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.