Equilibrium mass-dependent isotope fractionation of antimony between stibnite and Sb secondary minerals: A first-principles study

Atomic structure used to model antimony equilibrium isotope fractionation in Sb-bearing minerals — First-principles modelling of Sb-bearing mineral structures provides theoretical estimates of equilibrium isotope fractionation between stibnite and secondary antimony minerals.

Key points

What the study shows

This study provides the first theoretical estimates of equilibrium Sb isotope fractionation between stibnite and several secondary antimony minerals.

Secondary Sb minerals are predicted to be isotopically heavier than stibnite at equilibrium.

At approximately 22 °C, the calculated range of equilibrium isotope compositions spans about 2.3‰ between stibnite and Sb₂O₅.

Isotope fractionation is controlled by oxidation state, the nature of the first atomic neighbours around Sb and distortion of Sb–O coordination environments.

Summary

Predicting the isotope fingerprints of Sb minerals

Antimony released during the weathering and oxidation of primary ore minerals can be incorporated into a range of secondary Sb-bearing phases. These mineral transformations can potentially change isotope composition even in the absence of kinetic processes.

Understanding this equilibrium isotope fractionation is therefore essential for interpreting δ¹²³Sb signatures in ores, weathering products, mine wastes and contaminated environments.

This study uses first-principles calculations to estimate equilibrium mass-dependent Sb isotope fractionation among stibnite and several secondary minerals, including valentinite, senarmontite, cervantite and a fully oxidized Sb₂O₅ structure.

The calculations show a systematic tendency for secondary oxide minerals to display higher reduced partition-function ratios than stibnite. As a result, the oxidized and oxygen-bearing mineral phases are predicted to preferentially incorporate the heavier ¹²³Sb isotope at equilibrium.

The magnitude of this effect is controlled not simply by mineral identity, but by the local atomic environment of Sb: oxidation state, Sb–O versus Sb–S bonding, coordination and distortion of the polyhedra surrounding Sb all contribute to the predicted isotope composition.

Main result

Secondary Sb minerals are predicted to be isotopically heavier than stibnite at equilibrium. At about 22 °C, calculated lnβ values increase from approximately 0.71‰ for stibnite to 3.03‰ for Sb₂O₅, corresponding to a theoretical mineral-to-mineral range of about 2.3‰. This demonstrates that mineral transformation alone can generate substantial Sb isotope differences.

Theoretical results

A progressive enrichment in heavy Sb isotopes from sulfide to oxide minerals

The calculated reduced partition-function ratios increase systematically across the investigated minerals, revealing a strong relationship between atomic structure and equilibrium isotope composition.

Stibnite 0.71‰ Calculated lnβ at approximately 22 °C for Sb₂S₃, the primary Sb sulfide mineral.

Valentinite 1.64‰ Calculated lnβ for the orthorhombic Sb(III) oxide polymorph.

Senarmontite 1.80‰ Calculated lnβ for the cubic Sb(III) oxide polymorph.

Cervantite 2.20‰ Higher calculated isotope composition for a mixed-valence Sb(III)–Sb(V) oxide mineral.

Sb₂O₅ 3.03‰ Highest calculated lnβ among the investigated structures, corresponding to fully oxidized Sb(V).

Theoretical range ≈ 2.3‰ Difference between the calculated equilibrium isotope compositions of stibnite and Sb₂O₅ at about 22 °C.

Mineral comparison

Atomic structure controls equilibrium isotope fractionation

The comparison among Sb-bearing minerals shows that isotope composition is strongly related to the local bonding environment around antimony. Replacing sulfur neighbours with oxygen, increasing oxidation state and modifying the geometry of Sb–O polyhedra all affect bond stiffness and therefore the relative stability of the heavy and light Sb isotopes.

Comparison of calculated antimony isotope fractionation among stibnite and secondary Sb minerals — **Calculated equilibrium fractionation among Sb minerals** Comparison of theoretical isotope compositions for stibnite and secondary Sb-bearing minerals. The progressive increase from sulfide to increasingly oxidized oxide structures highlights the role of oxidation state and local atomic coordination in controlling equilibrium Sb isotope fractionation.

Geochemical significance

Why mineral transformations matter for Sb isotope tracing

Differences in Sb isotope composition between an ore, a weathering product and a secondary mineral do not necessarily indicate different Sb sources. Theoretical equilibrium fractionation shows that mineral transformation itself can generate per-mil-scale isotope differences as Sb moves from sulfide-bearing primary minerals into oxidized secondary phases. Accounting for mineralogy and oxidation state is therefore essential when δ¹²³Sb values are used to reconstruct sources, weathering pathways or contaminant transport in mining-impacted environments.

Publication

Scientific reference

Citation

Ferrari, C., Méheut, M., Resongles, E., Freydier, R., Casiot, C. (2022). Equilibrium mass-dependent isotope fractionation of antimony between stibnite and Sb secondary minerals: A first-principles study. Chemical Geology, 611, 121115.

DOI: 10.1016/j.chemgeo.2022.121115

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