"Rock of the Month # 37, posted for July 2004" --- This sample was obtained by Mark Stanley of Sumar Minerals (Norwood, Ontario: mark.stanley@bellnet.ca). Mark kindly made available small but critical fragments from this mass for ongoing research. The notes provided below are largely from examinations of four polished sections (sample 2220) in early 2001. The sample is from a large fulgurite developed in the Foymount area of southeastern Ontario.

It has not proved possible to examine the site of the strike directly, but a microscopic study of the sand adhering to the fulgurite, and entrained within the glass, provides a clear idea of the sandy soil that was evidently the protolith on which the tube developed.

The protolith and the glassy fulgurite

The sand contains roughly 62 volume percent quartz, mostly 0.03-0.3 mm in diameter. There are also plagioclase feldspar (12%), amphiboles (hornblende to actinolite, 8%), alkali feldspar (7%), magnetite (4%), garnet, biotite and limonite (2% each) and sphene (titanite: 1%), plus traces of other minerals including pyroxenes, epidote, zircon, scapolite, (?) kyanite and additional Fe-Ti oxides such as hematite, ilmenite and leucoxene. This mineralogy is broadly consistent with amphibolite facies regional metamorphism in the local bedrocks of the mid-Proterozoic Grenville province of the Canadian Shield. The quartz grains display a fracturing which may be linked to the intense heating and rapid cooling of the fulgurite formation. The fulgurite interior consists of visually inhomogeneous vesicular glass with complex internal structures and relict grains of the minerals described above.

A tree, rock or area of open sand or soil does not always show much evidence of a lightning strike, despite the violence of the event. Sometimes a few broken twigs may be the only clue. Surface moisture on the bark, and the proportions of moisture and mineral grains in the ground, appear to be key factors influencing the outcome. You can see a photo of a lightning-damaged tree in the Local Ecology pages.

Iron silicides

The most interesting feature, however, are rare mm-scale spheroids of anisotropic, highly reflective metallic phases resembling beads of aluminium. Preliminary electron microprobe analyses indicate that these beads contain curved crystallites of at least two iron silicides, highly reduced minerals seldom reported on Earth. Preliminary electron microprobe analysis suggests that the metal phase in the Foymount example is largely a mixture of Fe₃Si₇ and FeSi. See a later Rock of the Month for a more detailed account of the curious silicides.

Such silicides have previously been reported by Essene and Fisher (1986) in a landmark study on a giant fulgurite developed along the ridge of a moraine in Michigan. This paper is by far the most useful of 40-plus articles examined on the topic, dating back as far as 1884. Their study indicates coexistence of immiscible metallic and silicate liquids at temperatures in excess of 2000 K. They note that fulgurites are most easily developed in sand, but there are also rock and soil fulgurites, which are highly vesicular. Large (mm- to cm-scale) metallic spheroids were noted in dark glass near a charred tree root, which by microprobe analysis were identified as native silicon plus silicides such as Fe₃Si₇, FeSi and FeTi₂Si₂. The glass contains graphite, quartz and rare baddeleyite, and the glass itself is inhomogeneous with 82-99 wt.% SiO₂. The phase Fe₂Si was recently described in a lunar meteorite (Dhofar 280) in grains up to 0.035 mm in diameter, and named "hapkeite" for Bruce Hapke of the University of Pittsburgh (Martel, 2004).

References

ESSENE,EJ and FISHER,DC (1986) Lightning strike fusion: extreme reduction and metal-silicate liquid immiscibility. Science 234, 189-193, 10 October.

MARTEL,LMV (2004) New mineral proves an old idea about space weathering. Planetary Science Research Discoveries (PSRD), University of Hawaii, 7pp., 05 July.