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Gergovie 's plateau 15 in 20 million years ago, in the Miocene, a casting of basalt arrived in a crater lake at the place of the current plateau of Gergovie ( 63 ) Warm waters circulated in the cracks of the basalt and a change of the glass, the plagioclases, pyroxenes and olivines occurred. Waters were charged then in calcium and little in magnesium. This calcium settled by haste in cracks, in the form of carbonate of calcium. The calcite ( rhomboedric ) and the aragonite ( orthorhombic ) are 2 crystalline forms of the carbonate of calcium. The calcite is the stable form at low pressure (in surface). The aragonite is the stable stape in high pressure ( strong depth). It is for it that the immense majority of limestones are calcite and not aragonite. " In spite of this thermodynamic instability of surface, some concentrations in ions (Mg ++, Sr ++) or the action of the life can favor the growth of the crystals of aragonite, in depends of calcite. Corals for example make a test of aragonite; certain springs . It forms then crystals in the form of needles, often often deposited in bundles or radiant sheaves (called "sea urchins") in the opened cracks or to the emergence of the spring."(1) The aragonite is a metastable shape and the fate of these crystals fibro - crossed off is to be slowly transformed there calcite. The aragonite of Gergovie is thus brought to disappear in some million years. (1)text from Benoît URGELLI's work http://www.ens-lyon.fr/Planet-Terre/Hebdo/Images/pages/Img-11-2003.html     Product Name+   Price      Aragonite & Calcite   40.00€      Aragonite & Calcite   25.00€      Aragonite & Calcite   40.00€      Aragonite & Calcite   20.00€      Aragonite & Calcite   30.00€      Aragonite & Calcite   20.00€      Aragonite & Calcite   27.00€      Aragonite & Calcite   20.00€      Aragonite & Calcite   35.00€      Aragonite & Calcite   45.00€      Aragonite & Calcite   180.00€  Displaying 1 to 11 (of 11 products) Result Pages:  1

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