Understanding redox processes during iron precipitation in standing water: implications in formation of iron oxides minerals in the terrestrial planetary environment (especially Mars)

Abstract

Iron oxide minerals are found associated with sulfate rocks in Meridiani Planum, Mars and in Valles Marineris, Mars. Similar occurrence of iron oxide minerals, hematite and sulfate rock is found on Earth. Many experiments are performed over the time to understand formation of iron oxide minerals and genetic link between iron oxide mineral and associated sulfate rock. Here also, an iron precipitation experiment is performed in presence of various secondary minerals. In the present experiment, the iron precipitation in the gypsum and siderite vessels occurs in increasing acidic and oxidizing conditions. And, the iron precipitation in the calcite and magnesite vessels occurs in increasing alkaline and reducing conditions. The reactions occurring between ammonium ferrous sulfate and various sulfate, carbonate minerals in separate vessels lead to iron speciation. This is evident from Eh–pH diagrams plotted for various minerals experimental vessels. Reddish-brown precipitate present in various experimental vessels suggests ferric iron precipitation and the greenish color precipitate suggests reduced ferrous iron in the system. XRD studies of the precipitated material indicate the formation of iron oxides mineral, goethite from the oxidation of ammonium ferrous sulfate compound in all experimental vessels except experimental vessels 3 and 4 (calcite + mohr’s salt ± charcoal), which have another iron oxides mineral, magnetite. The present research studies support the martian hematite formation model based on iron oxide mineral precipitation in standing water.