Precipitation patterns formed by self-organizing processes in porous media

Abstract

ABSTRACT We compare laboratory and field examples of self-organized mineral precipitates in porous media. Laboratorytests of silver chromate precipitation in glass beads and glass bead⁄gel mixtures produce structures such as peri-odic banding and mm-size spherules. These are morphologically similar to the varied forms of iron oxide precipi-tates in the Jurassic Navajo Sandstone, Utah USA, that preserve records of former fluid redox boundaries in aporous and permeable sandstone. Experimental studies of periodic precipitates in porous media can providevaluable insight for understanding the diagenetic history of similar precipitates in natural environments.Key words: concretions, Liesegang bands, self-organizationReceived 23 April 2010; accepted 10 December 2010Corresponding author: L. M. Barge, Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109, USA.Email: Laura.M.Barge@jpl.nasa.gov. Tel: +18183938209. Fax: +18183934445.Geofluids (2011) 11, 124–133 INTRODUCTION In this study, we examine precipitates that form viaself-organizing processes in porous and permeable media.Self-organizing patterns in diffusion-controlled chemicalsystems have been studied for over a century, since the dis-covery by R. E. Liesegang in 1896 that the interactionbetween inter-diffusing silver and chromate ions in a gela-tin gel can produce periodically spaced bands of precipitate(Liesegang 1896). These patterns, termed ‘Liesegangbands’, have since been produced using various combina-tions of reactants (Henisch 2005, and references therein)and in many different types of gels, and the pattern mor-phology can vary widely depending on initial experimentalconditions. Liesegang bands are typically described as ‘peri-odic patterns’, and we will retain this description, althoughthe spacing of bands may not be strictly periodic in thespatial sense.The formation of Liesegang patterns has been attributedto various mechanisms including supersaturation–nucle-ation–depletion processes (in which the first band depletesthe immediate area of the least abundant reactant, resultingin the second band being deposited some distance fromthe first (Ostwald 1897; Smith 1984) and competitiveparticle growth (in which larger particles are less solubleand grow at the expense of smaller ones – a phenomenonknown as Ostwald ripening (Mu¨ller & Ross 2003; Sultan& Ortoleva 1993; Ortoleva 1982; Lifshitz & Slyozov1961). Liesegang band patterns tend to obey several exper-imental laws, for example, that the distance between con-secutive bands increases approximately according to ageometric series (Jablczynski 1923). Liesegang band spac-ing is a function of many factors including reactant con-centrations and thresholds for particle formation, andprecipitation patterns can be partially controlled by alteringsuch factors. Periodic precipitation in systems where homo-geneity of the medium or coagulation threshold is variedin the direction of diffusion, rather than kept as a constantvalue, can give rise to regularly spaced bands or ‘revert’patterns (where band spacing decreases with distance) (An-tal et al. 2007; Molna´r et al. 2008; Jahnke & Kantelhardt2010). Local concentrations of ions can be controlled by,for example, an applied time-dependent electric field acrossthe gel, resulting in band formation in predetermined loca-tions (Bena et al. 2008). The physical and chemical prop-erties of the diffusion medium can also have effects on thepattern formation as well. Gradients of factors such as tem-perature or pH that affect precipitation can give rise to