Detailed magnetic monitoring of the enhanced magnetism of ferrihydrite along its progressive transformation into hematite

Abstract

Under certain aging conditions, ferrihydrite evolves into hematite through intermediate products of enhanced magnetism. Although the magnetic properties of the end product, hematite, are to date satisfactorily known, those of ferrihydrite (and especially the products of its progressive aging) are not sufficiently explored. To this end, magnetic experiments, conducted mostly by using alternating magnetic fields, have been performed. The results reveal that two‐line ferrihydrite exhibits conspicuous low‐temperature spin glass behavior, a new finding that, on the other hand, could be expected given the previous microstructural observations that described this mineral as a very disordered gel. Upon aging, a progressive increase of the effective magnetic moment per iron ion is detected, in agreement with previous observations that ascribed ferrimagnetic character to an intermediate crystalline phase (sometime called ferriferrihydrite) that disappears just before the full transformation into hematite. Transmission electron microscopy observations suggest that this intermediate crystalline phase nucleates at the expense of the primordial gel, remaining physically attached to it until complete transformation into hematite. This microstructural picture appears well supported as, upon aging, the glassy magnetic dynamics found in ferrihydrite persists for the aged products even when dispersed in a nonmagnetic matrix, very likely because part of the so grown ferrimagnetic nanoparticles form aggregates within remains of the nontransformed gel. We propose a coherent model of ferrihydrite aging, based on integrated microstructural and magnetic observations, which is useful for the identification of aging products in geological and biological environments.

Simulation of Natural Iron Oxide Alteration in Soil: Conversion of Synthetic Ferrihydrite to Hematite Without Artificial Dopants, Observed With Magnetic Methods

“…Furthermore, production of hematite versus goethite is particularly sensitive to pH and Eh (Burrows et al, 2013; Das et al, 2011; Essington, 2015; Lukens et al, 2018; Penn & Soltis, 2014; Schellmann, 1959; Schwertmann & Murad, 1983). Synchronous production of primary goethite and hematite in our experiments, therefore, when compared to results obtained under similar humidity, temperature, and pH conditions that do not contain goethite (e.g., Cabello et al, 2009; Gutiérrez et al, 2016; Jiang et al, 2018; Michel et al, 2010; Soltis et al, 2016), testifies to the sensitivity and the relative stability of hematite and goethite as a function of environmental conditions. This observation, and subsequent variations of ambient conditions over time, provide caution against the assumptions placed on the goethite to hematite ratio as a geologically stable paleoprecipitation proxy.…”

“…The drop in room temperature remanence might reflect dissolution of minor amounts of ordered ferrihydrite or ferric gel (cf. Gutiérrez et al, 2016; Michel et al, 2010) concomitant with goethite crystallization (e.g., Fischer, 1971; Mackay, 1960). The TEM image of goethite crystals in the 0‐hr sample and its ability to carry remanence and the suggestion from Mössbauer spectroscopy that miniscule amounts of hematite might also be present clearly indicate that the material is not entirely amorphous at 0 hr, which suggests the possibility that some ferrihydrite clusters with ferrimagnetic ordering may exist in a metastable state.…”

“…In‐phase ( χ ′) and out‐of‐phase ( χ ″) mass‐normalized magnetic susceptibilities were measured on representative specimens as a function of temperature between 10 and 100 K, in AC fields of 238.7 A/m and frequencies of 1, 10, and 100 Hz. These settings were chosen for direct comparison with the measurements of Gutiérrez et al (2016). Magnetic susceptibility between 10 K and room temperature (300 K) was measured in 238.7 A/m fields and 1000 Hz.…”

“…Using one of the original samples from Michel et al (2010) and synthesizing its analog, Gutiérrez et al (2016) delineated a possible transformation path of ferrihydrite to hematite in the presence of ligands. They proposed a model for microstructure changes and compositions based on ferrihydrite being an antiferromagnetic ferric gel (as proposed by Coey & Readman, 1973a, 1973b).…”

Nano-Sized Fe(III) Oxide Particles Starting from an Innovative and Eco-Friendly Synthesis Method

“…In ref. [59] a similar and even more dramatic result, although under substantial different aging conditions, is observed and attributed to dipolar interaction between magnetic particles. A slight appearance of hysteresis is consistent with the formation of the hematite phase [55], as detected by the XRD patterns.…”