A general moment NRIXS approach to the determination of equilibrium Fe isotopic fractionation factors: application to goethite and jarosite

TL;DR

This paper introduces a new general moment approach using NRIXS data to accurately determine equilibrium Fe isotopic fractionation factors in minerals like goethite and jarosite, enhancing understanding of geochemical processes.

Contribution

The paper presents a novel general moment method to calculate isotope fractionation factors from NRIXS spectra, improving accuracy and applicability in geochemical studies.

Findings

Measured Fe isotope partition functions in goethite and jarosite.

Developed a new method to derive {eta}-factors from NRIXS moments.

Linked the first moment to the mean force constant of Fe bonds.

Abstract

We measured the reduced partition function ratios for iron isotopes in goethite FeO(OH), potassium-jarosite KFe3(SO4)2(OH)6, and hydronium-jarosite (H3O)Fe3(SO4)2(OH)6, by Nuclear Resonant Inelastic X-Ray Scattering (NRIXS, also known as Nuclear Resonance Vibrational Spectroscopy -NRVS- or Nuclear Inelastic Scattering -NIS) at the Advanced Photon Source. These measurements were made on synthetic minerals enriched in 57Fe. A new method (i.e., the general moment approach) is presented to calculate {\beta}-factors from the moments of the NRIXS spectrum S(E). The first term in the moment expansion controls iron isotopic fractionation at high temperature and corresponds to the mean force constant of the iron bonds, a quantity that is readily measured and often reported in NRIXS studies.