Concentric Fe-oxyhydroxide bands in dacite cobbles: Rates of buffering chemical reactions

'Liesegang patterns', rinds and bands are commonly observed in nature and form by self-organised periodic precipitation of Fe-oxyhydroxide following a nonlinear reaction-diffusion process. Although strictly Liesegang patterns consist of bands that increase in width with increasing distance from the source of the Fe that precipitated as Fe-oxyhydroxide, regular banded patterns are also sometimes observed that are otherwise similar to Liesegang patterns. However, the detailed process and time scale of regular Fe-oxyhydroxide bands development is still not fully understood. Here we describe an example of regular Fe-oxyhydroxide bands formed within dacite cobbles. Iron was provided to the outer surfaces of the cobbles by acidic water that diffused towards the cobbles' cores. The spatial distributions of Ca and Fe within the Fe-oxyhydroxide bands across the cobbles show that the rhythmic Fe-oxyhydroxide precipitation was controlled by pH buffering. The width of each band (L) and the expected diffusion coefficient of the rock matrix (D) provide the rate of reaction (V) and allow us to estimate the duration of Fe-oxyhydroxide band formation. A 'diffusion-reaction cross plot' implies that the rhythmic Feoxyhydroxide patterns formed very rapidly, within an order of 10(2)-10(3) years, considerably faster than previously estimated. The simplified model can be applied to estimate the reaction time in any similar rock if regular Fe-oxyhydroxide bands are observed.