STUDY OF SOME SPECIFIC FEATURES OF LIESEGANG PERIODIC PRECIPITATION, WITH APPLICATIONS TO GEOCHEMICAL PATTERN FORMATION

Abstract

Reaction-diffusion precipitation processes engender the formation of beautiful exotic strata of bands, in a way similar to the geochemical self-organization inherent in the morphology of some rocks. Liesegang patterning is one of the most well-known self- organization phenomena, governed by two pivotal processes which are ion diffusion and precipitation, taking place in a gel medium. An outer electrolyte reacts with an inner electrolyte in the gel during the interdiffusion of the two coprecipitates, exhibiting a beautiful display of rhythmic bands.

The present work is composed of three main sections: 1. A surface study of revert spacing in a 1D PbCrO4 system; 2. Synthesis and characterization of calcium carbonate mineral during a carbonation reaction, carried out in a slaked lime gel matrix (mimicking a portlandite rock mineral); and 3. Chaos studies in a 2D Liesegang experiment with PbCrO4 precipitation.

In the first section, we carry out surface and structural studies in a lead chromate Liesegang system exhibiting revert spacing in a 1D thin tube. Although the revert spacing phenomenon was fairly studied in the Literature, we demonstrated and confirmed in our study the ionic adsorption of chromate on the precipitate, and proved that the extent of adsorption increases with distance from the electrolytes junction. We used Scanning Electron Microscopy (SEM), Atomic Absorption Spectrometry (AAS), and Energy Dispersive X-ray (EDX) spectroscopic measurements. SEM micrographs revealed the presence of isolated particles at the bottom of a given band. These particles grow in size as we go down the tube, thus suggesting that adsorption increases. AA measurements confirmed the presence of free CrO42‒ closer to the bottom of a band than the remainder of any interband gel portion. EDX measurements and calculations showed that the bottom of a band is always enriched with excess CrO42‒, and hence negatively charged, which causes the Coulombic attraction of Pb2+, causing the next band to form closer and closer as we go down the pattern.

In the second section, we investigate the zonation of calcite [CaCO3] in a portlandite mineral [Ca(OH)2] putty, using a displacement reaction in the solid phase. The diffusion of aqueous carbonic acid into the portlandite-bearing putty causes the precipitation of calcium carbonate in a variety of patterns ranging from periodic (Liesegang) to mosaic (spherulites) patterns, attributed to the non-homogeneous distribution of calcium hydroxide particles after they first aggregate during the gelling. The differentiation of

the two minerals was characterized by pH measurements, Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and notably powder X-ray diffraction (XRD), yet also distinguished to the naked eye by the phenolphthalein indicator color.

Finally, in the last section, we conduct experiments on PbCrO4 precipitation in 2D Petri dishes, wherein the band distribution varies from periodic to deterministic chaos (yet to be rigorously characterized) within a flow rate range from 0.025 ml/day- 0.100 ml/day. Outside this range, periodic behavior was observed in the sense that 2D lead chromate pattern obeys the empirical scaling laws.