Ordering in FeII–III Hydroxysalt green rusts from XRD and Mössbauer analysis (chloride, carbonate, sulphate, oxalate...); about the structure of hydrotalcite-like compounds

Abstract

The domain of existence of Fe II-III hydroxysalt green rusts (GR) is characterised by the ratio x = [Fe III ]/[Fe total ] lying in [1/4, 1/3] as it is for any layered double hydroxide (LDH). Since Fe III cations balance the charges of intercalated anions, cation and anion distributions are correlated. However, the distribution of anions depends on their charge, shape and size. For Cl j and CO 3 2j , x belongs to the complete range whereas for C 2 O 4 2j and SO 4 2j it is limited to x = 1/4 and 1/3, respectively. Mö ssbauer spectra present in all cases two ferrous and one ferric doublets, which characterise each GR by their relative abundance. Intermediate values of x are in fact obtained by intrincate domains of two types, one where the cation periodicity is 2Âa 0 and the other one ¾3Âa 0 if a 0 is the lattice parameter.

The inevitable journey to being

“…As previously mentioned, green rust of the kind to be precipitated at the interface between the alkaline hydrothermal solution and acidulous phosphate and iron-bearing ocean water is likely to comprise [Fe 8 ) with the obligate intercalation of anions to maintain charge balance [160][161][162][163][164][165]. Such a nanocryst may have acted as the pseudo enzyme driving phosphate condensation in lieu of the membrane helices [100,165,167 -169].…”

“…Such a nanocryst may have acted as the pseudo enzyme driving phosphate condensation in lieu of the membrane helices [100,165,167 -169]. The monovalent interlayer anions drawn in to balance charge in these varieties of green rust are more easily exchanged than the divalent ions such as sulfate and carbonate [165,168], and are thus more susceptible to replacement by active phosphate sorbents [169,170]. We hypothesize that a proton flux, driven by the pH gradient amounting to five pH units, could translocate the orthophosphate anions from the ocean bathing the mound into the open subnanometric chambers walled by the crystal planes which tend to be oriented orthogonally to the outer and inner surface of the membrane (figures 4 and 5).…”

“…The transmembrane potential-part redox and part pH gradient-totals approximately 1 V [19,140,148] and provides the vectorial free energy that drives the system's inherently endergonic processes: (i) the generation and inward propagation of localized, polaron-based, channel dilations, (ii) the confinement of phosphate under conditions of reduced water activity within a superficially positioned local dilation, (iii) the subsequent condensation to pyrophosphate which in turn permits the mobilization of the polaron dilation and (iv) the 'pumping' of the pyrophosphate into the interior against its own gradient. The ferrous hydroxide comprising the outer margins of the precipitate membrane begins to be oxidized by nitrate to green rust which has the effect of opening the interlayers locally as the ferric ions on either side of the interlayer repel each other, the positive charges attracting counter-ions such as chloride, nitrate, carbonate and phosphate to take up the space [100,101,135,136,[163][164][165]. The changes in local redox state locally distort the crystal structure inducing, via phonon interactions, the formation of localized polaron quasi-particles [171,172].…”

Nitrogen Oxides in Early Earth's Atmosphere as Electron Acceptors for Life's Emergence

“…Brucite (Mg[OH] 2 ) is a major initial component of Lost City mounds, owing to the high concentration of Mg in the present-day ocean, whereas Hadean mounds would have had an iron-dominated mineralogical composition. The Hadean ocean was carbonic and rich in iron and other transition metals, so the porous precipitate mounds would have comprised amorphous to microcrystalline brucite-structured iron oxyhydroxides or green rusts (e.g., *Fe II 4 Fe III 2 (OH) 12 [CO 3 ] 3H 2 O) along with the iron sulfides mackinawite and greigite, dosed with nickel, cobalt, and molybdenum (Russell and Hall, 1997;Génin et al, 2005Génin et al, , 2006Mloszewska et al, 2012;Nitschke and Russell, 2013;Russell et al, 2014;White et al, 2015;Tosca et al, 2016;Halevy et al, 2017).…”

Reduction of N2 by Fe2+ via Homogeneous and Heterogeneous Reactions

“…Whether white rust would have been present in the early oceans is a question of pH and P CO 2 , though green rusts have been postulated (Russell and Arndt 2005). Not until the discovery of green rust was the role of the buffer's anion understood (Genin et al 2006;Genin et al 2001).…”