An experimental study on the preparation of tochilinite-originated intercalation compounds comprised of Fe1−xS host layers and various kinds of guest layers

Tochilinite represents a mineral group of ordered mixed-layer structures containing alternating Fe1−xS layers with mackinawite-like structure and metal hydroxide layers with Mg(OH)2-like structure. In this article, we report the preparation of a series of tochilinite-originated (or Fe1−xS-based) intercalation compounds (ICs). According to their preparation procedures, these ICs can be divided into four kinds. The first kind of IC was sodium tochilinite (Na-tochilinite), which was prepared by the hydrothermal reaction of metallic Fe particles with concentrated Na2S·9H2O aqueous solutions. The hydroxide layer of the Na-tochilinite was a mixed hydroxide of Na+ ions along with a certain amount of Fe2+ ions. When the hydroxide layer of the Na-tochilinite completely dissolved in aqueous solutions, a Fe-deficient mackinawite-like phase Fe1−xS was obtained, which was probably an electron-deficient p-type conductor. The second kind of ICs was prepared by ‘low-temperature direct intercalation in aqueous solutions, using Na-tochilinite as a parental precursor. When the Na-tochilinite was ultrasonicated in aqueous solutions containing Lewis basic complexing agents (like NH3, N2H4, 2,2′-bipyridine (bipy), and 1,10-phenanthroline (phen)), the Na+ ions of the Na-tochilinite were removed and the Lewis basic complexing agents entered the hydroxide layer of the Na-tochilinite and became coordinated with the Fe2+ ions, and the second kind of ICs was thus produced. The second kind of ICs includes NH3 IC, N2H4 IC, N2H4-NH3 IC, [Fe(bipy)3]2+-containing IC and [Fe(phen)3]2+-containing IC. The third kind of ICs, which includes NH3 IC, N2H4–NH3 IC and N2H4-LiOH (NaOH) IC, was prepared by the hydrothermal reaction of metallic Fe particles with (NH4)2S aqueous solution, S (elemental) + N2H4·H2O aqueous solution, and S + N2H4·H2O + LiOH (NaOH) aqueous solution, respectively. The third kind of ICs has a close relationship with the second kind of ICs both in composition and structure. The fourth kind of ICs was prepared by the oxidation and reduction of some of the N2H4-containing ICs mentioned above, which include N2H2 (diazene or diimide) IC, N2 (dinitrogen) IC and NH3 IC. The N2H2 IC was prepared by mild air oxidation of the N2H4-LiOH IC. The N2 IC was prepared by strong air oxidation of the N2H4-LiOH IC, however, we have not been able to separate the pure phase N2 IC. Hydrothermal reduction of the N2H4 IC made by the direct intercalation method in strong reducing environment by H2S + Fe (metal) led to the production of the NH3 IC of the fourth kind of ICs. The NH3 ICs prepared by the three methods had similar compositions and structures. As almost all the ICs reported in this paper were extremely sensitive both to air and to the electron beam, they were mainly characterized by XRD.The properties and interrelationships (or mutual transformations) of the Fe1−xS-based ICs revealed novel chemistry occurring in the sub-nanoscopic space between the micrometer- to nanometer-sized electron-deficient Fe1−xS layers. An important finding of this novel chemistry was that the Fe1−xS-based ICs tended to oxidize or reduce the intercalated species when the redox state of their environments varied. The results of our experiments potentially have many cosmochemical implications. The most important implication is that our experimental results, along with previous studies, strongly suggested that some of the ammonium salts, ammonia and carbonates existing in the matrix of the CM carbonaceous chondrites may have been formed by abiotic reactions employing molecular nitrogen as the nitrogen source and carbon monoxide as the carbon source and iron sulfide and/or iron hydroxide as catalysts.