geode - teaching hand specimen of a hollow geode with a tiny hole giving a hint of the inside
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 has a tiny opening, giving the temptation to see what's inside. We like it for the suspense, and as the quartz crystal lining is often unspectacular, we wouldn't open it. It gets across to students that geodes are bubble-fillings. It looks bubbly. We don't have locality data other than "Mexico."
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