Data for the journal article "Simulation of natural alteration of iron oxides in soil: conversion of synthetic ferrihydrite to hematite without artificial dopants, observed with magnetic methods"



We present the results of a new study on the conversion of pure, undoped synthetic ferrihydrite, wet-annealed at pH 6.56 and 90 ℃ without stabilizing ligands, to nanophase goethite, hematite and an intermediate magnetic phase, likely to be nanophase maghemite. Our analysis included magnetic field and temperature dependent properties and characterization by powder x-ray diffraction, Mössbauer spectra and high-resolution transmission electron microscopy. Unlike most other studies of ferrihydrite alteration we sampled alteration products after 0.5 hours, and then in a geometric progression to 32 hours, yielding an extremely detailed examination of the earliest alteration phases. There are many similarities to the latest studies of pure ferrihydrite alteration but with a significant difference: we observe very early appearance of oriented nanophase goethite along with a soft magnetic contribution, while rhombohedral hematite crystals are formed later, as reported in the previous studies. Our observations attest to the non-uniqueness of the magnetic enhancement process and to its strong dependence on environmental conditions, with important implications for the proposed hematite/goethite ratio as a reliable proxy for paleoprecipitation intensity.

Magnetic, pXRD and Mössbauer metadata used in the cited study on alteration of synthetic ferrihydrite. All magnetic data was collected at the Institute for Rock Magnetism on a Quantum Design MPMS and comprises magnetic susceptibility (in-phase and out-of-phase) as a function of temperature (10-300K) and frequency (1-1000 Hz), hysteresis loops measured at 5K and 300 K, and magnetic remanence data (pTRM, temperature and AF demagnetized, the "goethetite test") collected between 400 and 10 K.

Funding information
Sponsorship: Research was funded through the Instrumentation and Facilities program of the National Science Foundation Earth Sciences Division (NSF/EAR 1642268)
Date made available2020
PublisherData Repository for the University of Minnesota
Date of data production2016 - 2020

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