Monday, June 7, 2021

Liesegang Banding In Proterozoic Badami Sandstone

The Chalukya era (6th-8th CE) rock cut caves and sculptures at Badami in Karnataka are an archeological wonder. But there is plenty of geology there to admire. In January 2020, I spent some time wandering through Badami. The sandstone layers are 900 million years old river deposits. I wrote a long post about them, explaining the primary sedimentary structures that one can observe in these rocks, and what they tell us about the water depths and currents during deposition of the sediment. 

But these primary structures, i.e. sedimentary layer orientations that form during deposition, are not the only interesting features of these rocks. Chemical reactions in these sediments after their burial has overprinted an intriguing fabric on to the rock.

In the picture a very distinct dark and light banding is seen in one of the Badami rock surfaces. This is Liesegang banding. 

The dark bands are rich in iron oxide. The lighter bands have little or no iron oxide. Such banding forms by the mobilization of ions from one location in the sediment and their precipitation at another. Ions diffuse along a concentration gradient in the water filled pore spaces. Robert A. Berner's book, Early Diagenesis: A Theoretical Approach, has a good explanation for the formation of Liesegang banding. I am reproducing that below.

"Mobilization of different components of a substance can occur at two or more different locations. The best example of this is the formation of Liesegang banding.In Liesegang banding we have the interdiffusion of two dissolved ions which cab react with one another to form a relatively insoluble solid. The two ions can come from different sources and when their concentrations at a given site build  up, via diffusion, to sufficiently high values, precipitation of the insoluble solid occurs. This precipitation suddenly lowers concentration in the neighborhood of the solid, and as a result the diffusion profiles become altered. Continued interdiffusion results in a new build-up in concentration and precipitation at another site. Depending on the geometry of the situation, this process may result in Liesegang rings (3-dimensional), tubes (2-dimensional), or layers (1-dimensional). A common example of Liesegang phenomenoa are rhythmic bands of iron oxides often found in sandstones. In this case precipitation is most likely brought about by the interdiffusion of dissolved Fe++ (from an anoxic) source) and dissolved O2 (from an oxic source). Where the Fe++ and O2 meet, Liesegang banding occurs".

The iron (Fe++) would already have been present in the sediment perhaps in discrete grains of pyrite (FeS2), or trapped in carbonaceous plant debris.   Rainfall fed groundwater is the common source of oxygen.  As pyrite gets oxidized it releases Fe++ and sulphur ions. The ferrous ions get oxidized to ferric ions (Fe+++). These then nucleate to form iron oxide or hydroxides. Rapid diffusion of ions towards a growing crystal will eventually lower the concentration of ferric ions in the region surrounding the grain to below the nucleation threshold, at which point crystal growth stops. This threshold is reached at a different location where pyrite oxidation is releasing a fresh supply of Fe++. At this new location the concentration of ferric ions build up again to levels where they start nucleating into iron oxide. This migration of zones of dissolution (of pyrite) , diffusion, and nucleation results in the distinct banding. I've summarized this explanation from a paper by P. Ortoleva and colleagues on redox (reduction-oxidation) front propagation and formation of mineral banding.

Formation of redox fronts during the burial of a sedimentary rock can be economically important. For example, a certain type of sandstone hosted uranium deposit known as 'roll-front' occur where oxidizing fluids containing dissolved uranium meet reduced components such as pyrite or organic matter. 

Here is another close up of these Liesegang bands. They have a ring or a tube like geometry. The cross bedding indicated by the arrow is a primary structure formed by the movement of sand sculpted into ripples or waves on the river bed. The Liesegang bands have been imprinted over the cross beds subsequently. 

The chemical reactions that occur in sediment after their deposition are of great interest to geologists.  They play a large role in the reorganization of porosity and permeability through the dissolution and re-precipitation of minerals.Throughout the history of a sedimentary basin, fluids move through these pore networks mobilizing elements, and under favorable conditions, enriching them at particular locations. Geologists prospecting for metal and hydrocarbon deposits want to understand this process.

No comments:

Post a Comment