Brisbane: Liesegang rings
Tasks:
1. How are Liesegang rings formed in sandstone? Explain in your own words.
2. What colors do the Liesegang rings have, and what could influence their coloring?
3. You will have noticed that the Liesegang rings end abruptly at the separations of the individual slabs. Why might this be the case?
4. Please take a picture of yourself or an object on site to prove your visit and post it with your log.
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Here in Brisbane, in front of the City Hall, there is an area of sandstone pavement, and at GZ, it contains an interesting feature: Liesegang rings. These concentric patterns, visible in the sandstone, are a stunning result of natural chemical processes that have taken place deep within the rock over millions of years.
Picture: Atli Arnarson "Liesegang rings II" (https://www.flickr.com/photos/atlapix/47996179313)
How does sandstone form?
Sandstone is a sedimentary rock that forms through the consolidation and cementation of sand grains. This process begins with the deposition of sand grains, which are transported by water, wind, or ice. These deposits typically occur in various environments, such as seas, rivers, lakes, and deserts.
Over millions of years, layers of sediment accumulate on top of previously deposited sand. These layers are compressed by the pressure of the overlying sediments. Over time, loose sand solidifies through processes such as compaction and cementation into solid rock – sandstone. During cementation, the sand grains are bonded by minerals like quartz, calcite, or iron oxides, which precipitate from the surrounding water and act as a natural "glue."
This process can occur over different geological periods, leading to the presence of sandstones on all continents, sometimes far from where they were originally deposited, such as in mid-mountain ranges where they have been exposed after millions of years of geological uplift and erosion.
What is sandstone made of?
Sandstone is composed of interconnected sand grains that can be up to 2 mm in size. The main component of many sandstones is quartz, a mineral that is particularly resistant to chemical weathering. The density and hardness of sandstone vary depending on the size of the sand grains.
Sandstone can take on various colors – from gray, brown, and yellow to red. These colors result from mineral and organic impurities, particularly iron-containing minerals, which can cause a reddish or brown coloration.
What are Liesegang rings, and how do they form?
Liesegang rings are an example of natural chemical pattern formation. This phenomenon occurs when chemical reactions take place in porous media such as sandstone, creating periodic structures. The rings are named after the German chemist Raphael Liesegang, who discovered this phenomenon in 1896.
How do these rings form?
The formation of Liesegang rings is a complex process triggered by diffusion – the movement of molecules from areas of high concentration to areas of low concentration – in a porous medium. In sandstones, this occurs as follows:
1. Chemical weathering and diffusion: When water containing dissolved chemical substances such as iron penetrates the porous rock, it begins to diffuse. During this process, the dissolved substances can precipitate in certain areas of the rock, forming solid minerals.
2. Formation of concentric rings: These precipitations do not occur uniformly but periodically, creating the characteristic concentric rings. The causes of this periodicity are the subject of intense research and can range from the specific chemical properties of the solutions to the physical properties of the rock.
3. Influence of fractures and cracks: Liesegang rings often orient themselves according to pre-existing structures in the rock, such as fractures or cracks. These are created by tectonic stresses, pressure release, or diagenetic processes and provide preferred pathways for the diffusion of solutions.
These rings are particularly visible in rock sections and can be considered natural works of art.
https://www.mineralienatlas.de/lexikon/index.php/RockData?lang=de&rock=Sandstein
https://de.wikipedia.org/wiki/Sandstein
https://www.chemie.de/lexikon/Liesegangsche_Ringe.html
https://de.wikipedia.org/wiki/Liesegangsche_Ringe
Bild: https://www.flickr.com/photos/atlapix/47996179313