When you squeeze something, it should get smaller — that’s just common sense. You squeeze a sponge to make it smaller, so that water gets pushed out. But why doesn’t the water compress along with the sponge as you squeeze it? Why doesn’t the water stay in the sponge, but instead drips all over the place? It turns out that water is nearly incompressible. Even though water is trapped in the sponge, it doesn’t compress at the same rate, so the water has nowhere to go except out. Even at the bottom of the ocean, water basically has the same physical properties as the water on the surface. It hasn’t hardly compressed at all, even with the tremendous amount of weight pushing down on it. But under the right pressure conditions, pure water will eventually compress, as the water molecules are forced together into smaller and smaller spaces. Freezing water (a liquid) will expand to ice (a solid), a process that involves temperature and a physical phase state change. Water will freeze to ice at room temperature, but only if the pressure is around 145,000 psi (~1 GPa, gigapascal). At sea-level, our atmosphere pushes down on us at 14.7 psi.
To expand under pressure is a whole different story. It would be a strange sensation to squeeze a sponge only to see it get bigger as you put more pressure on it. However, this is what some minerals do when they get squeezed to very high pressures, like 300,000 psi (~2 GPa). In nature, these kinds of pressures are only reached when you get down deep in the Earth. These geologic conditions occur at subduction zones, where rocks (which are made of minerals) are pulled down back into the Earth. Approximately 77 miles down, minerals experience these extreme pressures, but instead of compressing (like most of the minerals such as quartz, olivine, feldspar) certain minerals will expand. There are only a few minerals that we know of that do this. You may think that this breaks the laws of thermodynamics, but there is a catch, and and that catch is chemistry.
The minerals of most interest for physical expansion are the ones that have water in their crystal structures, such as zeolites and clays. These minerals look a little like a sponge, where they have tunnels, tubes, channels, pore spaces, and lots of places for water to find a spot to fit. At this small molecular scale, you can’t really call the water inside the mineral water at all. Water is H2O in the liquid state, just like we call frozen H2O ice when it crystallizes, and vapor when H2O is in the gas state. None of those apply to H2O in minerals, so we simply call it H2O, where it has none of the physical properties of gas, water, or ice.
As these clay and zeolite minerals get squeezed, they do contract initially, but only to a point. Some of them break-apart and transform to a different mineral, and some of them do something (in my opinion) more interesting. From a recent high-pressure study, on which I was a co-author, graduate student Huijeong Hwang from Yonsei University compressed minerals and water in a diamond anvil cell to observe mineral/water interaction at high pressure. Huijeong found that the mineral nacrite, a clay mineral, takes up water and forms an atomic ice-like layer within the crystal structure of the mineral.
So, instead of water forming a chunk of ice minerals at these high pressures, the H2O moves into crystals of accommodating minerals to form a single layer of nearly hexagonal H2O sheets. No thermodynamic laws are broken, even though it seems like it, because the overall mineral chemistry changes, by allowing the H2O molecules to be absorbed. When nacrite absorbs the water, its volume increases by about 20%, which is significant. After the expansion, it continued to compress with the H2O locked up in its structure. That would be similar to the difference in volume increase from an average ‘large’ egg to an ‘extra-large’ egg.
An exciting outcome of this experiment was that when the mineral was released from the high-pressure chamber, it stayed expanded! It didn’t contract to its original state, and the ice-like structure was stuck inside the mineral, which is unlike other clay minerals (for example, kaolinite). We are still not totally sure why this is, could be something to do with the unique crystal symmetry of nacrite. In any case, the nacrite crystal structure stabilizes the H2O, and it is maintained at room temperature and pressure conditions. This means that if you found nacrite in this super-H2O state, then this would mean that the right chemical conditions and pressure/temperature conditions could be similar to a subduction zone. This is a fascinating way to measure if other planets and moons in our solar system used to (or still do) have plate tectonic activity like Earth. In theory, you could just look at the minerals and reconstruct how they got there.
Aaron Celestian is Curator of Mineral Sciences at the Natural History Museum of Los Angeles, Affiliate Research Scientist at NASA Jet Propulsion Lab, and member of the Origins and Habitability Lab. He researches how minerals interact with their environments and with living things, and how those minerals can be used to solve problems like climate change, pollution, and disease.
Hwang, Huijeong, et al. “Pressure-induced hydration and formation of bi-layer ice in nacrite, a kaolin-group clay.” ACS Earth and Space Chemistry (2019). Add a comment below if you would like to have a copy of this paper.
Colligan, Marek, et al. “High-pressure neutron diffraction study of superhydrated natrolite.” The Journal of Physical Chemistry B 109.39 (2005): 18223–18225. Add a comment below if you would like to have a copy of this paper.
You, Shujie, et al. “Pressure‐Induced Water Insertion in Synthetic Clays.” Angewandte Chemie International Edition 52.14 (2013): 3891–3895. This paper is open access.