The water carried these chemicals into the cavities, where the amethyst and other minerals grew due to cooling and pressure release. Glass breaks down over geologic time and makes silica and other chemicals available in a form that is readily soluble in water soaking through the rocks. In the lava flow these waters would have found volcanic glass. Uplift and tilting of the area about 70 million years ago would force water out of the aquifer into the porous areas of the overlying lava. Below the lavas is a large aquifer (the Botucatu aquifer) filled with ground water that closely resembles the fluid inclusion liquids. Trace element data from the fluid inclusions gives another important clue to the source of the mineral-forming fluid. For 60 million years these enormous cavities sat empty of crystals. The basalts formed about 130 million years ago, but the green celadonite, which makes up the rinds of the geodes, formed about 70 million years ago. What was the source of these fluids? An amazing story unfolds from the radiometric dating of the minerals. These cannot be fluids related to the magma that formed the lavas. This water had a temperature of no more than 100 degrees C, and possible less than 50 degrees C, during mineral formation. Analyses of the fluid inclusions in the amethyst, calcite and gypsum show them to be filled with slightly salty water. Fluid inclusions are treasure troves of information when studied with sophisticated instruments. These are samples of the mineral-forming liquids caught as the crystals grew. An important clue to this event is the presence of small gas and liquid bubbles (called fluid inclusions) trapped within these minerals. The second stage was the formation of the amethyst, plus celadonite, calcite and gypsum fillings. These cavities, though, were empty of crystals. The bulbous to tubular shapes thus point towards the top of the flow, a fact easily seen when the geodes are in place in the mines. The lava was cooling fast too, and soon became so thick and sticky that bubbles quite rising and were trapped. As gas bubbles emerged from the congealing lava (much as bubbles emerge when beer or soda pop is poured) they coalesced as they rose. Not every lava has enough dissolved gas to form such big openings. This was caused as volcanic gases were released from certain lavas as they cooled. In the first stage the large hollows form. proposed a 2-stage model for their formation. Of all these flows, however, only a few are known to host amethyst cathedral geodes. The lava outburst occurred as part of the opening of the South Atlantic Ocean during Cretaceous time about 130 million years ago. For comparison, this would be enough to cover Minnesota with a pile of basalt lava over 2 miles high. An estimated 800,000 cubic kilometers of lava extruded over an 11 million year time span. This was one of the largest outpourings of basalt lava known. The geodes are mined from several lava flows belonging to the Parana Continental Flood Basalt Province. Albert Gilg of Techniche University Munchen in Germany (Gilg, et. The genesis of deposits on the Brazil side of the border has recently been extensively researched by an international team of geochemists lead by H. These excellent geodes come from a region along the Brazil-Uruguay border. Have you ever been to a show and seen enormous amethyst geodes or crystals 3-5 feet or more in height? The tubular geodes are lined with deep purple gemmy amethyst crystals. Bill Cordua, University of Wisconsin-River Falls
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