Arsenic in hydrothermal apatite: oxidation state, mechanism of uptake, and comparison between experiments and nature

Weihua Liu, Yuan Mei, Barbara Etschmann, Joël Brugger, Mark Pearce, Chris G. Ryan, Stacey Borg, Jeremy Wykes, Peter Kappen, David Paterson, Ulrike Boesenberg, Jan Garrevoet, Gareth Moorhead, Gerald Falkenberg

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Element substitution that occurs during fluid–rock interaction permits assessment of fluid composition and interaction conditions in ancient geological systems, and provides a way to fix contaminants from aqueous solutions. We conducted a series of hydrothermal mineral replacement experiments to determine whether a relationship can be established between arsenic (As) distribution in apatite and fluid chemistry. Calcite crystals were reacted with phosphate solutions spiked with As(V), As(III), and mixed As(III)/As(V) species at 250 °C and water-saturated pressure. Arsenic-bearing apatite rims formed in several hours, and within 48 h the calcite grains were fully replaced. X-ray Absorption Near-edge Spectroscopy (XANES) data show that As retained the trivalent oxidation state in the fully-reacted apatite grown from solutions containing only As(III). Extended X-ray Fine Spectroscopy (EXAFS) data reveal that these As(III) ions are surrounded by about three oxygen atoms at an As[sbnd]O bond length close to that of an arsenate group (AsO4 3−), indicating that they occupy tetrahedral phosphate sites. The three-coordinated As(III)–O3 structure, with three oxygen atoms and one lone electron pair around As(III), was confirmed by geometry optimization using ab initio molecular simulations. The micro-XANES imaging data show that apatite formed from solutions spiked with mixed As(III) and As(V) retained only As(V) after completion of the replacement reaction; in contrast, partially reacted samples revealed a complex distribution of As(V)/As(III) ratios, with As(V) concentrated in the center of the grain and As(III) towards the rim. Most natural apatites from the Ernest Henry iron oxide copper gold deposit, Australia, show predominantly As(V), but two grains retained some As(III) in their core. The As-anomalous amphibolite-facies gneiss from Binntal, Switzerland, only revealed As(V), despite the fact that these apatites in both cases formed under conditions where As(III) is expected to be the dominant As form in hydrothermal fluids. These results show that incorporation of As in apatite is a complicated process, and sensitive to the local fluid composition during crystallization, and that some of the complexity in As zoning in partially reacted apatite may be due to local fluctuations of As(V)/As(III) ratios in the fluid and to kinetic effects during the mineral replacement reaction. Our study shows for the first time that As(III) can be incorporated into the apatite structure, although not as efficiently as As(V). Uptake of As(III) is probably highly dependent on the reaction mechanism. As(III)O3 3− moieties replace phosphate groups, but cause a high strain on the lattice; as a result, As(III) is easily exchanged (or oxidized) for As(V) during hydrothermal recrystallization, and the fully reacted grains only record the preferred oxidation state (i.e., As(V)) from mixed-oxidation state solutions. Overall this study shows that the observed oxidation state of As in apatite may not reflect the original As(III)/As(V) ratio of the parent fluid, due to the complex nature of As(III) uptake and possible in situ oxidation during recrystallization.

Original languageEnglish
Pages (from-to)144-159
Number of pages16
JournalGeochimica et Cosmochimica Acta
Volume196
DOIs
Publication statusPublished - 1 Jan 2017

Keywords

  • Apatite
  • Arsenic
  • Mineral replacement reaction
  • Molecular simulations
  • Oxidation state
  • Trace element partitioning
  • XANES spectroscopy

