geode - whopper unopened geode from Mexico - display specimen
Geodes are bubble fillings that weather out of a vesicular lava. As the flow cools, unescaped gases form voids in the otherwise solid flow. Subsequent hydrothermal activity fills these voids with mineral-rich hot water and as it cools, crystals grow on the surface of the roughly spherical opening. In the lava flow, the immediate zone around the now crystal-lined bubble is altered, becomes more resistant than the surrounding rock and subsequently weathers out. Often they are completely mineral-filled. Some are hollow.
This excellent display specimen clearly illustrates the bubble-in-a-lava-flow story. We don't have locality data other than "Mexico."
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