COMPLEX CEMENTATION TEXTURES AND AUTHIGENIC MINERAL ASSEMBLAGES IN RECENT CONCRETIONS FROM THE LINCOLNSHIRE-WASH (EAST-COAST, UK) DRIVEN BYFE(0) TO FE(II) OXIDATION

Concretions are common in some of the modem intertidal sediments on th e-lincolnshire coast of the Wash. The mineralogy and geochemistry of n umerous 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 d istinct sequence that is spatially developed around the metallic nucle us acid that are arranged from core to periphery: (a) a ferrous hydrox y chloride mineral (similar in bulk composition to akaganeite) togethe r with iron monosulphide (amorphous FeS and mackinawite), pyrite and p ossibly elemental sulphur; (b) ferroan carbonate cements (including si derite, ankerite and calcite); (c) mixed ferrous and ferric minerals ( ''green rust'' together with magnetite and possibly greigite); (d) ful ly oxidized minerals (including akaganeite, goethite, hematite, gypsum and a complex basic sulphate of Fe, Ca, Mg, Si and Al). This latter m aterial has not been possible to characterize fully but is probably an amorphous mixture. The initiating reaction for the precipitation of t hese cements is anaerobic corrosion of iron at zero Valence state with sulphate as an oxidant. Metal, hydroxide and monosulphide are found i n close spatial association, indicating the reaction: 4Fe(0) + SO42- 4H(2)O --> 3Fe(OH)(2) + 20H(-) + FeS Further away from the metallic n ucleus, carbonates cement the host elastic sediment. These are the pro ducts of reaction between the first-formed hydroxides and pore water s olutes (principally HCO3-, 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 c oncretions. The cementation process in these concretions is driven by the extreme instability of metallic iron (here, relic military armamen ts and shrapnel fragments) in contact with saline, anaerobic water and is indicative more of cathodic corrosion than the growth of ancient c arbonate-sulphide concretions.