Breaking rocks: a closer look

When I described in my last post how rocks can be broken up by volume-increasing reactions happening within them, I left you with several open questions in the end. One of them was whether reaction-driven fracturing can also occur when there is no stress from the outside and no fracture to start with. It is easy enough to imagine that minerals that grow in a crack may push against the walls of the crack, move them apart and cause further fracturing. But for this first crack, with which everything starts, we certainly need some forces from the outside that make the rock break. Or do we really?

You probably imagine a large crack like on the left of figure 1 in the beginning, but a smaller one would also work, would it not? And maybe even a small pit on the surface would be enough.

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Figure 1: From left to right: Fracture that could be widened if a mineral grew in it; smaller fracture; even smaller dissolution pit.

Such pits are very common: When a mineral is dissolved, it is often not dissolved by the same amount everywhere, and its surface becomes rough. When a new mineral, which has a higher volume than the old one, grows on this surface, the roughness of the surface causes it to push on some places more than on others. As soon as we have these kinds of stress concentrations, we can make fractures.

This is something we actually see in minerals: During my Master, I put small crystals of one mineral (scolecite) into a liquid (an NaOH solution), heated it up and let it react with the solution. After the reaction, I saw that a new mineral (tobermorite) had grown in small pits that had formed on the scolecite surface (figure 2A). When the tobermorite wedges became larger and reached a certain depth, they often developed fractures at their tips, where the stress was concentrated (figure 2B). As this is an experiment done in the lab, I know exactly what the crystal looked like before the experiment and can be certain that the fractures formed during the reaction and were not there before.

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Figure 2: A) Pyramid-shaped pits form on the surface of a dissolving scolecite crystal (dark grey), and tobermorite (light grey) grows in them. B) Fractures start at the tips of growing tobermorite wedges. (For details, see: Dunkel, K. G., and Putnis, A., 2014, Replacement and ion exchange reactions of scolecite in a high pH aqueous solution: European Journal of Mineralogy, v. 26, no. 1, p. 61-69.)

Similar observations have been made for serpentinisation, one of the reactions that are common in “searocks”, where olivine reacts with water to serpentine. Seawater dissolves olivine so that small pits form on its surface, serpentine grows in them and eventually breaks the olivine (figure 3).

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Figure 3: A) Fractures divide olivine grains (grey) into fragments. Serpentine (black) grows in the fractures and in pits on the olivine surface. B) Dissolution pits on fracture walls in olivine. The fractures were caused by the growth of serpentine in former pits on the surface of the grain. (Images by Oliver Plümper.)

Because olivine crystals have a regular structure (see post by Sofia here), the small pits on their surface will not be perfectly round, but have a rectangular cross-section. Therefore, the stress that a growing mineral exerts on the walls of the pits will not be the same in all directions, but highest at the corners. When the stress becomes higher than what the olivine can take, this is where a fracture will form. Along the fracture, the water can again dissolve the olivine, and the process repeats itself.

(For more information on fracturing during serpentinisation: Plümper, O., Røyne, A., Magrasó, A., and Jamtveit, B., 2012, The interface-scale mechanism of reaction-induced fracturing during serpentinization: Geology, v. 40, no. 12, p. 1103-1106.)

As you see, we can have reaction-induced fracturing without any previous fractures; we just need a bit of dissolution. At least on a small scale. If we wanted to react large rock volumes, this might be a bit slow, but on the scale of single olivine grains, this process is definitely relevant and can strongly enhance the effect of external fracturing.

On another note, the picture in figure 2B taught me how different people with different backgrounds can have very different perspectives. I used the picture to explain the importance of dissolution pits for reaction-induced fracturing here. However, that was not the reason why I did the experiment in the first place. At that time, I was studying the way ions are exchanged between water and zeolite minerals (such as the scolecite I used for the experiment). Zeolites are so called ion exchangers and are used in washing powder, for example. If there is a lot of calcium and magnesium in your water, you may get a problem with limescale (carbonate deposits that may clog pipes and cover heating elements) in your washing machine. Therefore, zeolites are put into washing powder because they take Ca2+ and Mg2+ ions out of the water and exchange them for other ions which do not cause problems.

One day, while I was sitting in my supervisor’s office and discussed my latest results with him, a visiting professor came in, saw the picture I showed you above, and explained that it was an amazing example of reaction-induced fracturing. At that time, I had never thought of this (cut me some slack, I was just a Master student) and neither had my supervisor (blame him all you want, he is an experienced researcher). The two of us found a lot of things interesting in this picture, but had never noticed the implications it had for fracturing. The visitor did, because he had a different background and had worked on mechanical aspects of geology such as fracturing of rocks and minerals before. He spent more than an hour discussing his theories with me and gave me a lot of exciting ideas.

So the moral is: do not focus too much on one specialty; learn a lot of different things, talk to people from other disciplines, and try to get as many perspectives as possible. If you need any further incentive to talk to other people: the visiting professor who was so interested in the picture back then is now my PhD supervisor in Oslo…

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