When nature plays Tetris to shape the composition of minerals

Who has never been surprised at the perfection of quartz crystals or such a neat cubic shape of pyrite crystals? The completeness of some crystals may suggest that the crystalline formation is governed by a flawless organization of atoms. A representation of absolute order. But this perfection is only apparent.

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[EN VIDÉO] crystal formation at the atomic scale
A salt crystal is filmed during its formation at a resolution never reached before: atoms! © 2021 American Chemical Society

It is generally believed that the crystallization process leads to the formation of Minerals is governed by a very systematic mechanism that can be planned as the addition of k blocks Construction Similar. The way these basic blocks are made up ofanion from or moleculeswill define crystal lattice, the true signature of a mineral, This stacking is not done randomly, it is a mechanism governed by rules chemistry and conversation between atoms and molecules. However, an increasing number of laboratory experiments suggest that this classical model of crystallization is not so simple and that other crystallization routes are possible in nature.

a crystallization far from perfect

A team of researchers led by Tomasz Stawski Federal Institute for Materials Research and Testing Closely studied anhydrite crystals from Berlin Naika Mine in Mexico, famous for its crystals gypsum Giants, some up to 11 meters long. The studied anhydrite crystals have the advantage of growing silently, especially under stable conditions and close to equilibrium. For millennia, therefore, there have been no obstacles to constrain their growth, which makes them particularly interesting examples for the study of crystallization mechanisms in natural environments.

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The crystal structure of these anhydrite crystals has been investigated in the laboratory using various microscopy techniques and analyses. Spreading From X-ray, results published in the journal PNAS, show that these crystals are far from perfect and that their structure is characterized by presence of defects Alignment at the nanometric scale, that is, of atoms. These defects, no matter how small, are far from trivial, as their presence within the crystal structure will have an effect on multiple length scales. Therefore, ultimately the crystal of centimeter size obtained will not be complete but will be made up of vacancies of different sizes. We are talking about mesostructured crystals, whose crystal structure, imperfect, is unique to each crystal.

a failed tetris game

to understand it crystallization process, we can imagine a basic block made up of a combination of atoms. Unlike the classic scheme in which the blocks are stacked perfectly, here the blocks will not fit together perfectly, creating points, lines, or even fault planes. If the amount and extent of these defects at the atomic scale are sufficient to form different crystal domains (we speak of grains), then Stuff Considered to be polycrystalline. When no further ordering is observed, it is said to be amorphous.

The crystals studied by the team of researchers do not clearly belong to any of these categories, but have characteristics common to these different groups. Some measurements indicate that the structure is that of a single crystal, others show a polycrystalline signature, and others show the presence of an amorphous phase. Therefore anhydrite crystals are called mesocrysts. This crystalline architecture comes from these small alignment defects that occur during the stacking of basic blocks.

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To schematize their concept, the authors give examples of nature to play the construction game Tetris, without succeeding in achieving the goal of achieving an ideal stacking from small basic bricks.

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