An international team of researchers, including Prof. Christoph Spötl from the Institute of Geology at the University of Innsbruck, Austria, has identified a previously unknown mechanism in the formation of the mineral aragonite. Nano-crystalline calcium carbonate was found in a cave in Tyrol, Austria, which provides insights into the formation of the widespread aragonite.
Aragonite occurrences in the Obstans Ice Cave in Tyrol: The surface of this white layer on the cave wall is covered with the newly discovered mAra, the precursor of aragonite.
Prof. Dr. Christoph Spoetl
Whether aragonite or calcite is present – both are forms of calcium carbonate (CaC03,) – depends on finest differences in pressure or dissolved elements. Aragonite is a crystalline form of calcium carbonate and occurs frequently on the earth's surface. However, according to thermodynamics, the more stable form, the mineral calcite, should actually be present under these conditions. It is still not fully understood why aragonite is so common instead of calcite.
Found in an Ice Cave
Scientists from Hungary, Italy and Austria have now discovered that there is a previously unknown nanocrystalline polymorphic phase of calcium carbonate, which plays a decisive role here. This mineral phase was found in the Obstans Ice Cave. It was found that some of these crystals contain significant amounts of magnesium, an element that actually does not fit into the crystal structure of aragonite, says Prof. Christoph Spötl. He emphasises: "Metastable aragonite preferably forms from aqueous solutions with a high magnesium to calcium ratio. Under such conditions, aragonite is formed in the ice cave - despite temperatures just above zero."
A New Type of Crystal
The particularity, however, was not only the magnesium, because in addition a number of electron diffraction characteristics were determined, which also do not belong to aragonite. A new type of crystal was identified in the form of a monoclinic aragonite (mAra), which has a lower symmetry than the aragonite crystallising in the rhombic system.
In mAra, magnesium atoms and hydroxyl groups can be located at the atomic positions where calcium and carbonate should be. There is a layer structure consisting of six units. The stacking order of the mAra units differs from that of the aragonite. It is assumed that mAra is the precursor of the metastable aragonite and does not only occur in this alpine cave. Thanks to the new, highly developed electron microscopes, such discoveries are now possible.
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