Chalk crystallizes differently from previously believed

January 25th, 2009 - 12:27 pm ICT by ANI  

Berlin, Jan 25 (ANI): In a new research, scientists at the Max Planck Institute of Colloids and Interfaces in Germany have determined that chalk crystallizes differently from previously believed. The researchers investigated the crystallization of calcium carbonate, known commonly as chalk, and found that stable nanoclusters form in water with a small quantity of dissolved calcium carbonate, which is not how it was assumed to happen in the past. The lime scale deposits are created from these tiny chalk particles. Previously, it was also an unknown fact that the structure of crystallized calcium carbonate depends on the alkalinity of the solution. These new findings might provide help in coping with the lime scale in washing machines, as well as help to explain the sophisticated structure of biominerals, and to better understand the role of the oceans as carbon dioxide (CO2) sinks. Calcium carbonate is the main constituent of marble, dolomite and many types of sediment, and it is also found in the shells of crabs, mussels, snails, sea urchins and in single-celled organisms. These biomaterials have properties that make them interesting for applications in medicine and building materials technology. The ingenious structure of their crystals at nanoscopic level makes them particularly robust. Researchers at the Max Planck Institute in Golm near Potsdam have now made a contribution to achieving this aim by demonstrating that calcium carbonate crystals are created differently from the way they were previously thought to form. When calcium and carbonate ions come together in a solution, they form stable nanoclusters consisting of around 70 calcium and carbonate ions, and they do that even in very soft water, a dilute solution from which chalk does not normally precipitate. If the concentration of dissolved calcium carbonate is increased, the clusters clump together and the mineral crystallizes. “It seems that it is already decided when the clusters form, which of the three anhydrous crystal structures calcium carbonate will assume,” said Helmut Colfen who headed the study. “We also observed that the crystal structure depends on the pH level,” he added. The pH level indicates the acidity or alkalinity of a solution. Under low alkaline conditions, calcium carbonate forms calcite, its most stable crystalline structure. In a more alkaline environment, it creates vaterite, a non-stable crystalline structure. The newly proposed mechanism of crystallization also has consequences for technology. “The stable clusters offer a new point at which to tackle lime scale deposits - not only in washing machines and dish washers, but also in industry,” said Colfen. (ANI)

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