Cite this

Liu, Weihua ; Mei, Yuan ; Etschmann, Barbara ; Brugger, Joël ; Pearce, Mark ; Ryan, Chris G. ; Borg, Stacey ; Wykes, Jeremy ; Kappen, Peter ; Paterson, David ; Boesenberg, Ulrike ; Garrevoet, Jan ; Moorhead, Gareth ; Falkenberg, Gerald. / Arsenic in hydrothermal apatite : oxidation state, mechanism of uptake, and comparison between experiments and nature. In: Geochimica et Cosmochimica Acta. 2017 ; Vol. 196. pp. 144-159.
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title = "Arsenic in hydrothermal apatite: oxidation state, mechanism of uptake, and comparison between experiments and nature",
abstract = "Element substitution that occurs during fluid–rock interaction permits assessment of fluid composition and interaction conditions in ancient geological systems, and provides a way to fix contaminants from aqueous solutions. We conducted a series of hydrothermal mineral replacement experiments to determine whether a relationship can be established between arsenic (As) distribution in apatite and fluid chemistry. Calcite crystals were reacted with phosphate solutions spiked with As(V), As(III), and mixed As(III)/As(V) species at 250 °C and water-saturated pressure. Arsenic-bearing apatite rims formed in several hours, and within 48 h the calcite grains were fully replaced. X-ray Absorption Near-edge Spectroscopy (XANES) data show that As retained the trivalent oxidation state in the fully-reacted apatite grown from solutions containing only As(III). Extended X-ray Fine Spectroscopy (EXAFS) data reveal that these As(III) ions are surrounded by about three oxygen atoms at an As[sbnd]O bond length close to that of an arsenate group (AsO4 3−), indicating that they occupy tetrahedral phosphate sites. The three-coordinated As(III)–O3 structure, with three oxygen atoms and one lone electron pair around As(III), was confirmed by geometry optimization using ab initio molecular simulations. The micro-XANES imaging data show that apatite formed from solutions spiked with mixed As(III) and As(V) retained only As(V) after completion of the replacement reaction; in contrast, partially reacted samples revealed a complex distribution of As(V)/As(III) ratios, with As(V) concentrated in the center of the grain and As(III) towards the rim. Most natural apatites from the Ernest Henry iron oxide copper gold deposit, Australia, show predominantly As(V), but two grains retained some As(III) in their core. The As-anomalous amphibolite-facies gneiss from Binntal, Switzerland, only revealed As(V), despite the fact that these apatites in both cases formed under conditions where As(III) is expected to be the dominant As form in hydrothermal fluids. These results show that incorporation of As in apatite is a complicated process, and sensitive to the local fluid composition during crystallization, and that some of the complexity in As zoning in partially reacted apatite may be due to local fluctuations of As(V)/As(III) ratios in the fluid and to kinetic effects during the mineral replacement reaction. Our study shows for the first time that As(III) can be incorporated into the apatite structure, although not as efficiently as As(V). Uptake of As(III) is probably highly dependent on the reaction mechanism. As(III)O3 3− moieties replace phosphate groups, but cause a high strain on the lattice; as a result, As(III) is easily exchanged (or oxidized) for As(V) during hydrothermal recrystallization, and the fully reacted grains only record the preferred oxidation state (i.e., As(V)) from mixed-oxidation state solutions. Overall this study shows that the observed oxidation state of As in apatite may not reflect the original As(III)/As(V) ratio of the parent fluid, due to the complex nature of As(III) uptake and possible in situ oxidation during recrystallization.",
keywords = "Apatite, Arsenic, Mineral replacement reaction, Molecular simulations, Oxidation state, Trace element partitioning, XANES spectroscopy",
author = "Weihua Liu and Yuan Mei and Barbara Etschmann and Jo{\"e}l Brugger and Mark Pearce and Ryan, {Chris G.} and Stacey Borg and Jeremy Wykes and Peter Kappen and David Paterson and Ulrike Boesenberg and Jan Garrevoet and Gareth Moorhead and Gerald Falkenberg",
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Liu, W, Mei, Y, Etschmann, B, Brugger, J, Pearce, M, Ryan, CG, Borg, S, Wykes, J, Kappen, P, Paterson, D, Boesenberg, U, Garrevoet, J, Moorhead, G & Falkenberg, G 2017, 'Arsenic in hydrothermal apatite: oxidation state, mechanism of uptake, and comparison between experiments and nature' Geochimica et Cosmochimica Acta, vol. 196, pp. 144-159. https://doi.org/10.1016/j.gca.2016.09.023

Arsenic in hydrothermal apatite : oxidation state, mechanism of uptake, and comparison between experiments and nature. / Liu, Weihua; Mei, Yuan; Etschmann, Barbara; Brugger, Joël; Pearce, Mark; Ryan, Chris G.; Borg, Stacey; Wykes, Jeremy; Kappen, Peter; Paterson, David; Boesenberg, Ulrike; Garrevoet, Jan; Moorhead, Gareth; Falkenberg, Gerald.

In: Geochimica et Cosmochimica Acta, Vol. 196, 01.01.2017, p. 144-159.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - Arsenic in hydrothermal apatite

T2 - oxidation state, mechanism of uptake, and comparison between experiments and nature

AU - Liu, Weihua

AU - Mei, Yuan

AU - Etschmann, Barbara

AU - Brugger, Joël

AU - Pearce, Mark

AU - Ryan, Chris G.

