Fe-bearing phases

Available textbooks and reviews provide a detailed introduction to MB spectroscopy (e.g., Hawthorne 1988 Burns 1993). MB spectra provide quantitative information about the distribution of Fe among its oxidation and coordination states (e.g., octahedrally coordinated Fe3+), identification of Fe-bearing phases, relative distribution of Fe among those phases, and can help to constrain crystallinity and particle size. 

Only one naturally relevant abiotic Se(VI) reduction process has been documented to date. Se(Vl) can be reduced to Se(TV) and ultimately to Se(0) by green rust , an Fe(II)- and Fe(lll)-bearing phase with sulfate occupying interlayer spaces (Myneni et al. 1997). Johnson and Bullen (2003) obtained an ese(vi)-se(iv) value of 7.4%o ( 0.2) for the Se(VI) reduction reaction. The result was not sensitive to changes in pH or solution composition within the ranges over which green rust is stable. 

In complex systems that involve multiple Fe-bearing species and phases, such as those that are typical of biologic systems (Tables 1 and 2), it is often difficult or impossible to identify and separate all components for isotopic analysis. Commonly only the initial starting materials and one or more products may be analyzed for practical reasons, and this approach may not provide isotope fractionation factors between intermediate components but only assess a net overall isotopic effect. In the discussions that follow on biologic reduction and oxidation, we will conclude that significant isotopic fractionations are likely to occur among intermediate components. 

Hints for super- or undersaturation of minerals can be found in the last paragraph of the initial solution calculations entitled saturation indices . Graphical representation of saturation proportions is often done by means of bar charts, whereas SI = 0 marks the point of intersection between the x-axis and the y-axis, and the bars of super-saturated phases point upwards and those for undersaturated phases downwards (example Fe-bearing mineral phases Fig. 37). 

The total abundance of each condensate is limited by the abundance of the least abundant element in the condensate. For example, the mineral schreiber-site Fe3P forms by reaction of P-bearing gases with Fe metal at about 1300 K and 10 4 bar total pressure. Phosphorus has an atomic abundance of 8,373 atoms, which is about 1% of the atomic abundance of iron. There are 3 Fe atoms in each molecule of schreibersite. Thus Fe3P formation consumes only 3% of the total iron abundance, while removing all phosphorus from the gas phase. Likewise, troilite formation removes all sulfur from the gas because its abundance is only 53% of that of iron, while unreacted Fe metal remains present at lower temperatures until it is consumed by formation of Fe-bearing oxides and silicates. 

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