Experimental Tests of Micro-concretion Nucleation in Porous Media

Introduction: Here we examine nucleation of self-organized spherical precipitates in porous and permeable media. Spheroidal concretions are common in geological environments and can arise through various geological or biological mechanisms, such as growth around an already existing nucleus (e.g. organic matter). Iron oxide concretions, however, often lack any obvious nucleus [1] and their presence in sandstone environments may result from self-organized nuclea-tion and precipitation. Spherical iron oxide concretions with a self-organized distribution have also been observed at the MER Opportunity landing site at Merid-iani Planum, Mars. These "blueberries" are found distributed throughout the section and their distribution and chemistry indicates that they were formed through aqueous diagenetic processes [2]. The growth of concretions via advection or diffusion of additional reactants has been studied theoretically [3] and experimentally [1], and much previous work, both experimental and theoretical, exists regarding the formation of periodic precipitates in diffusion-controlled systems [4]. The mechanisms that lead to the formation of periodic bands in gels are well understood , and the positions of the bands of precipitate in a homogenous medium can be derived from solutions of Fick's laws of diffusion, assuming certain boundary conditions [4, 5]. A similar approach can be taken for calculating the formation of periodically distributed spherical precipitates in a heterogeneous sandstone, and this has been modeled by [1]. Experimentally, however, the nucleation and growth of spherical concretions in porous media requires further investigation. We have performed diffusion experiments in combined glass beads and gel media , to study the effects of grain size, pore size, and pore distribution on the morphology of silver chromate precipitates. Precipitates in our experimental systems range from spherical to "finger fluid fronts", and these resemble various types of iron precipitation morphologies that are also observed in natural environments (e.g., the Navajo Sandstone [1]). Experimental: Our approach involves diffusion of silver and chromate ions through different types of gels and glass beads. The silver ion (Ag