Self-organized chemical precipitates: Laboratory and field studies

Self-organized patterns can be formed in diffusion experiments where two interdiffusing electrolytes react to form an insoluble precipitate in a medium (such as gelatin) that permits diffusive motion of ions, but prevents product particles from moving from their site of formation. Various pattern morphologies such as periodic bands, dendritic crystals, and continuous precipitates can be formed in many types of diffusion media. We hypothesized that interfering ions that are not part of the dominant precipitation reaction could affect the formation of self-organized patterns in a reproducible fashion, and that the same precipitation reaction occurring in different diffusion media would produce different pattern morphologies (depending on the physical and chemical properties of that medium).;To initially characterize the patterns formed by various diffusion/reaction systems, we performed experiments with ferrous carbonate, phosphate, and hydroxide precipitation in agarose gel. Ferrous compounds were precipitated under reducing conditions, forming self-organized patterns such as periodic bands, and were subsequently oxidized as atmospheric oxygen diffused into the gel. Mineral replacement occurred upon introduction of a reactant that formed a more insoluble compound than the one already present, and after replacement the banding pattern remained unaltered, allowing identification of the original precipitate.;We also characterized the effects of the diffusion medium and interfering ions on self-organized precipitation in silver nitrate/potassium chromate and silver nitrate/potassium dichromate reaction systems. We observed that precipitate morphology was characteristic of the reactants that were initially present and the type of gel medium in which they precipitated. It was found that soluble impurities in agarose gel were responsible for the slight banding produced in silver chromate precipitation experiments. We then tested the effects of organic compounds (simple amino acids and their Nacetylated derivatives) on morphologies of silver chromate precipitates in purified agarose gel. High concentrations of pure amino acids caused periodic banding of crystals, and N-acetyl amino acids were more effective than pure amino acids, because of their negative charge. We hypothesized that the length and orientation of the neutral side chain of the amino acids, as well as charge, affects the degree of binding to the crystal surface, and hence the ability to induce banded patterns.;To better apply these studies to self-organized patterns that form in natural systems, where the diffusion medium may be inhomogeneous (such as sand or sandstone), we studied laboratory and field examples of self-organized mineral precipitates in porous media. Silver chromate precipitation experiments in tubes of glass beads and glass bead/gel mixtures produced structures such as finger fluid fronts, periodic banding, and millimeter-size spherules. The spheroidal precipitates produced in our experiments nucleated via self-organizing processes throughout the glass bead medium, and bear morphological resemblance to iron oxide concretions formed via self-organizing processes in the Navajo Sandstone, UT, that preserve records of fluid flow events in a porous and permeable sandstone.