A Raman Spectroscopic Study of Lightning‐Induced Glass Produced From Five Mineral Phases.

Lightning‐induced volcanic spherules (LIVS) are glasses produced by the rapid melting and solidification of molten volcanic ash grains. High temperatures generated by lightning will alter the physical and chemical properties of minerals exposed to the discharge. Laboratory experiments reveal that LIVS glass composition varies depending on the starting material, exhibiting heterogeneous compositional features common in other glasses created by cloud‐to‐ground lightning, nuclear explosions, and high velocity impact events. This study uses scanning electron microscopy, energy dispersive spectroscopy, and Raman spectroscopy to investigate the structure and Raman signatures of lightning‐induced glass spherules manufactured from five igneous minerals (<32 μm powders of albite, labradorite, augite, hornblende, and magnetite). LIVS were created through high‐current impulse experiments using peak currents of 25 and 40 kA. Analysis of the post‐experimental albite, labradorite, augite, and hornblende LIVS reveal primarily homogeneous silicate or aluminosilicate glasses with limited heterogeneity. Their amorphous Raman spectra are comparable to rhyolitic and mafic natural glasses along with Na2O‐K2O‐Al2O3‐SiO2, CaCO3‐Al2O3‐SiO2, and CaO‐MgO‐SiO2 synthetic glass networks. A few of the augite and hornblende LIVS spectra exhibit premelting effects, which occur below the melting point and represent the onset of cation disordering in phases that remain crystalline. Magnetite samples produced crystal‐rich, glass‐poor LIVS characterized by the growth of dendritic microcrystals and crystalline spectra that also contain a few bands alluding to the composition of their silicate–phosphate glass matrix. By understanding these chemical changes induced by lightning, we can extract information from other types of glasses produced during high temperature, short duration events. Plain Language Summary: High temperatures generated by volcanic lightning melt and fuse airborne volcanic ash grains consisting of mineral crystals and glassy fragments. Molten grains will rapidly quench into solid or hollow glass spheres termed lightning‐induced volcanic spherules, thereby altering the chemical and structural properties of the ash grains. This study utilizes Raman spectroscopy to characterize the structural properties of glass spherules produced from five common minerals (albite, labradorite, augite, hornblende, and magnetite) through lightning simulation experiments. Results reveal that lightning can alter these mineral samples irrevocably by transforming them into glass with complex compositions and Raman signatures corresponding to mineral and glass phases. The four minerals with a higher abundance of silica, a glass‐forming element, primarily produced chemically complex glass spheres with compositions representing a mixture of different melted minerals, and Raman signals similar to synthetic and naturally occurring glasses. In contrast, the post‐experimental spheres created from the silica‐poor, iron‐rich mineral magnetite are composed of newly formed iron micrometer‐sized crystals, limited amounts of glass, and Raman signals indicative of crystalline material. Although specific to volcanic ash, the impact of lightning on mineral fragments has broader implications for a wide range of geoscientists and planetary scientists. Key Points: Lightning simulation experiments conducted on powdered minerals produce particles containing both glass and crystalline phasesRaman spectroscopy of lightning‐induced spherules reveal spectra comparable to naturally occurring and synthetic multicomponent glassesCrystalline spectra derive from surviving mineral fragments and lightning‐induced crystallization of iron oxide dendrites [ABSTRACT FROM AUTHOR]