Mackinawite, formation

McNeil MB, Little BJ (1990) Technical Note Mackinawite formation during microbial corrosion. CORROSION 46(7) 599-600... 

Many mineral species are known to be selectively crystallized by the presence of bacteria. Carbonate minerals, such as calcite, aragonite, hydroxycalcite, and siderite oxide minerals, such as magnetite and todorokite oxalate minerals, such as whewellite and weddellite sulfide minerals, such as pyrite, sphalerite, wurtzite, greigite, and mackinawite and other minerals, such as jarosite, iron-jarosite, and g3q>sum, are known to precipitate in the presence of bacteria. Therefore, investigations have been developed to analyze the formation of banded iron ore by the action of bacteria, and to analyze the ancient environmental conditions of the Earth through the study of fossilized bacteria. 

In marine and lacustrine muds, the initial sulfide phase precipitated during early diagenesis is mackinawite (FeS09) which is subsequently converted to greigite (Fe3S4) and pyrite (FeS2) (85-89). This reaction path leads to the formation of framboidal pyrite (88.90). However, in salt marsh sediments under low pH and low sulfide ion activity conditions, direct precipitation of pyrite by reaction of ferrous iron with elemental sulfur without the formation of iron monosulfides as intermediates has been reported (85-87.89.91.92). This reaction is one possible pathway for the precipitation of pyrite as single crystals (89). 

Distribution of dissolved Fe(II), as well as in the case of dissolved Mn(II), is characterized by the increasing of its content in the redox zone and by formation of an intermediate maximum within the dissolved Mn(II) maximum. Fe(II) is oxidized rapidly in the presence of oxygen to Fe(III) that exists as oxides and hydroxides with low solubility. The dissolved Fe(II) appears at a = 16.2 kgm-3. Its concentration increases toward the maximum (about 0.3 pM at oq = 16.5-16.6 kgm 3), and then decreases to 0.05-0.07 pM at or = 16.8 kg m-3. The deep concentrations appear to be controlled by solubility with FeS (mackinawite) or Fe3S4 (greigite), even though FeS2 (pyrite) is more insoluble and is present in the water column [71]. 

With the exception of Equation (10), all these pathways involve a precursor iron monosulfide phase. Direct precipitation of pyrite from solution (Equation (10)) is strongly inhibited. This is due to the difficulty of direct nucleation of pyrite, leading to very large supers aturation with respect to pyrite in experimental and natural solutions (Schoonen and Barnes, 1991a). Experimental studies have thus focused on the role of one or more iron monosulfide precursors to pyrite, which have long been recognized as intermediates in sedimentary pyrite formation (see review by Morse et al., 1987). Poorly crystalline mackinawite is the initial product of reaction of H2S with aqueous or solid... 

Reactions involving mackinawite and an oxidized sulfur species have been repeatedly shown to lead to pyrite formation (e.g., Bemer, 1969 Rickard, 1969, 1975). In addition, Wilkin and Bames (1996) and Penning et al. (2000) have shown that pyrite formation is exceptionally rapid when the mackinawite is pre-oxidized (e.g., exposed briefly to air) prior to the experiment. Based partly on X-ray photoelectron and Auger spectroscopy results of pyrrhotite oxidation (Mycroft et al., 1995), Wilkin and Bames (1996) hypothesized that this oxidative exposure initiates an iron-loss pathway similar to Equation (13). In sulfidic solutions, Fe(II) oxyhydroxides, shown as a product in this reaction, would not accumulate, but instead would undergo reductive dissolution by a reaction similar to Equation (14) ... 

The loss of one-fourth of the iron from mackinawite with simultaneous oxidation of one-half of the initial iron leads to formation of greigite (Wilkin and Barnes, 1996 Equation (15)). Iron loss (as opposed to sulfur gain— Equation (11)) is energetically favored, because mackinawite and greigite share the same close-packed sulfur sublattice ... 

Fig. 17. PPO4 versus pS for upper 8 cm at FOAM, NWC, and DEEP where pS was detectable. The line drawn is the equilibrium relation between vivianite (Fe3(P04)2 8H2O) and mackinawite (FeS). , FOAM O. NWC A, DEEP. All seasons plotted. The formation of sulfides is generally favored, but regions exist near the sediment-water interface where coexistence of phases or phosphate dominance are possible.

Six iron-sulfur minerals are stable enough to exist in nature all contain the ferrous (Fe +) form of iron. Those minerals with iron-to-sulfur ratios of 1 1 (FeS) are mackinawite and pyrrhotite. Those with iron-to-sulfur ratios of 3 4 0 0384) are greigite and smythite. All the iron sulfides with ratios of 1 1 and 3 4 are soluble in mild acids with formation of H2S. Those minerals with iron-to-sulfur ratios of 1 2 (FeSj) are pyrite and marcasite. Pyrite and marcasite are distinguished from the other four iron sulfides by their insolubility in concentrated HCl. Pyrite is highly pH- and temperature-stable. Due to this, it is found often in nature. This inertness also makes pyrite a desirable reaction product for sulfide removal using an iron-based scavenger. Various iron sulfides can be formed chemically from iron compounds reacting with soluble sulfides at ambient conditions in aqueous systems. The specific reaction conditions control both the products formed and the rate of reaction. 

The stoichiometric monotelluride CuTe, the yellow-bronze colored mineral vulcanite, on the other hand, does crystallize in a true layer structure which, however, represents a fairly distorted version of the mackinawite type (Figure 74). The formerly planar square net of the tetrahedrally coordinated metal atoms is now a slightly puckered Cu plane. Moreover, the lattice is compressed along [100] which leads to the formation of straight Te chains along the a-axis with Te—Te =... 

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