A recent paper published in the journal Minerals reviews the presence of iron silicides in fulgurite samples to explain the connection between geological environments where these minerals are found.
Study: Iron Silicides in Fulgurites. Image Credit: Wirestock Creators/Shutterstock.com
Iron silicides (Fe-Si) are rare Earth minerals that are commonly found in extraterrestrial rocks such as meteorites. Their rare existence in terrestrial rocks is due to a requirement of extremely reducing conditions to support their formation.
Such 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 the rock, soil, and/or sand to such an extent that a fusion occurs.
A recent paper published in the journal Minerals reviews the presence of iron silicides in fulgurite samples as a way to explain the core connection between geological environments wherein these minerals are found. Furthermore, the researchers believe that there may be a correlation between constraining the conditions of silicide formation which could provide a basis for understanding how phosphides may have also formed and been present on early Earth.
“Copious amounts of reduced state phosphorus compounds, such as phosphides and phosphites, could possibly have been formed by lightning strikes on early Earth, thus increasing the availability of the element on Earth’s surface,” explains corresponding author professor Matthew A. Pasek at the Department of Geology, University of Florida, USA.
Iron Silicide Minerals
Amongst ‘Iron Silicide Minerals’, iron monosilicide is the best known which is used for the production of various alloys including silicon-containing alloys with special physicochemical 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 the last few decades. Additionally, little is known about the true origin or formation of these unique minerals.
As mentioned, the reason for the rare occurrence of iron silicide minerals on Earth is the formation conditions, which require an extremely reducing environment and extraordinarily high temperatures, which are rare events relative to the terrestrial processes.
Fulgurite-Forming Conditions
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. Accordingly, iron silicides have been detected in some fulgurites, including the Houghton Lake fulgurite, Michigan which was discovered in 2020.
The researchers state in the paper that the fulgurite was believed to have been the result of a natural formation, not an artificial source such as a downed power line, which was the case of another Michigan-based fulgurite.
Due to the reduced environment that occurs commonly in lightning strikes, iron silicides are frequent accessory minerals in different types of fulgurites
Matthew A. Pasek, Department of Geology, University of Florida
Moreover, iron silicides also have a cosmic extraterrestrial origin having been found in Ureilites which are a kind of stony, carbon-rich meteorite that experience extreme temperatures upon impact or from radioactive decay. “This iron silicide mineral formation route in our solar system requires low oxygen fugacity and ultrahigh-temperature conditions, which are like the fulgurite-forming conditions,” explains Pasek.
Comparing Terrestrial and Extraterrestrial Silicides
The researchers were able to access samples of both terrestrial and extraterrestrial iron silicides in order to compare the compositional makeup and determine if there are any differences or similarities in order to better understand these rare Earth minerals.
The team determined that the major differences between the two kinds of silicides were mostly compositional, with the terrestrial silicides usually showing an enriched state of titanium and in some cases, there was a formation of aluminum.
It is believed that the presence of aluminum may indicate an artificial fulgurite formation from the melting of a powerline conductor as these are usually made of aluminum.
The researchers observed that both terrestrial and cosmochemical silicides contain a small amount of (0.1–0.5 wt. %) of Phosphorous as phosphide substituting for Si, which is possibly the result of phosphorus being similarly abundant in cosmochemical material as it is in soil.
This discussion of iron silicide minerals in fulgurites could help identify fulgurites as being sources of these minerals, instead of necessarily indicating an extraterrestrial origin
Matthew A. Pasek, Department of Geology, University of Florida
The relevance of iron silicides and how they can help gain a better understanding of petrologic processes on Earth and elsewhere have yet to be exploited to the full, though future work and investigation into silicide minerals in other fulgurites may help constrain the processes of reduction required for their formation.
References:
Feng, T.; Abbatiello, J.; Omran, A.; Mehta, C.; Pasek, M.A. Iron Silicides in Fulgurites. Minerals 2021, 11, 1394. https://www.mdpi.com/2075-163X/11/12/1394
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