Magnetite sedimentation rate

However, for this process to be realized in nature a combination of several conditions is necessary. Kinetic factors—high rate of crystallization of the oxide compared to silicates or carbonates—would favor the deposition of iron in the form of magnetite. For Fe(OH)3 to convert to Fe304, the presence of a sufficient amount of a reducing agent, for instance organic matter, in the lower water layer or directly in the sediment is necessary, but in that case the final product more likely will be siderite or greenalite ... 

Reduction of oxide sediments is possible if the content of organic matter or free carbon is sufficient. The production of certain mineral forms depends on the activity of sulfur and pH. The formation of diagenetic magnetite is presumed only in neutral environments with low activity of sulfur and a slow rate of crystallization of the silicates. A rise in temperature (to 150-180°C) leads to the appearance of a magnetite stability field in silicate sediments. 

These observations indicate that reduction of As(V) to As(III) does not, in itself, result in the mobilization of arsenic. This conclusion is supported by laboratory adsorption studies showing similar affinities of As(III) and As(V) for hydrous ferric oxide, goethite, and magnetite.16 However, outstanding questions remain regarding the factors that control the rate and extent of the reductive dissolution of iron in these sediments and whether the arsenic (and iron) that is released into the porewater is (re)sorbed onto the residual iron oxyhydroxides in... 

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