HR: 17:20h
AN: OS14A-06 INVITED     [Abstracts]
TI: Non-paragenesis of Authigenic Sulfide Minerals: Mackinawite and Greigite are Not Precursors of Sedimentary Pyrite
AU: * Morse, J W
EM: morse@ocean.tamu.edu
AF: Texas A&M University, Oceanography Department MS 3146, College Station, TX 77843 United States
AU: Rickard, D
EM: SGLDR@cardiff.ac.uk
AF: Cardiff University, School of Earth, Ocean and Planetary Sciences, Cardiff, CF103YE United Kingdom
AB: Sedimentary sulfide minerals have traditionally been "operationally" divided into pyrite and acid volatile sulfide (AVS) minerals. AVS minerals have generally, with very scant direct evidence, been held to be comprised of mackinawite (tetragonal FeS) and greigite (cubic Fe$^{}_{3}$S$^{}_{4}$). They are also often referred to in the literature generically as FeS or iron monosulfides. Based largely on experimental studies at elevated temperatures over almost a third of a century and their metastability relative to pyrite, it has become almost dogma among sedimentary geochemists that mackinawite and or griegite are necessary precursors for pyrite formation in sediments. Being precursors to pyrite necessarily implies that they must be formed before pyrite and the normal paragenesis is supposed to be mackinawite to greigite to pyrite. The implication is that in their absence no authigenic sedimentary pyrite can be produced. This is not supported by many observations of pyrite formation in sediments where, during early diagenesis, abundant pyrite is commonly produced in the absence of any detectable AVS. Reactions for pyrite formation such as FeS$^{}_{(s)}$ + S$^{0}_{(s)}$ = FeS$^{}_{2(s)}$ or FeS$^{}_{(s)}$ + H$^{}_{2}$S = FeS$^{}_{2(s)}$ + H$^{}_{2}$, for example, represent the net mass balances and do not describe the process. More recently, two reaction mechanisms for pyrite formation, the "polysulfide" and "H$^{}_{2}$S" pathways, have gained wide acceptance. The reaction mechanisms involve dissolved species.If present at all, mackinawite and greigite contribute to pyrite formation via their dissolution which provides reactive components to solution. However, these dissolved components do not necessarily require mackinawite or greigite as their source. For the polysulfide pathway for pyrite formation the net reaction is better expressed as Fe$^{2+}$ + S$^{2-}_{n}$ = FeS$^{}_{2(s)}$ + S$^{2-}_{n-2}$ and for the H$^{}_{2}$S pathway the reaction is FeS$^{}_{(aq)}$ + H$^{}_{2}$S = FeS$^{}_{2(s)}$ + H$^{}_{2}$. FeS$^{}_{(aq)}$ represents aqueous FeS clusters that are true solution components formed by the reaction between Fe(II) and S(-II) and not simply mackinawite nanoparticles or colloids. Pyrite and, possibly, mackinawite and greigite are all formed at the same time or in no particular order. Non-paragenesis in the sedimentary iron-sulfur system is enhanced by continual S(-II) production through microbial activity in the deep biosphere which results in production of any or all of the three minerals at any time during the evolution of the sediment.
DE: 4825 Geochemistry
DE: 4851 Oxidation/reduction reactions
DE: 4802 Anoxic environments
DE: 1045 Low-temperature geochemistry
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting