Most geodes—hollow, crystal-lined rocks—can suit the palm of your hand. However the huge Pulpí Geode, which has to do with half the size of a small bedroom, fills part of a deserted mine in southeastern Spain. Now, scientists have actually evaluated some of its crystals to determine its age—and how this real-life Fortress of Solitude came to be so huge.
The 11-cubic-meter geode—the largest in the world, the scientists state—was found in 1999, in a long-closed mine near its name town. Some of the crystals are a number of meters long and are so pure that they’re transparent, in spite of their density.
Although the geode is embedded in rocks that have to do with 250 million years of ages, the crystals themselves are much more youthful than that. Radioactive dating of some of the earliest recommends they formed less than 5.6 million years ago however probably no more than 2 million years ago, the scientists report today in Geology.
Although the cavern is now dry, when the geode was growing, its cavity was filled with hot, mineral-rich water. The earliest layer of crystals, that include the mineral barite (barium sulfate), formed at temperature levels of about 100°C. Subsequent layers, that include crystals of celestine (strontium sulfate), grew in waters someplace around 70°C. The youngest crystals of plaster (hydrated calcium sulfate) formed at temperature levels of about 20°C a minimum of 60,000 years earlier—well prior to the coldest part of the last glacial epoch.