Physics
Scientific paper
Dec 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.2615m&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 50, Issue 12, p. 2615-2629.
Physics
16
Scientific paper
Pyrite and marcasite were precipitated by both slow addition of aqueous Fe2+ and SiO32‑ to an H2S solution and by mixing aqueous Fe2+ and Na2S4 solutions at 75°C. H2S2 or HS2‑ and H2S4 or HS4‑ were formed in the S2O32‑ and Na2S4 experiments, respectively. Marcasite formed at pH < pK1 of the polysulfide species present (for H2S2, pK1 = 5.0; for H2S4, pK1 = 3.8 at 25°C). Marcasite forms when the neutral sulfane is the dominant polysulfide, whereas pyrite forms when mono-or divalent polysulfides are dominant. In natural solutions where H2S2 and HS2 are likely to be the dominant polysulfides, marcasite will form only below pH 5 at all temperatures.The pH-dependent precipitation of pyrite and marcasite may be caused by electrostatic interactions between polysulfide species and pyrite or marcasite growth surfaces: the protonated ends of H2S2 and HS2 are repelled from pyrite growth sites but not from marcasite growth sites. The negative ions HS2 and S22‑ are strongly attracted to the positive pyrite growth sites. Masking of 1πg* electrons in the S2 group by the protons makes HS2 and H2S2 isoelectronic with AsS2‑ and As22‑, respectively (TOSSELLet al., 1981). Thus, the loellingitederivative structure (marcasite) results when both ends of the polysulfide are protonated.Marcasite occurs abundantly only for conditions below pH 5 and where H2S2 was formed near the site of deposition by either partial oxidation of aqueous H2S by O2 or by the reaction of higher oxidation state sulfur species that are reactive with H2S at the conditions of formation e.g., S2O32‑ but not SO42‑. The temperature of formation of natural marcasite may be as high as 240°C (HANNINGTON and SCOTT, 1985), but preservation on a multimillion-year scale seems to require post-depositional temperatures of below about 160°C (RISING, 1973; MCKIBBEN and ELDERS, 1985).
Barnes Hubert L.
Murowchick James B.
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