geode - teaching hand specimen of a typical geode
Geodes are bubble fillings that weather out of a vesicular basalt. As the basalt 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 void. In the basalt, the immediate zone around the now crystal lined bubble is altered and becomes more resistant than the basalt of the flow. These mineral lined geodes weather out of the basalt. Often they are completely mineral-filled. Others are crystal-lined.
This good-sized geode is best left unopened, as the quartz crystal lining is often unspectacular, and these get across to students that they are bubble-fillings. They look bubbly. This one has a flat side where it was joined to another as a double-bubble filling. We don't have locality data other than "Mexico."
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