The Aggregated Precipitation of Iron Minerals in Three Systems: Tubular Growth, Liesegang Patterns, and Interfacial Cementation

Abstract

My research has focused on the precipitation of iron minerals, mostly oxides and hydroxides, in aqueous systems across steep pH and Eh gradients. Unlike most work in this area, which involves loose precipitates filtered out of solutions, I have focused on precipitated aggregates and, more specifically, on those that are self-organized into dis-crete structures or patterns. This topic is actually quite narrow because such types of natu-ral material organization are rare within the geochemical realm compared with the mor-phological richness of crystals, not to mention the phantasmagoria of life.My investigation of iron-based examples has included three types of physical sys-tems: 1) growth of tubular structures around bubbles coming off a charged cathode in a free solution where convection dominates; 2) development of Liesegang patterns within gelled solutions due to reactions dominated by diffusion; and 3) formation of a cement-ing matrix within the aqueous interface between particles of silica. The third case in-volves physical characteristics of the first two in that it is primarily a tightly packed, dif-fusion-limited process, but at least initially the generation of gases can create mechani-cally driven flows through the interstitial spaces.All three systems and studies are inextricably related for both tubular ('vermi-form') structures and Liesegang patterns are commonly found in natural iron-cemented sediments such as massive laterite, ironstone deposits, and banded iron formations. They are also found on a much smaller scale within discrete 'concretions' and represent the two poles of the gradient between convection-based and diffusion-based systems. As Seilacher (2001) states concerning concretions, "the distribution and precipitation of dis-solved constituents, such iron and manganese, proceeds in two radically different mor-phospaces, which are typified by dendrites [and I would include tubes and other linear growth] on the one hand and Liesegang rings on the other." Both have been observed in my lab creations with surprising frequency and tenacity even in systems thought to be in-hibitory.