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The second sentence in the statement "In terms of its atomic structure, each Fe center is linked to three As atoms and three S atoms. The material can be described as Fe3+ with the diatomic trianion AsS3−." may be somewhat misleading, implying the presence of ferric iron in a mineral otherwise stable in reducing regimes. <ref>Schaufuss et al. (American Mineralogist 85:1754–1766, 2000)</ref>, although in agreement with the Fe, Ar, and S coordination mentioned here, describe the arsenopyrite dianion as (As-S)^(2-), thus making the iron ferrous by balance.
Oxidation states are a great (or terrible) simplification, as is the classification that it is stable in "reducing regimes." These sulfides are semiconductors so localized models are imperfect. A classic WP:SECONDARY source is the book by Craig and Vaughan who indicate that marcasite is Fe(II) with long Fe---Fe distances (3.4A), arsenopyrite is related but Fe(III) but with alternating Fe-Fe bonds (2.9A) and non-bonds (3.5A) along one axis, and loellingite is Fe(IV) with all short Fe-Fe contacts (2.85 A) along the same axis. All are diamagnetic. Again, these are all very ionic descriptions, but within the ionic model, AsS2- would be weird since we do not have molecular analogues. --Smokefoot (talk) 18:04, 13 May 2015 (UTC)