While we imagine a crystal is always solid and non-deformable, a new study shows that some minerals, particularly hydrated salts, can become soft on the surface while maintaining their crystal structure at depth. This behavior occurs when the crystal begins to melt.
A crystal is a solid whose atomic structure is both ordered and periodic. Here is the current definition. In general, when we think of crystals, we imagine solids that are rigid, non-deformable (under surface conditions) and have a distinctive geometry that reflects the well-ordered underlying molecular structure. A picture of quartz crystals is a good representation of what the crystalline form can be.
However, scientists have just discovered that some crystals can behave in a completely different way: they can become soft and deformable under certain conditions. We are not talking here about what happens at high pressure and high temperature, deep in the Earth’s mantle, but about a phenomenon observed at ambient pressure and temperature.
Strange behavior of hydrated salts during dissolution
By studying the behavior of some salts as they dissolve, researchers have noted these amazing physical adaptations. Not just any salt though. It was the hydrated salts that caught the attention of scientists. These salts, such as admiration (Na2Unavailable410 h2O) contain a large amount of water within its 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 moist environment and slowly begin to decompose. Because unlike crystals of so-called anhydrous salts (without water), which maintain their crystalline shape and remain solid during dissolution, mirabilite crystals become soft, deform and lose their geometry.
By studying these crystals, the researchers realized that they simultaneously behave in two different ways. The core of the crystal already retains its basic properties, while its surface behaves like a liquid. On the other hand, the crystal structure is preserved, while the surfaces undergo a certain molecular motion.
Mobile water molecules within the crystal structure
These results have been published in Nature CommunicationsHighlighting the influence of the presence of water on the crystal structure of these minerals. The behavior of mirabilite during its slow dissolution will actually be due to the mobility of water molecules within the crystal structure. Thus, the surface defects caused by the atomization phenomenon will be filled rapidly and spontaneously, resulting in some softening of the crystal surface.
Hydrated salts, such as mirabilite, are minerals that are very present in nature. We even see it on Mars. Metals have long been considered interesting for storing thermal energy. In fact, mirabilites have a high latent heat of fusion, that is, when a mineral transitions from a solid to a liquid state at a constant temperature and pressure, it absorbs a significant amount of heat. However, in order to exploit this thermochemical process, it is necessary to control the behavior of the metal during its dissolution phase. Therefore, these results should open new horizons in this field.
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