Both mechanochemistry and abiogenesis are each pretty deep subjects, and this essay will not due either of them justice. This is just a nudge to the curious out there.

I follow a Facebook page called “Abiogenesis”. It usually posts links to the primary literature and recently to a very intriguing paper. It is titled Mineral-mediated carbohydrate synthesis by mechanical forces in a primordial geochemical setting, The link is –https://doi.org/10.1038/s42004-020-00387-w. and is published by Nature.

Should the unthinkable arise and you, the gentle reader, are unfamiliar with mechanochemistry, I would suggest a free ACS pdf download (Free! Will yah look at that!) with some basics. Ms Google can offer many more links. This was new to me but not to the light of day.

Sorting out abiogenesis is difficult enough in the solution phase, but these workers have included geological surfaces into the mix. And directed to carbohydrates no less. It is ambitious but the field needs ambitious experimental work.

My guess is that most chemists coming through the undergraduate ACS core curriculum are as unfamiliar with mechanochemistry as am I. A chemistry or geochemistry faculty member involved in it would be needed to teach an elective for it. I’ll hazard a guess that mechanochemists are somewhat scarce on most chemistry faculty rosters at present. Geology departments offer geochemistry courses, but it is unlikely to overlap with the chemistry department. How often do chemistry faculty mix (or party) with the geology department? I know that before I began to study it, I dismissed geochem as boring dirty water chemistry. But, as I am finding out, the dirty water is quite interesting.

Oh to be a geologist, merrily tramping around in the weeds and the sunshine, swinging a rock hammer and collecting pretty rocks with a legitimate purpose.

Mechanochemistry definitely expands one’s horizons in chemistry. But what is mechanochemistry? The Wikipedia link above does a fair description of it. The thing is, rocks are frozen mixtures of minerals, usually with very low water permeability over short time spans and at sometimes at temperatures we ordinary chemists are unaccustomed to working with.

An analogy

I trained new employees in electrostatic discharge (ESD) safety at our chemical plant. We look at the friction of two surfaces in physical contact sliding past one another. It is easiest to think about dissimilar substances in this picture. As contacting surfaces slide, the surface electrons have the opportunity to stay put or jump to the other, more attractive surface. Electrons migrating away from a nonconducting or electrically isolated conductor surface leave positive charge behind. This is called the triboelectric effect. Furthermore, such mechanical friction generates heat as we all know, especially when one or both surfaces have irregular surface topography such as bumps, crystalline or amorphous protuberances and ridges at the submicroscopic scale. Friction is magnified when external forces concentrate at regions of pronounced submicroscopic topographies, meaning where the lumpy, edgy or mountainous features move past one another.

Have you ever generated a spark when two objects make a glancing blow? A good example would be using a flint and steel to light a campfire. The sparks come from small flakes of metal catching fire in the air. The small flying pieces are very hot but are exhausted rapidly. Cigarette lighters have been doing this for decades by rolling a fixed roughened wheel across a flint. The sparks generated are white hot as judged by their momentary brightness.

Well, that’s nice but what is the connection to mechanochemistry? Generating incandescent bits of metal is an example of how a great deal of energy can be applied to a very small solid surface feature. Now let’s back off the energy a bit and consider the rocks in a rock polisher. A rock polisher works by prolonged tumbling of rocks with small bits of very hard material like carborundum or cerium oxide abrasives. As the rocks tumble, they collide with one another. If during the collision there are abrasive particles at the point of contact, a small amount of chipping of both rocks might occur. The surface chipping will plateau at a particular surface smoothness depending on the size of the abrasive particle. Smaller and smaller abrasive particles will produce finer roughness until such point as humans might regard the rock as “smooth or polished.”

We’ve seen that fracturing of a rock surface can occur with an input of mechanical energy over an extended period of time. Highly localized individual surface features might be subject to mechanical energy input that is high in magnitude. In polishing, mechanical energy has disrupted the molecular structure of the surface matrix of the mineral. Fresh surfaces of rock are free to adsorb water or metal ions may swap anions if the atomic radii are similar and the ionic charge is the same. In swapping metals of like charge or just opening a molecular or atomic coordination site, a chemical transformation has happened. The chemical identity of a mineral structure at newly opened surfaces are different from those that have been weathered. This is because water can coordinate and alter the surface composition as a hydrate to begin with. The interior may be anhydrous, but the new surface becomes hydrated. Anhydrous substances are slightly different from their hydrated form. Hydration has several levels- coordinated water bou8nd to a metal ion, water of crystallization within the crystal lattice but not coordinated, or discrete water between crystals. The word “hydration” itself is not very specific, but the context may be written to infer more precise meaning.

Some organic chemicals like explosives are shock sensitive. A good example is nitroglycerine. Shock sensitivity is the application of mechanical energy to the substance, in the case of nitroglycerine, undergoing a chemical transformation begins with the breaking of the weakest covalent bond. This leads to a rapid cascade of transformations and the evolution of hot gases. Oh, and a shock wave too. Notably, these rapidly evolving hot gases occupy more space that did the liquid nitroglycerine, famously doing pressure-volume work on the surroundings.

This long, drawn out explanation leads to a point- Chemical change can and does occur at mineral surfaces. The application of mechanical forces to a solid can result in contact surfaces receiving a large input of mechanical energy. There are mechanical and chemical consequences as well as triboelectrical effects. In the case of a flint and steel, combustion can occur if just momentarily.

Back to Abiogenesis-

Minerals can be altered chemically somewhat through impact and sliding friction. The mineral itself can be altered but more to the point, if prebiotic substances are present near the contact they might be subject to the energy input which can manifest as localized and momentary heating.

Let’s not forget the mineral surfaces themselves. A given mineral may be an ionic substance where the ions are locked into a lattice. Furthermore, the distinct crystallographic surfaces may be chemically reactive in their interactions with the environment.

This is as far as I’m willing to go down this dendritic Google hole. More to follow. Have a good day!