Abstract

Concretions are common in some of the modern intertidal sediments on the Lincolnshire coast of the Wash. The mineralogy and geochemistry of numerous examples of these concretions have been studied in detail. The majority have metallic nuclei and those that do not, exhibit textures which suggest that they originally did so. Petrographic observations indicate that the cements within these concretions precipitated in a distinct sequence that is spatially developed around the metallic nucleus and that are arranged from core to periphery: (a) a ferrous hydroxy chloride mineral (similar in bulk composition to akaganeite) together with iron monosulphide (amorphous FeS and mackinawite), pyrite and possibly elemental sulphur; (b) ferroan carbonate cements (including siderite, ankerite and calcite); (c) mixed ferrous and ferric minerals (“green rust” together with magnetite and possibly greigite); (d) fully oxidized minerals (including akaganeite, goethite, hematite, gypsum and a complex basic sulphate of Fe, Ca, Mg, Si and Al). This latter material has not been possible to characterize fully but is probably an amorphous mixture.

The initiating reaction for the precipitation of these cements is anaerobic corrosion of iron at zero valence state with sulphate as an oxidant. Metal, hydroxide and monosulphide are found in close spatial association, indicating the reaction:  
4Fe0+SO42+4H2O3Fe(OH)2+2OH+FeS

Further away from the metallic nucleus, carbonates cement the host clastic sediment. These are the products of reaction between the first-formed hydroxides and pore water solutes (principally HCO−3, Mg2+ and Ca2+). When oxygen gains access to the growing concretion, Fe(II) minerals are replaced very rapidly by akaganeite and either goethite or hematite. Gypsum and other sulphate minerals also precipitate as new cements around the periphery of the concretions.

The cementation process in these concretions is driven by the extreme instability of metallic iron (here, relic military armaments and shrapnel fragments) in contact with saline, anaerobic water and is indicative more of cathodic corrosion than the growth of ancient carbonate–sulphide concretions.

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