Chemical Model for Pattern Formation in Rocks via Periodic Precipitation of Iron Oxide Minerals

Abstract

The zebra-like striped patterns that spontaneously appear in rocks are characterized by periodic self-organized red-brown bands consisting of iron oxide minerals. Although the formation of periodic iron oxide precipitates has been discussed in terms of the Liesegang mechanism, the details remain unclear. Herein, we used chemical experiments and numerical simulations to reproduce the periodic precipitation of iron oxide minerals in a gel medium. The diffusion flux of NaOH into the gel medium containing ferrous ions resulted in the periodic precipitation of Fe­(OH)2. Based on the periodicity and dependence on the electrolyte concentration, these Fe­(OH)2 precipitates formed via a Liesegang-type mechanism, as indicated by the spacing law and Matalon–Packter law. Further aerobic oxidation converted the Fe­(OH)2 pattern into an FeOOH pattern without changing the periodic bands. Although our experimental conditions did not allow geothermal conditions to be reproduced, further oxidation at high temperatures is expected to yield an Fe2O3 pattern. Thus, it was concluded that the Liesegang-like Fe2O3 patterns form via initial salt formation and precipitation of Fe­(OH)2, which is subsequently oxidized into FeOOH and Fe2O3. The Fe­(OH)2 pattern formation process was also obtained numerically using a reaction–diffusion simulation that considered a continuous flux of the ferrous ions into the reaction field. These experiments and simulations offer an improved understanding of the formation of rocks with periodic self-organized patterns of iron oxide minerals via the Liesegang mechanism.

Periodic Precipitation in a Confined Liquid Layer

“…This empirical rule is known as the spacing law. This type of structure is often observed as geological patterns, , and some researchers have been working to explain the formation mechanism of such geoscientific patterns by the Liesegang phenomenon. , Therefore, exploring the LP formation mechanism from a physicochemical standpoint is an essential scientific proposition.…”