Experimental study on the growth process of iron rind of Fe-oxide concretions

Abstract

Spherical Fe- oxide concretions are found in the Navajo Sandstone Formation in Utah, USA, which range in size from a few millimeters to a few centimeters. The interior of the concretions is filled with sand and the surface is covered with an Fe oxide rind. The process that formed the Fe rinds, the timescales of formation, and the effect of pores in the concretions and the dissolved oxygen concentration on the rind formation were studied experimentally.

Fe oxide concretions were artificially prepared in a petri dish. An experimental system was established to capture images of the forming Fe rind (brown band). The chemical distribution was investigated by X-ray fluorescence analysis. The results suggest the following two points about the formation of natural Fe oxide concretions. (1) In the early stages of Fe rind formation, there is a rapid increase in the width of the rind. The Fe rind grows rapidly there are many pores in the concretion. (2) Fe oxide concretions with clear rinds were formed at lower dissolved oxygen concentrations.