Study on Structure and Properties of La2O3-Doped Basaltic Glasses for Immobilizing Simulated Lanthanides

The La2O3-doped basaltic glass simulated high-level waste form (HLW) was prepared by the solid-state melt method. The simulated waste La2O3 maximum loading and the doping effect on structure, thermal stability, leaching behavior, density, and hardness of basaltic glasses were studied. XRD and SEM results show that the simulated waste loading of La2O3 in basaltic glass can be up to ~46 wt.%, and apatite (CaLa4(SiO4)3O) precipitates when the content of La2O3 reaches 56 wt.%. Raman results indicate that the addition of La2O3 breaks the Si–O–Si bond of large-membered and four-membered, but the number of A13+ involved in the formation of the network increase. Low content of La2O3 can help to repair the glass network, but it destroys the network as above 26 wt.%. DSC results show the thermal stability of simulated waste forms first increases and then decreases with the increase of La2O3 content. With the increase of La2O3 content, the density of the simulated waste form increases, and the hardness decreases. The leaching chemical stability of samples was evaluated by the ASTM Product Consistency Test (PCT) Method, which show that all the samples have good chemical stability. The leaching rates of La and Fe are three orders of magnitude lower than those of the other elements. Among them, L36 has the best comprehensive leaching performance.

Melt degassing triggered by magma injection?

<p>Explosive eruptions of silicic magmas depend mainly on the amount and the degassing behavior of soluble volatile components like H<sub>2</sub>O and CO<sub>2</sub>. The injection of a hot mafic magma into a cooler volatile-rich rhyolitic magma chamber might initiate mingling and mixing processes at the interface of the two melt reservoirs (Paredes-Marino et al. 2017). An accompanying increase in temperature and a buoyant ascent of the H<sub>2</sub>O-saturated rhyolitic melt may cause a sufficiently high decrease in solubility at pressures < 300 MPa (e.g. Holtz et al. 1995) to trigger vesicle formation. Furthermore, the interface between different melt compositions might act as a site for enhanced vesicle formation. To test this hypothesis, bimodal decompression experiments were conducted. Basaltic and rhyolitic compositions similar to the Askja eruption 1875 in Iceland (Sparks and Sigurdsson 1977) were used for this purpose. For the preparation of the experiments, rhyolitic and basaltic glass cylinders were molten and hydrated separately in an internally heated argon pressure vessel with H<sub>2</sub>O excess at 200 MPa and 1523 K for 96–168 h and then isobarically quenched with 16 K∙s<sup>‑1</sup>. The hydrated glass samples were cut perpendicular to the cylinder axis. The cylinder faces were polished to enable a perfect contact of the rhyolite cylinder with the basalt cylinder. An additional decompression experiment with two contacted hydrated rhyolite cylinders was conducted as a reference to test the experimental setup.</p><p>Each pair of cylinders was heated isobarically with 25 K·s<sup>-1</sup> to 1348 K at 210 MPa and equilibrated for 10 min. To simulate the magma ascent, three bimodal samples and the reference sample were decompressed with rates of 0.17 MPa∙s<sup>-1 </sup>or 1.7 MPa∙s<sup>-1</sup> to the final pressure of 100 MPa and then quenched with 44 K∙s<sup>-1</sup>. H<sub>2</sub>O vesicle number and spatial distribution as well as the H<sub>2</sub>O contents in the decompressed samples were analysed by microscope, quantitative BSE image analysis, and FTIR-spectroscopy, respectively.</p><p>All decompression experiments resulted in vesiculated samples. In the rhyolite reference experiment, the H<sub>2</sub>O vesicles are homogeneously distributed within the whole sample. The former interface of the cylinders is no longer visible. This confirms that the former contact plane of the cylinders does not influence the degassing behaviour during decompression.</p><p>Optical examination and electron microprobe analysis of oxide diffusion profiles of the decompressed bimodal samples expose the development of a hybrid melt zone between the rhyolite and the partially crystallized basalt, documenting mixing processes during the decompression experiments (Petri 2020). The hybrid zone in the rhyolitic compositional dominated region is decorated with an enhanced number of H<sub>2</sub>O vesicles compared to the rhyolitic and basaltic glass volumes. This suggests that the injection of a basaltic melt into a rhyolitic melt reservoir may lead to significantly enhanced homogeneous H<sub>2</sub>O vesicle formation in the hybrid zone and, therefore, enhanced degassing with the concomitant triggering of explosive eruptions.</p><p> </p><p>Holtz F. et al. (1995) American Mineralogist 80: 84-108.</p><p>Paredes-Marino J. et al. (2017) Scientific Reports 7: 16897.</p><p>Petri P. (2020) Master thesis University of Tübingen.</p><p>Sparks S.R.J. and Sigurdsson H. (1977) Nature 267: 315-318.</p>

Preparation and Dielectric Properties of the Amorphous Basaltic Glass

Due to the continuous basalt fiber is a kind of amorphous material, only the basaltic glass without crystallization can represent its dielectric properties. To explore the dielectric properties of the continuous basalt fiber, amorphous basaltic glass must be prepared. The present research focuses on the influence of chemical component on the preparation process of the amorphous basaltic glass and its dielectric properties. The basaltic rocks from different places of China were melted at 1500℃, then the melt was poured into the mould, at last glass sample was annealed at 650℃ for 2h. Ten groups of basaltic rocks were studied, and the results showed that the melt viscosity of basaltic rocks with 55-58% SiO2 was poor at 1500℃. The high-content of Fe2O3 and TiO2 in basaltic rocks was found to enhance the formation of magnetite (Fe3O4) crystal during the annealing process. The other five groups of basaltic rocks were suit to the amorphous basaltic glass. At 1MHz, the best dielectric constant of amorphous basaltic glass is 6.55, the dielectric loss is 4.034×10-3.