Magnetite Synthesis in the Presence of Cyanide or Thiocyanate under Prebiotic Chemistry Conditions

Abstract

Magnetite is an iron oxide mineral component of primitive Earth. It is naturally synthesized in different ways, such as magma cooling as well as olivine decomposition under hydrothermal conditions. It is probable magnetite played a significant role in biogenesis. The seawater used in the current work contained high Mg2+, Ca2+ and SO42− concentrations, unlike the seawater of today that has high Na+ and Cl− concentrations. It is likely that this seawater better resembled the ion composition of the seas of the Earth from 4 billion years ago. Cyanide and thiocyanate were common molecules in prebiotic Earth, and especially in primitive oceans, where they could act on the magnetite mechanism synthesis via Fe2+ interaction. In this research, magnetite samples that were synthesized under prebiotic conditions in the presence of cyanide or thiocyanate, (both with and without artificial seawater), showed that, besides magnetite, goethite and ferrihydrite can be produced through different Fe2+-ion interactions. Cyanide apparently acts as a protective agent for magnetite production; however, thiocyanate and seawater 4.0 Gy ions produced goethite and ferrihydrite at different ratios. These results validate that Fe3+ oxides/hydroxides were possibly present in primitive Earth, even under anoxic conditions or in the absence of UV radiation. In addition, the results show that the composition of water in early oceans should not be neglected in prebiotic chemistry experiments, since this composition directly influences mineral formation.

Study of the formation of ferrihydrite under prebiotic chemistry conditions: artificial seawater 4.0 Gy and ammonium thiocyanate

“…In addition, Samulewski et al . (2020) also observed that when magnetite was synthesized in the presence of SCN − anion, goethite was also obtained. For Fh-DW-NH 4 SCN-1 and Fh-DW-NH 4 SCN-2 samples, the (hematite, (018)) and (magnetite, (311)) peaks should be the highest intensity, but probably due to orientation, they are not (Fig.…”

“…However, the FT-IR spectra of Fh-DW-NH 4 SCN-2, Fh-SW-NH 4 SCN-1 and Fh-DW-NH 4 SCN-1 samples showed bands at 890 cm −1 (O-H bending in plane) and 794 cm −1 (O-H bending out of the plane) which are characteristic of goethite (Table 1) (Cornell and Schwertmann, 2003). In addition, Samulewski et al (2020) also observed that when magnetite was synthesized in the presence of SCN − anion, goethite was also obtained. For Fh-DW-NH 4 SCN-1 and Fh-DW-NH 4 SCN-2 samples, the (hematite, (018)) and (magnetite, (311)) peaks should be the highest intensity, but probably due to orientation, they are not (Fig.…”

“…Artificial seawater 4.0 Gy only had an effect on the crystallinity of ferrihydrite, since instead of 6-line ferrihydrite, 2-line ferrihydrite was obtained. However, Samulewski et al ., observed that in the synthesis of magnetite, besides magnetite, goethite (26.9%) and gypsum (19.4%) were also obtained (Samulewski et al ., 2020). The Fh-SW-NH 4 SCN-2 sample showed lower crystallinity when compared to the Fh-6 sample, probably due to the high quantity of salts present in seawater 4.0 Gy that interferes with the formation of the ferrihydrite structure.…”

“…Since the Fe 3+ ion is considered a hard acid, the metal coordinates with the SCN − ion through nitrogen to form isothiocyanate complexes (Pearson, 1963). It is well known that goethite is formed when sulphur compounds interact with the Fe 3+ cation (Cornell and Schneider, 1989; Cornell et al ., 1989; Cornell and Schwertmann, 2003; Vu and Moreau, 2015; Carneiro et al ., 2013; Samulewski et al ., 2020). Therefore, this may be an indication that Fe 3+ interacted with the SCN − ion also by sulphur.…”

“…In addition, in the synthesis of magnetite in the presence of SCN − anion, Samulewski et al (2020) did not observe sorption/precipitation of this anion in the synthesized products. Furthermore, the adsorption of SCN − anion onto magnetite did not occur in a wide pH range (Samulewski et al, 2020). Although ferrihydrite adsorbs SCN − anion at a neutral pH, high Mg 2+ , Ca 2+ and SO 4 2− concentrations are necessary (Zaia et al, 2020).…”