A proposed new type of arsenian pyrite: Composition, nanostructure and geological significance

Artur P. Deditius, Satoshi Utsunomiya, Devon Renock, Rodney C. Ewing, Chintalapalle V. Ramana, Udo Becker, Stephen E. Kesler

Research output: Contribution to journalArticle

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Abstract

This report describes a new form of arsenian pyrite, called As3+-pyrite, in which As substitutes for Fe [(Fe,As)S2], in contrast to the more common form of arsenian pyrite, As1--pyrite, in which As1- substitutes for S [Fe(As,S)2]. As3+-pyrite has been observed as colloformic overgrowths on As-free pyrite in a hydrothermal gold deposit at Yanacocha, Peru. XPS analyses of the As3+-pyrite confirm that As is present largely as As3+. EMPA analyses show that As3+-pyrite incorporates up to 3.05 at % of As and 0.53 at. %, 0.1 at. %, 0.27 at. %, 0.22 at. %, 0.08 at. % and 0.04 at. % of Pb, Au, Cu, Zn, Ni, and Co, respectively. Incorporation of As3+ in the pyrite could be written like: As3 + + y Au+ + 1 - y (□) ⇔ 2 Fe2 +; where Au+ and vacancy (□) help to maintain the excess charge. HRTEM observations reveal a sharp boundary between As-free pyrite and the first overgrowth of As3+-pyrite (20-40 nm thick) and co-linear lattice fringes indicating epitaxial growth of As3+-pyrite on As-free pyrite. Overgrowths of As3+-pyrite onto As-free pyrite can be divided into three groups on the basis of crystal size, 8-20 nm, 100-300 nm and 400-900 nm, and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals. The Yanacocha deposit, in which As3+-pyrite was found, formed under relatively oxidizing conditions in which the dominant form of dissolved As in the stability field of pyrite is As3+; in contrast, reducing conditions are typical of most environments that host As1--pyrite. As3+-pyrite will likely be found in other oxidizing hydrothermal and diagenetic environments, including high-sulfidation epithermal deposits and shallow groundwater systems, where probably kinetically controlled formation of nanoscale crystals such as observed here would be a major control on incorporation and release of As3+ and toxic heavy metals in oxidizing natural systems.

Original languageEnglish
Pages (from-to)2919-2933
Number of pages15
JournalGeochimica et Cosmochimica Acta
Volume72
Issue number12
DOIs
Publication statusPublished - Jun 15 2008
Externally publishedYes

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Nanostructures
pyrite
Chemical analysis
Poisons
crystal
Crystals
Gold deposits
epithermal deposit
Arsenic
Heavy Metals
Epitaxial growth
Crystal lattices
X-ray spectroscopy
Vacancies
trace metal
Groundwater
arsenic

All Science Journal Classification (ASJC) codes

  • Geochemistry and Petrology

Cite this

A proposed new type of arsenian pyrite : Composition, nanostructure and geological significance. / Deditius, Artur P.; Utsunomiya, Satoshi; Renock, Devon; Ewing, Rodney C.; Ramana, Chintalapalle V.; Becker, Udo; Kesler, Stephen E.

In: Geochimica et Cosmochimica Acta, Vol. 72, No. 12, 15.06.2008, p. 2919-2933.

Research output: Contribution to journalArticle

Deditius, Artur P. ; Utsunomiya, Satoshi ; Renock, Devon ; Ewing, Rodney C. ; Ramana, Chintalapalle V. ; Becker, Udo ; Kesler, Stephen E. / A proposed new type of arsenian pyrite : Composition, nanostructure and geological significance. In: Geochimica et Cosmochimica Acta. 2008 ; Vol. 72, No. 12. pp. 2919-2933.
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abstract = "This report describes a new form of arsenian pyrite, called As3+-pyrite, in which As substitutes for Fe [(Fe,As)S2], in contrast to the more common form of arsenian pyrite, As1--pyrite, in which As1- substitutes for S [Fe(As,S)2]. As3+-pyrite has been observed as colloformic overgrowths on As-free pyrite in a hydrothermal gold deposit at Yanacocha, Peru. XPS analyses of the As3+-pyrite confirm that As is present largely as As3+. EMPA analyses show that As3+-pyrite incorporates up to 3.05 at {\%} of As and 0.53 at. {\%}, 0.1 at. {\%}, 0.27 at. {\%}, 0.22 at. {\%}, 0.08 at. {\%} and 0.04 at. {\%} of Pb, Au, Cu, Zn, Ni, and Co, respectively. Incorporation of As3+ in the pyrite could be written like: As3 + + y Au+ + 1 - y (□) ⇔ 2 Fe2 +; where Au+ and vacancy (□) help to maintain the excess charge. HRTEM observations reveal a sharp boundary between As-free pyrite and the first overgrowth of As3+-pyrite (20-40 nm thick) and co-linear lattice fringes indicating epitaxial growth of As3+-pyrite on As-free pyrite. Overgrowths of As3+-pyrite onto As-free pyrite can be divided into three groups on the basis of crystal size, 8-20 nm, 100-300 nm and 400-900 nm, and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals. The Yanacocha deposit, in which As3+-pyrite was found, formed under relatively oxidizing conditions in which the dominant form of dissolved As in the stability field of pyrite is As3+; in contrast, reducing conditions are typical of most environments that host As1--pyrite. As3+-pyrite will likely be found in other oxidizing hydrothermal and diagenetic environments, including high-sulfidation epithermal deposits and shallow groundwater systems, where probably kinetically controlled formation of nanoscale crystals such as observed here would be a major control on incorporation and release of As3+ and toxic heavy metals in oxidizing natural systems.",
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AU - Ramana, Chintalapalle V.

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