Comparison of iron silicide in fulgurites to an extraterrestrial sample

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A recent article published in the journal Minerals reviews the presence of iron silicides in fulgurite samples to explain the connection between the geological environments where these minerals are found.

To study: Iron silicides in fulgurites. Image Credit: Creators of Wirestock/Shutterstock.com

Iron silicides (Fe-Si) are rare earth minerals commonly found in extraterrestrial rocks such as meteorites. Their rare existence in terrestrial rocks is due to a requirement of extremely reducing conditions to favor their formation.

These extreme reducing conditions are those that lead to the formation of fulgurites – commonly known as fossilized lightning – which are glasses that form when cloud-to-ground lightning heats rock, soil and/or sand to such an extent that a merger occurs. .

A recent article published in the journal Minerals reviews the presence of iron silicides in fulgurite samples as a means of explaining the central connection between the geologic environments in which these minerals are found. Additionally, the researchers believe there may be a correlation between the stresses of silicide formation conditions, which could provide a basis for understanding how phosphides may also have formed and been present on early Earth.

Large amounts of reduced-state phosphorus compounds, such as phosphides and phosphites, could have been formed by lightning in the early Earth, increasing the element’s availability on the Earth’s surface.explains the corresponding author, Professor Matthew A. Pasek of the Department of Geology at the University of Florida, USA.

Iron silicide minerals

Among “iron silicide ores”, iron monosilicide is the best known which is used for the production of various alloys, including silicon-containing alloys with special physico-chemical properties, as well as finding use in microelectronics.

However, iron silicides in nature are extremely rare, little known, and researchers have only been able to access good samples of these minerals in recent decades. Additionally, little is known about the true origin or formation of these unique minerals.

As mentioned, the reason for the scarcity of iron silicide minerals on Earth is due to the conditions of formation, which require an extremely reducing environment and extraordinarily high temperatures, which are rare events compared to terrestrial processes.

Conditions of formation of fulguritis

A reducing environment or atmosphere is when the removal of oxygen and other oxidizing gases or vapors results in an atmospheric condition in which oxidation is prevented. As a result, iron silicides have been detected in some fulgurites, including the fulgurite from Houghton Lake, Michigan which was discovered in 2020.

The researchers state in the paper that the fulgurite would have been the result of a natural formation, not an artificial source such as a downed power line, which was the case with another Michigan-based fulgurite.

Due to the reduced environment that commonly occurs during lightning strikes, iron silicides are frequent accessory minerals in different types of fulgurites.

Matthew A. Pasek, Department of Geology, University of Florida

Additionally, iron silicides also have a cosmic extraterrestrial origin having been found in Ureilites which are a kind of stony, carbon-rich meteorite that experiences extreme temperatures upon impact or radioactive decay. “This pathway of iron silicide mineral formation in our solar system requires low oxygen fugacity and ultra-high temperature conditions, which resemble the conditions for fulgurite formation,” says Pasek.

Comparison of terrestrial and extraterrestrial silicides

Researchers were able to access samples of terrestrial and extraterrestrial iron silicides to compare composition composition and determine if there are any differences or similarities to better understand these rare earth minerals.

The team determined that the main differences between the two types of silicides were primarily in composition, with terrestrial silicides generally showing a titanium-enriched state and, in some cases, aluminum formation.

It is believed that the presence of aluminum may indicate an artificial formation of fulgurite from the melting of a power line conductor as these are usually aluminum.

The researchers observed that both terrestrial and cosmochemical silicides contain a small amount (0.1-0.5% by weight) of phosphorus in the form of phosphide substituting for Si, which is possibly the result of the same abundance of phosphorus in the cosmochemical material than in the soil.

This discussion of iron silicide minerals in fulgurites may help identify fulgurites as sources of these minerals, rather than necessarily indicating an extraterrestrial origin.

Matthew A. Pasek, Department of Geology, University of Florida

The relevance of iron silicides and how they can help to better understand petrological processes on Earth and elsewhere has yet to be fully explored, although future work and research on silicide minerals in other fulgurites can help to limit the reduction processes necessary for their formation.

The references:

Feng, T.; Abbatiello, J.; Omran, A.; Mehta, C.; Pasek, MA Iron silicides in fulgurites. Minerals 2021, 11, 1394. https://www.mdpi.com/2075-163X/11/12/1394

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