Dear Mr. Lin,

the system is more complex than being depicted by a simple ratio like the Fe/Ca ratio. Keep in mind there are other elements such as bivalent Mg and Mn competing for the same position in the lattice of carbonate minerals. They are conducive to a wide range of solid solution series in the dolomite-, siderite- and calcite system (see e.g. Fe calcite). Temperature  plays a role as important as the presence of sulfur with its variability as to the valence state. Without a full-blown overview of your anoxic system as to the mineralogy and chemical composition, in my opinion, every answer will be rather premature and will conduce to a horse-tailing of the discussion.

Please give full particulars as to the points raised, and I am sure that you will get rather sound answers. Calcite is a common mineral in the "mineral kingdom", but associated with other minerals often can be very "rebellious".

Best regards

H.G.Dill

Thank you very much for the kind reply Professor Harald. Actually, we are working on siderite concretion found in the salt marsh of UK. Quartz ~ 50%, orthoclase ~ 15%, siderite ~ 20%, Mg-calcite ~ 10%, gypsum, albite and pyrite are minor. Amorphous minerals such as FeS, goethite are also present. pH of the pore fluid are ~ 7, Ca ~ 5mM, Mg ~ 25mM, Fe ~ 1mM, SO4 ~ 20 - 40 mM (vary with depth of core sediment), Mn is rather trace.

Dear Chin Yik Lin

Actually what the Prof Harald notes based on  geological and ore mineral  provenance is true, but in marine environment as you have mentions on salt or tidal marsh, first i do not know how length the sediment core profiling you had  taken. If the long just short around 30-50cm i believe the production of ferum carbonate or siderite could  much but thus caused  of decomposition  anaerobic reduction bacteria activity even affected of ion HS-. The anoxide conditions increasing the release  and transition of Mn  from pore interstitial  to surface sediment of salt marsh and generating more Fe2+ as  reducing of  Fe3+  to supply compared to Ca2+ and there fore no precipitate calcite or appear  fadely . And again the ratio Fe2+/Ca2+ =0.5 normally  in marine and  based on your results showed thus might be increasing close to coastal, mangrove  and river environment.

Tq,

Regard's

Zolhizir bin Daud