This month’s post from our Community Geologist, Dr Ian Kille discusses ice-cream, sandstone and Mystery Rock 3 which featured in last month’s WallCAP newsletter. If you’d like to receive our monthly newsletter and get involved with our Stone Sourcing activities, sign up as a volunteer here.

One of the crucial questions which has come up over the last few weeks as I’ve reached down through the stratigraphy of my freezer towards the Palaeozoic, is why old ice-cream is crunchy?

Knowing why my ice-cream is not in tip top condition is high up my agenda. I was therefore delighted to find that there might be a relationship between this and what happens to some sandstones as they get older (the Mystery Rock 3 for last month’s newsletter included).

On the Northumberland Coast and along the Hadrian’s Wall corridor, there are many examples of sedimentary rocks which have either bands or balls in them which cross-cut sedimentary layering. That these features cross-cut the sedimentary layering tells us they grew after the sediments forming the rock were laid down. The balls are more formally known as concretions (or nodules) and the banding is more formally called, err, banding. Both the banding and the concretion formation are associated with ground water permeating the sediments. The mechanisms for this are enigmatic – scientific code for, “its complex and we’re not really exactly sure how this happened”.

There are some interesting options though, which explain at least in part what is going on.

There are two gentlemen, chemists working at the turn of the twentieth century, whose names are associated with these phenomena: Raphael Liesegang and Wilhelm Ostwald, Liesegang for his rings and Ostwald for his ripening.

Liesegang seems to have stumbled on his rings by accident. He was involved in some of the early work on photography, using the light sensitive chemical silver nitrate. He also used potassium chromate which will react with a solution of silver nitrate to produce the insoluble precipitate silver chromate.  One day, Liesegang had a petri dish of silver nitrate dissolved in a gel and inadvertently dropped a crystal of potassium chromate onto it. To his surprise, as the potassium dissolved and diffused through the gel, rather than producing a growing circle of precipitating silver chromate, it produced a series of concentric rings.

Liesegang was excited by this and made lots of observations which he published. He also got in touch with a colleague, Wilhelm Ostwald who worked out an explanation for this phenomenon. Ostwald proposed that as the potassium chromate diffuses outwards through the gel, nearby silver nitrate heads inwards towards it, anxious to react with it. This creates a band outside the reacting ring depleted in silver nitrate. As the potassium chromate continues to diffuse through the gel, it must cross this depleted band before it can find more silver nitrate to react with and create more precipitating silver chromate. The same process then happens all over again with a ring of precipitate forming, along with a band depleted in silver nitrate outside of it. The Liesegang rings are unsurprisingly often referred to as a diffusion depletion phenomenon.

One of the features of the rings is that they become progressively further apart as you move away from the source. This is a defining feature of the rings and the rate at which they get further apart can be mathematically calculated. Looking at Mystery Rock 3 (figure 2) you can see that there are a series of concentric rings which may be explained in this way.

What about the concretions though? Ostwald we have already seen was interested in how chemical reactions work. One of the areas he studied was the way in which larger crystals tend to grow at the expense of smaller crystals. The atoms at the surface of a crystal are less stable than those in the middle of a crystal, as those at the edge are bonded to fewer atoms. This means that larger crystals, which have a lower proportion of atoms at their surface, are more stable. As atoms move in and out of solution, they will preferentially head towards the larger crystals which will grow and replace the smaller crystals over time. This process, now known as Ostwald ripening, can be seen in many different situations, from the way that copper sulphate crystals grow from solution, to the way that igneous phenocrysts form in magmas and to the way that foam bubbles tend to become larger over time. Not least, this process happens in ice-cream, where the larger ice-crystals will grow at the expense of smaller ones over time. Ice-cream technology is devoted to finding ways of ensuring that this happens as slowly as possible.

In sedimentary rocks a common expression of Ostwald Ripening is in the formation of concretions. These are often formed of calcium carbonate (see figure 6) but may also be formed of iron oxides and other minerals. Concretions vary in size from a few millimetres to many metres across and vary in shape from almost perfect spheres to some curious intersecting sphere and doughnut shapes (figure 7).

It would be satisfying if it were possible to end here offering the rings and ripening as a complete explanation of what we see in the rocks. However, the banding seen in some rocks and the relationship between bands and concretions is more complex than can be explained just by these phenomena.  Even in Mystery Rock 3 it is not clear how the multiple series of bands are formed.

Then there are rocks, known as wonder-stones; a picture of one of these from Birling Carrs near Alnmouth can be seen below (figure 8). Recent research has shown that the darker bands of iron, cementing the sandstone, may have an organic origin. Microbes use the energy from oxidation and concentrate the resulting iron oxide into the bands where they live. The wonder-stones may therefore have formed by a combination of organic activity (microbes) and the self-organising processes described by Liesegang and Ostwald. Part of the reason for the research interest in these easily observed patterns in these sandstones is that they can indicate the presence of life and so used to help understand if there really is life on Mars.

Perhaps this blog post should have started with the question, “what is the relationship between crunchy ice-cream and life on Mars?”. Regardless, the patterns produced by diagenetic iron and other minerals are beautiful in their own right and are potentially useful indicators of the provenance of these stones, both in an archaeological and a geological sense.

@Northumbrianman

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