The occurrence and chemical composition of Recent Fe-rich mixed carbonates in a river bog

Abstract A deposit of recent iron carbonate in a river bog is described macroscopically and field pH/Eh measurements have been made. X-ray diffraction and chemical analysis revealed the existence of an Fe-rich mixed carbonate with considerable Mn and Ca content. Also vivianite (Fe 3 (PO 4 ) 2 .8H 2 O) was found while the phosphate fraction possibly also contains Mn and Ca. The accurate composition of the carbonate and phosphate fractions remains, however, unclear. The paragenesis of vivianite and siderite is predictable from thermodynamic data and is found in all occurrences of iron carbonate in bogs. Carbon isotope analysis of the carbonate minerals yields values of Ca - 18%delta 13 C, indicating that an important part of the carbon in the carbonate is of organic origin. Greigite (cubic Fe 3 S 4 ) was identified in the minor sulfidic fraction of the sediment and is believed to form simultaneously with siderite and vivianite. The pH is believed to be the major controlling factor on the stability of the iron carbonate. A model for the formation of the iron carbonate is presented based on mixing of groundwater with different origin and composition, but processes occurring within the bog will be of importance.