While we imagine that a crystal is always hard and undeformable, a new study shows that certain minerals, in particular hydrated salts, can become soft on the surface while maintaining their crystalline structure in depth. This behavior takes place when the crystal begins to dissolve.
A crystal is a solid whose atomic structure is ordered and periodic. Here is the current definition. Generally, when we think of crystals, we therefore imagine solids that are hard, non-deformable (under surface conditions) and possess a characteristic geometric shape that reflects the underlying well-ordered molecular structure. The image of quartz crystals is a good representation of what a crystallized form can be.
And yet, scientists have just discovered that certain crystals can behave in a very different way: they could become soft and deformable under certain conditions. We are not talking here about what happens at high pressure and high temperature, in the depths of the Earth’s mantle, but about a phenomenon that is observed at ambient pressure and temperature.
The strange behavior of hydrated salts during dissolution
It was by studying the behavior of certain salts during their dissolution that the researchers observed these astonishing physical modifications. Not just any salt though. It was the hydrated salts that caught the attention of scientists. These salts, such as mirability (Na2SO4·10H2O), contain a large amount of water within their crystal structure. In a dry environment, mirabilite crystals behave quite normally: they have identifiable faces and are not deformable. Things change when they are placed in a humid environment and their dissolution begins slowly. Because unlike crystals of so-called anhydrous salts (without water), which keep their crystalline form and remain hard during dissolution, crystals of mirabilite become soft, deform and lose their geometry.
By studying these crystals, the researchers realized that they behaved simultaneously in two different ways. The core of the crystal indeed retains its basic properties, while its surface behaves like a liquid. On the one hand, the crystalline structure is therefore maintained, while the surfaces are subject to a certain molecular mobility.
Mobile water molecules within the crystal structure
These results, published in Nature Communications, highlight the effect of the presence of water in the crystalline structure of these minerals. The behavior of mirabilite during its slow dissolution would indeed be due to the mobility of water molecules within the crystalline structure. Thus, the surface defects produced by the dissolution phenomenon would be quickly and spontaneously filled in, resulting in some softening of the surface of the crystal.
Hydrated salts, such as mirabilite, are minerals very present in nature. We even see it on Mars. They have long been considered interesting minerals for the storage of thermal energy. Indeed, mirabilite has a high latent heat of fusion, that is to say that when the mineral passes from the solid state to the liquid state at constant temperature and pressure, it absorbs a large amount of heat. However, in order to exploit this thermochemical process, it is essential to control the behavior of the mineral during its dissolution phase. These results should therefore open new perspectives in this field.