AU - Borg, Stacey

AU - Wykes, Jeremy

AU - Kappen, Peter

AU - Paterson, David

AU - Boesenberg, Ulrike

AU - Garrevoet, Jan

AU - Moorhead, Gareth

AU - Falkenberg, Gerald

PY - 2017/1/1

Y1 - 2017/1/1

N2 - Element substitution that occurs during fluid–rock interaction permits assessment of fluid composition and interaction conditions in ancient geological systems, and provides a way to fix contaminants from aqueous solutions. We conducted a series of hydrothermal mineral replacement experiments to determine whether a relationship can be established between arsenic (As) distribution in apatite and fluid chemistry. Calcite crystals were reacted with phosphate solutions spiked with As(V), As(III), and mixed As(III)/As(V) species at 250 °C and water-saturated pressure. Arsenic-bearing apatite rims formed in several hours, and within 48 h the calcite grains were fully replaced. X-ray Absorption Near-edge Spectroscopy (XANES) data show that As retained the trivalent oxidation state in the fully-reacted apatite grown from solutions containing only As(III). Extended X-ray Fine Spectroscopy (EXAFS) data reveal that these As(III) ions are surrounded by about three oxygen atoms at an As[sbnd]O bond length close to that of an arsenate group (AsO4 3−), indicating that they occupy tetrahedral phosphate sites. The three-coordinated As(III)–O3 structure, with three oxygen atoms and one lone electron pair around As(III), was confirmed by geometry optimization using ab initio molecular simulations. The micro-XANES imaging data show that apatite formed from solutions spiked with mixed As(III) and As(V) retained only As(V) after completion of the replacement reaction; in contrast, partially reacted samples revealed a complex distribution of As(V)/As(III) ratios, with As(V) concentrated in the center of the grain and As(III) towards the rim. Most natural apatites from the Ernest Henry iron oxide copper gold deposit, Australia, show predominantly As(V), but two grains retained some As(III) in their core. The As-anomalous amphibolite-facies gneiss from Binntal, Switzerland, only revealed As(V), despite the fact that these apatites in both cases formed under conditions where As(III) is expected to be the dominant As form in hydrothermal fluids. These results show that incorporation of As in apatite is a complicated process, and sensitive to the local fluid composition during crystallization, and that some of the complexity in As zoning in partially reacted apatite may be due to local fluctuations of As(V)/As(III) ratios in the fluid and to kinetic effects during the mineral replacement reaction. Our study shows for the first time that As(III) can be incorporated into the apatite structure, although not as efficiently as As(V). Uptake of As(III) is probably highly dependent on the reaction mechanism. As(III)O3 3− moieties replace phosphate groups, but cause a high strain on the lattice; as a result, As(III) is easily exchanged (or oxidized) for As(V) during hydrothermal recrystallization, and the fully reacted grains only record the preferred oxidation state (i.e., As(V)) from mixed-oxidation state solutions. Overall this study shows that the observed oxidation state of As in apatite may not reflect the original As(III)/As(V) ratio of the parent fluid, due to the complex nature of As(III) uptake and possible in situ oxidation during recrystallization.

AB - Element substitution that occurs during fluid–rock interaction permits assessment of fluid composition and interaction conditions in ancient geological systems, and provides a way to fix contaminants from aqueous solutions. We conducted a series of hydrothermal mineral replacement experiments to determine whether a relationship can be established between arsenic (As) distribution in apatite and fluid chemistry. Calcite crystals were reacted with phosphate solutions spiked with As(V), As(III), and mixed As(III)/As(V) species at 250 °C and water-saturated pressure. Arsenic-bearing apatite rims formed in several hours, and within 48 h the calcite grains were fully replaced. X-ray Absorption Near-edge Spectroscopy (XANES) data show that As retained the trivalent oxidation state in the fully-reacted apatite grown from solutions containing only As(III). Extended X-ray Fine Spectroscopy (EXAFS) data reveal that these As(III) ions are surrounded by about three oxygen atoms at an As[sbnd]O bond length close to that of an arsenate group (AsO4 3−), indicating that they occupy tetrahedral phosphate sites. The three-coordinated As(III)–O3 structure, with three oxygen atoms and one lone electron pair around As(III), was confirmed by geometry optimization using ab initio molecular simulations. The micro-XANES imaging data show that apatite formed from solutions spiked with mixed As(III) and As(V) retained only As(V) after completion of the replacement reaction; in contrast, partially reacted samples revealed a complex distribution of As(V)/As(III) ratios, with As(V) concentrated in the center of the grain and As(III) towards the rim. Most natural apatites from the Ernest Henry iron oxide copper gold deposit, Australia, show predominantly As(V), but two grains retained some As(III) in their core. The As-anomalous amphibolite-facies gneiss from Binntal, Switzerland, only revealed As(V), despite the fact that these apatites in both cases formed under conditions where As(III) is expected to be the dominant As form in hydrothermal fluids. These results show that incorporation of As in apatite is a complicated process, and sensitive to the local fluid composition during crystallization, and that some of the complexity in As zoning in partially reacted apatite may be due to local fluctuations of As(V)/As(III) ratios in the fluid and to kinetic effects during the mineral replacement reaction. Our study shows for the first time that As(III) can be incorporated into the apatite structure, although not as efficiently as As(V). Uptake of As(III) is probably highly dependent on the reaction mechanism. As(III)O3 3− moieties replace phosphate groups, but cause a high strain on the lattice; as a result, As(III) is easily exchanged (or oxidized) for As(V) during hydrothermal recrystallization, and the fully reacted grains only record the preferred oxidation state (i.e., As(V)) from mixed-oxidation state solutions. Overall this study shows that the observed oxidation state of As in apatite may not reflect the original As(III)/As(V) ratio of the parent fluid, due to the complex nature of As(III) uptake and possible in situ oxidation during recrystallization.

KW - Apatite

KW - Arsenic

KW - Mineral replacement reaction

KW - Molecular simulations

KW - Oxidation state

KW - Trace element partitioning

KW - XANES spectroscopy

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