An international team made of Hungarian, Austrian and Italian researchers discovered a new natural form of CaCO3, a well-known formula already associated with three polymorphic minerals: calcite, aragonite and vaterite. The new form was sampled in the first precipitates of percolating water inside a dolomitic Alpine cave, where it appears as sub-micrometric needles that naturally turn into conventional aragonite with time. Enrico Mugnaioli and Mauro Gemmi, of the Center for Nanotechnology Innovation@NEST (Istituto Italiano di Tecnologia), were able to unveil the atomic structure of these tiny crystals using 3D electron diffraction, a cutting-edge technique that allows structure determination of crystals too small for conventional X-ray methods. The structure of the new polymorph can be imagined as a modulated sequence of conventional aragonite layers, from which derived its name monoclinic-aragonite (mAra). mAra results to be a straight precursor of common aragonite explaining why this mineral, that should otherwise not form on Earth surface conditions, is so common among the carbonates playing a major role in bio-mineralizations.
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