Iron Silicides in Fulgurites

Iron silicide minerals (Fe-Si group) are found in terrestrial and solar system samples. These minerals tend to be more common in extraterrestrial rocks such as meteorites, and their existence in terrestrial rocks is limited due to a requirement of extremely reducing conditions to promote their formation. Such extremely reducing conditions can be found in fulgurites, which are glasses formed as cloud-to-ground lightning heats and fuses sand, soil, or rock. The objective of this paper is to review reports of iron silicides in fulgurites, note any similarities between separate fulgurite observations, and to explain the core connection between geological environments wherein these minerals are found. In addition, we also compare iron silicides in fulgurites to those in extraterrestrial samples.

Structural stabilities of iron silicides at high pressures and temperatures

Iron silicides provide extensive applications in functional device and earth science. Very little is known about these alloys at high pressures and temperatures. Here, combining swarm-intelligence-based structure prediction methodology and quasi-harmonic approximation (QHA), we investigate Fe-Si compounds at pressures of up to 500 GPa. Evolutionary simulations correctly find the known B20-FeSi structure and some other experimentally observed compounds at atmospheric pressure and finite temperature. It is shown that among the possible iron silicides, only B2-FeSi is thermodynamically stable at high pressures, and the increasing temperature tends to stabilize B2-FeSi compound rather than other iron silicides.

The occurrence of iron silicides in a fulgurite: Implications for fulgurite genesis

ABSTRACT Rapidly formed eutectic textures are observed in Fe silicides in a fulgurite from Michigan. The 14 cm-diameter fulgurite was formed in sandy glacial till in 2014 near Houghton Lake, Michigan. Spherical droplets of iron silicides up to ∼200 μm in diameter were found in the natural glass. Back-scattered electron images of some droplets show a eutectic intergrowth texture of two iron silicides with individual crystals up to ∼1 μm in maximum dimension. X-ray diffraction study showed the specimens to be an intergrowth of naquite (FeSi) and linzhiite (FeSi2) or naquite and xifengite (Fe5Si3). Droplets also contain minor native silicon, Fe-Ti silicides, and/or other Ti- rich phases which were discovered during TEM observations. It is important to note that the lower-temperature phase luobusaite (Fe3Si7) was not observed in any droplets, indicating rapid quenching of the fulgurite, consistent with a natural origin during a lightning strike as opposed to an artificial origin, e.g., resulting from a downed power line.

SLAGS OF THE METALLURGICAL PLANTS OF ARMENIA AT FINE GRINDING

In connection with the development of copper and molybdenum production in Armenia, it becomes necessary to develop a technology for processing the slags of the functioning metallurgical plants to extract valuable metals from these slags. It concerns the copper slags of Alaverdi Copper-Smelting Plant with a content of FeO ~50 % and the molybdenum slags of Yerevan “Pure Iron” Plant with a content of SiO2 ~80 %. These slags are obtained at high temperatures (with FeO·SiO2 , CaO·SiO2 , Fe3 O4 ) and, therefore, they are less active to be used later. Meanwhile, the mentioned slags are rich in iron and silicon oxides and can serve as a cheap raw material for producing iron silicides. The iron silicides can be used in micro- and nanoelectronics, as well as in metallurgy as an alloying additive in the production of steels of special physical and chemical properties. The production of such valuable silicides from an inexpensive raw material is important for Republic of Armenia and is of not only economic, but also ecological significance. To obtain iron silicides, a method of combined aluminothermal reduction of primarily mechanoactivated copper and molybdenum slags is proposed. The preliminary mechanoactivation allows to purposefully affect the structure of the reaction mixture and parameters of the self-propagating high-temperature synthesis (SHS), thus ensuring the possibility of regulating the structure and phase composition of the synthesized silicides. This work considers issues on the impact of transformation and phase formation on the morphology of slags of copper and molybdenum production in the functioning Armenian metallurgical plants at mechanochemical activation by the method of fine grinding in a vibromill. It is shown that at fine grinding (up to 10 mcm), the slags, containing small-reactivity complex compounds of iron and silicon (fayalite, magnetite, quarzite) undergo profound chemical changes, transforming into amorphous oxides. The obtained activated oxides can serve as a raw material for producing iron- and silicon-containing alloys – iron silicides.