|Location:||South of the Iguazu Falls, on the road to the historical Jesuit mission of San Ignacio Mini, Misiones|
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An enormous basaltic formation was created by the eruption of a volcanoe about 150Ma ago. The layer covers part of the Misiones province in Argentina, part of the artigas department in Uruguay, part of south Brazil and Paraguay. The specialty of this basaltic rock are very big holes, which were formed by gases like steam and carbon dioxide. The gasses were exhaling from the still liquid lava, forming gas bubbles, and when the lava then became solid the bubbles were holes in the rock. The ground water in the basalt transported minerals, mostly quartz (SiO2). The minerals were redeposited in the bubbles in form of crystals, mostly amethyste.
The erosion of the basalt under tropical conditions is a chemical decomposition which produces red clay. This is the residual of the rock, clays with thr red colour of oxidized iron. But the minerals of the gas bubbles are more resistant and not easily destroyed, so they are found almost unharmed in the clay. The rounded concretions are called geodes, which means a hollow rock with a crystal-lined interior.
The geodes contain mostly amethyste, which is a violet variety of quartz. Much less frequent but still common are topazes, agates, jaspers and rock crystals, which are actually all varieties of quartz with different colours.
The Minera Wanda (Mines of Wanda) are world famous gemstone mines. The amount and size of the geodes is extraordinary. According to legend the mines were named afer a Polish princess Wanda. She was washing her cloth at the river, when she suddenly cut her feet at the rocks on the ground. She told her men to smash the sharp rocks, when they discovered the crystals. Actually this story is rather weird, what should a Polish princess do in Argentinia, why should a princess wash her own cloth? However, there is some poetic truth in the red colour of the clay, which actually looks like blood in the water.
The geodes were first found in the soft red clay. They are easily extracted, but unfortunately they are also often damaged by erosion. The better quality geodes are found in the rock which is mined in a huge open cast. Several tunnels, which are wrongly called caves, were built to follow the most profitable veins of geodes.
The mines are visited by organized day trips, which generally also include the nearby historical Jesuit mission of San Ignacio or the world famous Iguazu Falls. The visitors are guided by local guides which explain the mining and geologic background. As this is a working mine, the mining goes on while the groups visit the mine, including the occasional blast. There is a shop selling the minerals.