skip to main content

Title: 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‐SiO2synthetic 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.

more » « less
Author(s) / Creator(s):
 ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Earth and Space Science
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    The physical properties of minerals are modified by the high temperatures of volcanic lightning during explosive eruptions. Alteration involves rapid heating and volatilization, melting, and fusion of ash grains within the discharge channel, followed by rapid quenching into new glassy textures. High current impulse experiments reveal that lightning alters not only the morphology and mineralogy of volcanic ash but also its magnetic properties. We investigate lightning‐induced magnetic changes in five igneous minerals (<32 μm powders of albite, labradorite, augite, hornblende, and magnetite) by comparing hysteresis parameters before and after impulse experiments conducted at peak currents of 25 and 40 kA. Both the paramagnetic and ferrimagnetic behaviors of the samples were altered, which we interpret as a superposition of two processes. (a) Rapid melting allows iron contained within inclusions of Fe‐oxides and Fe‐bearing silicates to diffuse into the newly formed melt, thereby increasing the paramagnetism of the resulting glass. (b) Nucleation and growth of magnetite from the newly formed melt increases the ferrimagnetic behavior of the post‐experimental samples. Nominally non‐Fe‐bearing silicates like albite and labradorite have significantly increased paramagnetism and ferrimagnetism. Fe‐bearing silicates like augite and hornblende contain higher concentrations of ferrimagnetic inclusions, from which Fe diffuses into the newly formed melt, thereby increasing paramagnetism while decreasing ferrimagnetism. Experiments conducted on magnetite produced new magnetite crystals with dendritic habits. Although specific to volcanic ash, these results provide important insights into the magnetism of other materials affected by lightning on Earth, the Moon, and throughout the solar system.

    more » « less
  2. Abstract

    Infrared (IR) and Raman spectroscopic features of silicate glasses are often interpreted based on the analogy with those of smaller molecules, molecular clusters, or crystalline counterparts; this study tests the accuracy and validity of these widely cited peak assignment schemes by comparing vibrational spectral features with bond parameters of the glass network created by molecular dynamics (MD) simulations. A series of sodium silicate glasses with compositions of [Na2O]x[Al2O3]2[SiO2]98−xwithx = 7, 12, 17, and 22 were synthesized and analyzed with IR and Raman. A silica glass substrate and a crystalline quartz were also analyzed for comparison. Glass structures with the same compositions were generated with MD simulations using three types of potentials: fixed partial charge pairwise (Teter), partial diffuse charge potential (MGFF), and bond order‐based charge transfer potential (ReaxFF). The comparison of simulated and experimental IR spectra showed that, among these three potentials tested, ReaxFF reproduces the concentration dependence of spectral features closest to the experimentally observed trend. Thus, the bond length and angle distributions as well as Si–Qnspecies and ring size distributions of silica and sodium silicate glasses were obtained from ReaxFF‐MD simulations and further compared with the peak assignment or deconvolution schemes—which have been widely used since 1970s and 1980s—(a) correlation between the IR peak position in the Si–O stretch region (1050‐1120 cm−1) and the Si–O–Si bond angle; (b) deconvolution of the Raman bands in the Si–O stretch region with theQnspeciation; and (c) assignment of the Raman bands in the 420‐600 cm−1region to the bending modes of (SiO)nrings with different sizes (typically, n = 3‐6). The comparisons showed that none of these widely used methods is congruent with the bond parameters or structures of silicate glass networks produced via ReaxFF‐MD simulations. This finding invokes that the adequacy of these spectral interpretation methods must be questioned. Alternative interpretations are proposed, which are to be tested independently in future studies.

    more » « less
  3. Abstract

    The 4.5 km deep IDDP‐2 drill hole was drilled at Reykjanes, Iceland, in an active seawater‐recharged hydrothermal system on the landward extension the Mid‐Atlantic Ridge. Drilling targeted a well‐defined hydrothermal up‐flow zone feeding the Reykjanes geothermal reservoir, which produces fluids compositionally equivalent to basalt‐hosted deep sea hydrothermal vents. Spot cores recovered from depths between 3,648 to 4,659 m depths consist of pervasively altered sheeted diabase dikes. The shallowest core has an alteration mineral assemblage similar to the producing geothermal reservoir but is being overprinted by the underlying Hornblende Zone assemblage dominated by labradorite and hornblende. The deepest cored section (4,634–4,659 m) retains a diabasic texture, but all primary minerals are replaced or have changed composition. Plagioclase ranges from An30to An99and igneous augite is replaced by intergrown hornblende, clinopyroxene, and orthopyroxene. Hydrothermal biotite and potassium feldspar formed locally due to reaction with a phase‐separated brine, indicated by co‐existing vapor‐rich and salt + vapor‐rich fluid inclusions. Seawater‐derived supercritical hydrothermal fluid entering the bottom of the bore hole actively phase separates due pressure drop controlled by the fluid levels in the drill hole. Felsic veins, present in trace amounts, record a continuous transition from magmatic to hydrothermal conditions, including incipient hydrous melting. The vertical changes observed in mineralogy and mineral chemistry indicate that fluids from the deep high temperature reaction zone can undergo significant cooling and reaction with host rocks along their path to the seafloor.

    more » « less
  4. Abstract

    High pressure and temperature experiments were carried out on the oxide mixtures corresponding to the bridgmanite stoichiometry under the hydrous shallow lower mantle conditions (24–25 GPa and 1673–1873 K with 5–10 wt. % of water in the starting material). Oxide mixtures investigated correspond to MgSiO3, (Mg, Fe)SiO3, (Mg, Al, Si)O3, and (Mg, Fe, Al, Si)O3. Melting was observed in all runs. Partitioning of various elements, including Mg, Fe, Si, and H is investigated. Melting under hydrous lower mantle conditions leads to increased (Mg + Fe)O/SiO2in the melt compared to the residual solids. The residual solids often contain a large amount of stishovite, and the melt contains higher (Mg,Fe)O/SiO2ratio than the initial material. (Mg + Fe)O‐rich hydrous melt could explain the low‐velocity anomalies observed in the shallow lower mantle and a large amount of stishovite in the residual solid may be responsible for the scattering of seismic waves in the mid‐lower mantle and may explain the “stishovite paradox. Since stishovite‐rich materials are formed only when silica‐rich source rock (MORB) is melted (not a typical peridotitic rock [bulk silicate Earth]), seismic scattering in the lower mantle provides a clue on the circulation of subducted MORB materials. To estimate hydrogen content, we use a new method of estimating the water content of unquenchable melts, and also propose a new interpretation of the significance of superhydrous phase B inclusions in bridgmanite. The results provide revised values of water partitioning between solid minerals and hydrous melts that are substantially higher than previous estimates.

    more » « less
  5. Abstract

    Natural volcanic ashfall samples were examined, and high‐current (~100 kA) electrical impulse experiments were conducted to reveal the changes in grain size that can occur during lightning discharge. Experiments on pseudo ash samples manufactured from volcanic deposits of both rhyolitic and basaltic composition show that aggregates of very fine grained ash particles (<32 μm) melt and degas to form vesiculated pumice fragments >100 μm in size. In some cases, bubbles <5 μm in diameter expand and detach from the outer surface of the pumice to form hollow spheres of glass, one type of lightning‐induced volcanic spherule, while other bubbles fragment. Volcanic ashfall from the 2009 Redoubt eruption and the 2016 Pavlof eruption contains both pumiceous grains and individual spherules. Results of this study reveal that volcanic lightning will alter the grain size distribution of ash through melting, vesiculation, and fragmentation of individual particles or ash aggregates.

    more » « less