raman spectroscopy, UV–Vis and XANES. Clearer Raman bands were observed when samples were oriented with plane A facing laser source. NIR and UV–Vis analysis could benefit from the complementarity of spectra measured from both orientations. Discrepancies between XANES spectra measured from the two orientations could lead to uncertainties in obtained values of oxidation states.

Fe2+ is the most abundant iron species in magmas and in many slags. Its structural role in these liquids is poorly understood, largely because of the difficulty of studying melts at high temperatures by direct structural methods. Whether Fe2+ behaves as a network modifier, network former or free-ion complex has not been adequately resolved, yet this is fundamental to understanding the properties of silicate liquids. Also uncharacterized are the structural changes accompanying the melt-to-glass transition in Fe-bearing silicate melts. Here we report the results of a high-temperature synchrotron-based X-ray absorption study of Fe in silicate glasses and melts of compositions near Na2FeSi3O8 and K2FeSi3O8. The glasses were also analysed by 57Fe Mossbauer spectroscopy. We conclude that Fe2+ is a four-coordinated network former in these melt/glass systems and that little structural relaxation occurs at the iron site during the melt-to-glass transition. These results and structural data for Fe22+SiO4 melts1 suggest the possibility of a pressure-induced change from four- to six-coordination for Fe2+ in magmas in the Earth's upper mantle.

Using synchrotron radiation, Fe x‐ray absorption spectra have been collected from a Mg0.88Fe0.12SiO3 perovskite quenched from 50 GPa and ∼ 2000 K. Least‐squares fits of the Extended X‐ray Absorption Fine Structure (EXAFS) data yield an Fe coordination number of 5.5 (±0.8) oxygens, an Fe‐O distance of 214 (± 4) pm, and a static and thermal disorder parameter of 147 pm². The absorption edge spectrum shows a weak pre‐edge feature suggesting little deviation from centrosymmetry for the Fe site. These results indicate that Fe in this silicate perovskite partitions predominantly into the six‐coordinated site and may provide the first evidence for Si occupying an 8‐12 coordinated site.

Potassium K-EXAFS and XANES measurements were made using synchrotron radiation on selected glasses along the NaAlSi3O8-KAlSi3O8 (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 KAlSi2O6 (leucite) than in crystalline KAlSi3O8 (orthoclase) or KNa3Al4Si4O16 (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 albite50-orthoclase50. 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.

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.