A quick post about some quirky geology today.
Geodes are crystal-filled masses inside a larger body of rock. They are awesome. Scientists generally know how they form. Voids or bubbles in volcanic or sedimentary rock (particularly basalt/lava) were inundated with hot, mineral rich, fluids circulating in the ground. Crystals grew from the fluids – like rock candy grows on a string in a saturated sugar solution.
Geodes are typically small. Quartz is the most common crystals that grow in geodes. My grandfather brought me a round geode he got in Alaska that he called a thunderstone. I had a geology professor use a rock saw to cut it open to reveal a crystal bridge inside.
But some geodes can be massive. I recall seeing, in person, at a rock shop in Trump Tower in NYC in the 90s, an elongated amethyst geode several feet high. Amethyst is quartz that has atoms of iron substituted in the crystal lattice or has been exposed to radiation. I could not imagine how this massive cluster of crystals could have formed. Its price reflected its rarity. Giant geodes like this are typically from Brazil and Uruguay.
The most interesting amethyst geode I ever heard about was a heart shaped crystal cluster that was found by miners near the border of Uruguay and Brazil, where the geodes began forming over 130 million years ago. The rock was split to reveal two sides of the geode that had a clear quartz edge and a deep purple center.
Come on! That’s pretty cool!
Amethyst is a key economic resource in northern Uruguay where it has been mined for over 150 years. This area is recognized as a geological heritage site because of the amethyst here has such a deep violet color and high quality, as well as the giant size of some geodes that can be several meters in length. Even though scientists know generally how the geodes are formed, a recent study in this area of Uruguay suggests additional details. The scientists studied the fluids in the geodes and in the surrounding rocks and determined that the amethysts crystallized at low temperatures from groundwater-like fluids. This may help discover new areas of exploration.