Chapter 2 1 Introduction

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Chapter 2

2.1 Introduction

As discussed in chapter 1, the reduction of silicate material by impacts and lightning strikes is an unusual but interesting process on planetary surfaces. Comparing reduced samples to their unmelted starting materials is the best way to understand and quantify the changes caused by high temperature processing. Unfortunately, it is difficult to gather definite starting materials for tektites. The Australasian source crater is unknown, and millions of years of erosion and weathering have removed or changed the original surfaces near the other three craters. The surface areas of the source regions and the resulting volumes of tektite material are large, and the tektites land far from the craters. A definitive match of a particular tektite to an exact source is very difficult.

Fulgurites have none of these limitations. Fulgurite materials are geologically very young, and sometimes can be collected within days of formation. They are found in situ, and commonly have unmelted starting materials cemented into the fulgurite glass. Fulgurites are numerous and occur in a variety of soil and rock types. They are ideal samples for investigating the effects of high temperature processing.

Previous studies of fulgurites have included a wide range of techniques but few total samples. A scanning electron microscope study of Australian quartz sand fulgurites observed a quartz glass with partially melted zircon and ilmenite grains (Pye, 1982). The authors also observed unmelted but fractured quartz grains inside the glass, and they hypothesized that expanding bubbles of vapor insulated some quartz grains from the extreme temperatures, although thermal shock caused fracturing and fragmentation. Reduced phases were not directly observed, but the authors suggested that dark linings of some vesicles might be due to thin coatings of elemental Si.

Weeks et al. (1980) investigated sand dune fulgurites using electron magnetic resonance spectroscopy. The authors found that 90% of the Fe in the fulgurite glass is concentrated in metallic spherules whereas the majority of the Fe in the desert sand is oxidized to Fe2O3. They also estimated a formation temperature of 3500 K assuming atmospheric oxygen pressure of 0.2 atm.

Essene and Fisher (1986) discovered extremely reduced phases in a fulgurite formed from unconsolidated glacial till. They reported spherical metallic globules containing Si, FeSi, Fe3Si7, and the new mineral TiP and FeTiSi2 (Fig. 2 .1). Like Pye (1982), they also found unmelted crystals of quartz and zircon inside the glass. The fulgurite was found near charred tree roots, and graphite crystals were present in the glass. The authors suggest that the oxidation of carbonaceous material contributed to the extreme reduction. Other reduction mechanisms suggested by Essene and Fisher (1986) are the vaporization of oxygen during boiling with removal driven by the flow of electrons through the melt, as in electrolysis, and the scavenging of oxygen to form nitrous oxides from nitrogen in the ambient atmosphere.

Fig. 2.1:Backscattered electron photographs of the fulgurite from Essene and Fisher (1986). (A) Spheroidal metallic globules in silicate glass. (B) Intergrowths of FeSi (white), Fe3Si7 (dark gray), and laths of TiP and FeTiSi2 (light gray) (C) Globule containing Si (dark) and Fe3Si7. (D) Silicon encasing submicrometer-sized blebs of gold and encased in a matrix of FeSi, Fe3Si7, and TiP. Long scale bar is 100 m.

The contribution to reduction by electron flow through the melt is difficult to quantify; however, if this were a major reduction mechanism, it would suggest that the degree of reduction should be correlated to fulgurite size. Larger, more energetic lightning strikes would keep the melt fluid for a longer time allowing for extended electron flow and enlarging the melted area. Too few fulgurites have currently been studied in detail to evaluate this hypothesis.

The oxidation of nitrogen in the atmosphere may be a contributor to reduction in fulgurites, but the volume of atmosphere contained in the upper soil layers is smaller than that needed to reduce a large volume of Fe and Si to metals. For example, in the Black Rock, Utah, fulgurite (Section 2.3.5), the sand starting material contains 80% Fe3+ and 20% Fe2+ (out of 0.14 wt% Fe total). In comparison, the fulgurite glass contains 37% Fe3+, 15% Fe2+, and 48% Fe0 metal. In 1cm3 of glass, 8.7x10-4 moles of O2 have been removed. In comparison, 1cm3 of sand with 40% porosity contains 1.4x10-6 moles of N2 at standard temperature and pressure. Assuming the upper limit that all of the N2 is oxidized to NO2, 2.8x10-5 moles of O2 could be lost from the starting material. The oxidation of atmospheric N2 can only account for ~3% of the reduction of the Black Rock, Utah fulgurite as an upper limit.

Oxidation of the carbonaceous plant material found charred near the fulgurite certainly contributed to the reduction seen by Essene and Fisher (1986), but plant material and graphite crystals were not observed in the reduced fulgurite examined by Weeks et al. (1980). Jones et al. (2005) experimentally determined that the presence of carbon is not necessary to reduce a fulgurite by inducing lightning strikes into pure binary oxides. The authors found that NiO was reduced to Ni metal by the action of the lightning strike alone. MnO affected by the same lightning strike melted but did not form metal. The reduction mechanism that best explains these results is that oxides become thermodynamically unstable at high temperatures. At 1 bar pressure, thermodynamic data indicate that NiO decomposes into Ni meal and O2 gas at ~2575 K and MnO decomposes at ~5275 K. A lightning temperature between these values would induce NiO but not MnO to break down. Oxide decomposition temperatures are also dependent on the ambient pressure. The pressure may briefly be higher than 1 bar as the atmosphere and molten material rapidly expand away from the heat of the strike. As the then superheated material adiabatically expands and cools, the pressure in the wake of the strike will likely be lower than 1 bar until the ambient atmosphere refills the void.

The current study presents the results of an electron microprobe and Mössbauer spectroscopy study of ten fulgurites of varied compositions and source localities as well as nuclear bomb glass. A systematic study of multiple fulgurites and their country rock will determine if reduction is intrinsic to the high temperature processing of silicates and help to evaluate the various reduction mechanisms. A previous Mössbauer spectroscopy study of a single basalt rock fulgurite could not distinguish a difference between the oxidation states of the fulgurite glass and the country rock (Ablesimov et al., 1986). The authors did not report the error of their technique; however, the resolution of Mössbauer spectroscopy has greatly improved in the last 20 years and can now discern much smaller differences between samples.

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