Hydrothermal formation of Fe-oxide bands in zebra rocks from northern Western Australia

Coefficients of Reaction‐Diffusion Processes Derived From Patterns in Rocks

“…A model that combines both field observations and theoretical modeling involving fluid chemistry and fluid flow in porous media has been suggested to explain similar patterns found in Utah, and Arizona (Wang et al., 2015). The mineral hematite is a late‐stage transformation product of Fe‐Oxyhydroxide which has been identified in the case of the Zebra rock as the precipitate of the ferrous ions in an acidic geothermal environment according to the following chemical reaction (Kawahara et al., 2022) $$$\begin{array}{rl}\hfill \underset{\text{dolomite}}{\underbrace{2(\text{Ca},\,\text{Mg}){\left({\text{CO}}_{3}\right)}_{2(s)}}}& +\underset{\text{Fe}\mbox{-}\text{bearing}\,\text{acidic}\,\text{fluid}}{\underbrace{{\left[4\,{\text{Fe}}^{2+}+2{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{O}}_{2}\right]}_{(aq)}}}\to \hfill \\ \hfill & \to \underset{\text{Fe}\mbox{-}\text{Oxyhydroxide}}{\underbrace{4{\text{FeO}(\text{OH})}_{(s)}}}+{\left[2{\text{Ca}}^{2+}+2{\text{Mg}}^{2+}+4{\text{CO}}_{2}\right]}_{(aq)}\hfill \end{array}$$$ …”

“…Relevant studies have suggested potential chemical and hydrothermal environments during the pattern formation of Zebra rocks. Loughnan and Roberts (1990) proposed that the presence of dickite in some specimens is significant as it rules out a weathering alteration and points to an acid-sulfate alteration compatible with hydrothermal events as proposed by Kawahara et al (2022). They found that pH-fluctuations induced by the oxidation decomposition of pyrite would create a favorable chemical environment to convert ferrous to colloidal hematite (Fe 2 O 3 ).…”

Coefficients of reaction-diffusion processes derived from patterns in rocks

“…Self-organization develops autonomously in reaction-diffusion systems and is observed in many disciplines (e.g., pattern formation in biology (Cooper, 2012), corrosion processes and electrochemical deposition in chemistry (Christoph et al, 1999;Nakouzi & Sultan, 2011;Arguello, Labanda, et al, 2022;, and Zebra striped formation and rock textures in geology (Wang et al, 2015;Msharrafieh et al, 2016;Kelka et al, 2017;Abrajevitch et al, 2018;May et al, 2019;Al-Ghoul & Sultan, 2019;Regenauer-Lieb et al, 2021)). Two necessary conditions are required to induce self-organization: (i) the governing chemical reactions and diffusion processes are far from the equilibrium; and (ii) at least two processes actively interact in the reactiondiffusion system, which is an open system.…”

“…The concentration growth hence results in Liesegang striping due to the periodic coagulation of an autocatalytic nucleation mechanism. Kawahara et al (2022) criticized this model as pyrite is found in Zebra rocks suggesting an acidic fluid environment. They thereby proposed a new formation mechanism arguing that the pH fluctuations are due to the neutralization reactions between an Fe 2+ -bearing acidic hydrothermal fluid and the carbonate minerals in the sedimentary rock.…”

“…Here the concept of phase separation is employed for interpretation and parameter inversion of the rhythmically banded and spotted patterns of East Kimberley Zebra rocks, which can be readily extended to the general classification of Mississippi Valley Type deposits (Wang et al, 2015;Kelka et al, 2017).…”