Relation of pH and oxidation potential to sedimentary iron mineral formation

Abstract

This paper reports the results of a series of experiments performed to test the validity of the theoretical relations among several iron minerals. Equilibrium diagrams are drawn showing stability fields of hematite, siderite, and pyrite as functions of pH and oxidation potential for various low temperature environments, such as sea water or other relatively dilute salt solutions. The experiments show that ferric hydroxide (ferric oxide) comes to equilibrium with its environment in a few hours, ferrous carbonate requires several weeks, but reacts at an easily detectable rate; ferrous sulfide is not formed in detectable quantities over a period of several weeks by a change from an oxidizing to a reducing environment in an inorganic system. Ferrous hydroxide is easily precipitated from deoxygenated solutions; it oxidizes very rapidly to ferric hydroxide in air. The theoretical and empirical chemical relations of these iron compounds seem applicable to natural conditions; observed iron mineral associations have a reasonable interpretation within this framework. The work indicates that iron, if transported in solution to the site of deposition, is carried in mildly acid waters, and that any deficiency in aeration of these waters contributes further to iron solubility. Another implication is that the accumulation of thick layers of iron-rich rocks requires a trap between the source of iron and the depositional area if associated elastics are to be removed from iron-rich waters.