Fe–Ni-bearing serpentines from the saprolite horizon of Caribbean Ni-laterite deposits: new insights from thermodynamic calculations

Cristina Villanova-de-Benavent, Cristina Domènech, Esperança Tauler, Salvador Galí, Santiago Tassara, Joaquín A. Proenza

Research output: Contribution to journalArticle

Abstract

Fe–Ni-bearing serpentine from the saprolite horizon is the main Ni ores in hydrous silicate-type Ni laterites and formed by chemical weathering of partially serpentinized ultramafic rocks under tropical conditions. During lateritization, Mg, Si, and Ni are leached from the surface and transported downwards. Fe2+ is oxidized to Fe3+ and fixed as insoluble Fe-oxyhydroxides (mostly goethite) that incorporate Ni. This Ni is later leached from goethite and incorporated in secondary serpentine and garnierite. As a result, a serpentine-dominated saprolite horizon forms over the ultramafic protolith, overlapped by a Fe-oxyhydroxide-dominated limonite horizon. The serpentine from the protolith (serpentine I) is of hydrothermal origin and yields similar Ni (0.10–0.62 wt.% NiO) and lower Fe (mostly 1.37–5.81 wt.% FeO) concentrations than the primary olivine. In contrast, Fe–Ni-bearing serpentine from the saprolite (serpentine II) shows significantly higher and variable Fe and Ni contents, typically ranging from 2.23 to 15.59 wt.% Fe2O3 and from 1.30 to 7.67 wt.% NiO, suggesting that serpentine get enriched in Fe and Ni under supergene conditions. This study presents detailed mineralogical, textural, and chemical data on this serpentine II, as well as new insights by thermodynamic calculations assuming ideal solution between Fe-, Ni- and Mg-pure serpentines. The aim is to assess if at atmospheric pressure and temperature Fe–Ni-bearing serpentine can be formed by precipitation. Results indicate that the formation of serpentine II under atmospheric pressure and temperature is thermodynamically supported, and pH, Eh, and the equilibrium constant of the reaction are the parameters that affect the results more significantly.

Original languageEnglish
Pages (from-to)979-992
Number of pages14
JournalMineralium Deposita
Volume52
Issue number7
DOIs
Publication statusPublished - 12 Oct 2017

Fingerprint

Bearings (structural)
laterites
saprolite
laterite
horizon
Atmospheric temperature
Deposits
thermodynamics
deposits
Thermodynamics
Atmospheric pressure
Silicates
Equilibrium constants
Weathering
Ores
Rocks
atmospheric temperature
protolith
goethite
atmospheric pressure

Keywords

  • Caribbean
  • Fe–Ni-serpentine
  • Ni-laterite deposits
  • Saprolite
  • Thermodynamics

Cite this

Villanova-de-Benavent, Cristina ; Domènech, Cristina ; Tauler, Esperança ; Galí, Salvador ; Tassara, Santiago ; Proenza, Joaquín A. / Fe–Ni-bearing serpentines from the saprolite horizon of Caribbean Ni-laterite deposits : new insights from thermodynamic calculations. In: Mineralium Deposita. 2017 ; Vol. 52, No. 7. pp. 979-992.
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Fe–Ni-bearing serpentines from the saprolite horizon of Caribbean Ni-laterite deposits : new insights from thermodynamic calculations. / Villanova-de-Benavent, Cristina; Domènech, Cristina; Tauler, Esperança; Galí, Salvador; Tassara, Santiago; Proenza, Joaquín A.

In: Mineralium Deposita, Vol. 52, No. 7, 12.10.2017, p. 979-992.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Fe–Ni-bearing serpentines from the saprolite horizon of Caribbean Ni-laterite deposits

T2 - new insights from thermodynamic calculations

AU - Villanova-de-Benavent, Cristina

AU - Domènech, Cristina

AU - Tauler, Esperança

AU - Galí, Salvador

AU - Tassara, Santiago

AU - Proenza, Joaquín A.

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AB - Fe–Ni-bearing serpentine from the saprolite horizon is the main Ni ores in hydrous silicate-type Ni laterites and formed by chemical weathering of partially serpentinized ultramafic rocks under tropical conditions. During lateritization, Mg, Si, and Ni are leached from the surface and transported downwards. Fe2+ is oxidized to Fe3+ and fixed as insoluble Fe-oxyhydroxides (mostly goethite) that incorporate Ni. This Ni is later leached from goethite and incorporated in secondary serpentine and garnierite. As a result, a serpentine-dominated saprolite horizon forms over the ultramafic protolith, overlapped by a Fe-oxyhydroxide-dominated limonite horizon. The serpentine from the protolith (serpentine I) is of hydrothermal origin and yields similar Ni (0.10–0.62 wt.% NiO) and lower Fe (mostly 1.37–5.81 wt.% FeO) concentrations than the primary olivine. In contrast, Fe–Ni-bearing serpentine from the saprolite (serpentine II) shows significantly higher and variable Fe and Ni contents, typically ranging from 2.23 to 15.59 wt.% Fe2O3 and from 1.30 to 7.67 wt.% NiO, suggesting that serpentine get enriched in Fe and Ni under supergene conditions. This study presents detailed mineralogical, textural, and chemical data on this serpentine II, as well as new insights by thermodynamic calculations assuming ideal solution between Fe-, Ni- and Mg-pure serpentines. The aim is to assess if at atmospheric pressure and temperature Fe–Ni-bearing serpentine can be formed by precipitation. Results indicate that the formation of serpentine II under atmospheric pressure and temperature is thermodynamically supported, and pH, Eh, and the equilibrium constant of the reaction are the parameters that affect the results more significantly.

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