1. Left: The exterior of a thick portion of the fulgurite. The frosted white exterior and pale blue interior glass is especially striking, and probably indicates a relatively pure sand host.

2. Right: A cross-section of one of the branching tubes. Despite its white colour and frosted, even frothy appearance, the heft of the sample indicates that much of the interior is filled by the solid pale blue glass seen here.

"Rock of the Month #123, posted for September 2011" --- This sample, and the smaller sample shown below, was obtained by Mark Stanley of Sumar Minerals (Norwood, Ontario: mark.stanley@bellnet.ca). The find was made south of Ashby in Grant county. A small section from the Nebraska find was donated by Mark for a forthcoming petrographic examination. Stanley sample 1194.

Nebraska is clearly a favourable state in which to seek fulgurites! Fulgurites can form on rock, and in soil or sand. Pye (1982) described typical fulgurite tubes developed in dune fields in Queensland, Australia. The scientific literature is not particularly extensive, but dates back three centuries and contains examples from many parts of the world. Barbour (1925) observed that fulgurites appear to be quite common in sandy areas of Nebraska, and are exposed where winds blow the sands away. He attributes to Oscar E. Hans the suggestion that the moisture content of sand (and thus the season) influences the development and extent of fulgurites. Fulgurites taper rapidly, and often at 10-15 feet (m) end in flattened glass bubbles (lightning bubbles or "fulgurphysae"). The tubes themselves are often ridged or cross-shaped in cross-section (the `fulgurite cross').

Anderson (1925) wrote an excellent review of fulgurite morphology, using material from an unusual concentration of large and complete specimens recovered in Holt and Stanton counties, Nebraska. The compact tubes of spongy glass, formed by almost instant melting of sand, may be 15 feet (4.6 m; 40 feet / 12.2 m in one case) in length. The fulgurite tubes often display a cross-shaped cross-section. The surfaces of the inner tubular cavity or lumen are smooth and glassy, fully melted. Anderson used thin (50-micron) and thick (1/16", 1.6 mm) sections as an aid with tracing flow textures along the fulgurite (ibid., p.68). He noted that quartz grains may show reduced birefringence and granulation along fracture planes.

This thermal effect on quartz structure was affirmed recently (Pasek et al., 2010; Joseph et al., 2010) in fulgurite samples from Greensboro, North Carolina. They describe "toasted quartz" with reduced refractive index, increased crystal lattice volume, and shifts in the Raman bands observed in Raman spectroscopy, which lies in an intermediate zone between the edge of the tube and the core of totally melted material, glass with good homogeneity. Elements like Na, Mg, Zn and Sn are lost in fulgurite development, while others like P, Fe, Si, Al, Ni, Mn and Zr are retained.

The effects of a lightning strike are sudden (perhaps <1 millisecond) and extreme in both shock (pressure) and heat. Rogers (1946) studied fulgurites formed on granodiorite in California. He observed the melting of quartz to lechatelierite (glass), melting of biotite mica and feldspars to dark and pale glasses, respectively, and occurrence of shattered quartz grains cemented by silica glass. He estimated the transient peak temperature attained at ~1800°C.

One further curio from Nebraska is a small conical mass of metal 11 cm in length, described by Riley (1959). Riley's sample, recovered in a farm yard in April 1955, is rough and porous with a friable granular surface, composed largely of sub-mm spheres, filaments and ribbons of iron. The metal contains non-metallic grains, <0.1 mm in size, such as quartz and feldspar, all plausibly derived from the underlying Pliocene Ogallala Formation. An origin by lightning is the preferred explanation, the "protolith" possibly an iron artefact in the farm yard.

Below is a second example of fulgurite, a smaller but still very solid and instructive piece from the Monashee Mountains (Okanagan Valley district) of southern British Columbia. Stanley sample 1697. Note the solid green glass core, the bubbly outer zone, and the grey, sandy outer margin of the tube.

Two views of the Y-shaped junction of fulgurite tubes constituting this specimen. The interior and exterior hues of this sample are reminiscent of the Ontario example (see link below). The core is a dense green glass which has the hue of peridot (olivine), surrounded by a porous zone and then the outer crust.

References

Anderson,AE (1925) Sand fulgurites from Nebraska, their structure and formative factors. Nebraska State Museum, Vol. 1, Bull.7, 49-86.

Barbour,EH (1925) Notes on Nebraska fulgurites. Nebraska State Museum, Vol. 1, Bull.6, 45-48.

Joseph,ML, Atlas,Z, Royall,D and Pasek,MA (2010) Geochemical analysis of a type II fulgurite. Meteoritics & Planetary Science 45, A97.

Pasek,MA, Collins,GS, Carter,EA, Melosh,HJ and Atlas,Z (2010) Shocked quartz in a fulgurite. Meteoritics & Planetary Science 45, A163.

Pye,K (1982) SEM observations on some sand fulgurites from northern Australia. J.Sed.Petrol. 52, 991-998.

Riley,CM (1959) An iron fulgurite from Nebraska. Nebraska State Museum, Vol.5, Bull. 4, 82-88.

Rogers,AF (1946) Sand fulgurites with enclosed lechatelierite from Riverside county, California. J.Geol. 54, 117-122.