The Mystery of the Giant Crystals: How the 36-foot Geode of Pulpí Formed

The geological history that led to the formation of giant crystals and a 36-foot geode has been revealed by scientists. By looking at the abandoned mine where the geode of Pulpí was found, they were able to show the conditions it formed in hundreds of thousands of years ago.

The geode of Pulpí was discovered in 1999 in the Mina Rica silver mine in southeast Spain. The walls of the cave are covered with huge crystals that are so transparent they look like blocks of ice. It is thought that the geode of Pulpi is one of the largest in the world, with the interior measuring about 390 cubic feet.

Huge crystal caves have been discovered across the globe in recent decades, including the El Teniente mine in Chile and, most famously, the Cave of Crystals in Naica, Mexico. Explaining how these geological features formed, however, is difficult—they grow very slowly and, in most cases, the systems in which they developed are no longer active.

"To reveal their formation has been a very tough task because unlike in the case of Naica, where the hydrothermal system is still active, the large geode of Pulpí is a fossilized environment," Juan Manuel García-Ruiz, from Spain's Higher Council for Scientific Research (CSIC)," said in a statement.

García-Ruiz and colleagues were hoping to find out how the geode of Pulpí grew by creating geological maps of the area and performing mineralogical and geochemical analysis to work out the geological history that led to its formation. Their findings have been published in Geology.

In previous research, scientists showed how the Naica crystals were gypsum. This is an evaporite mineral composed of calcium sulfate with two water molecules. It is commonly found in layered sedimentary deposits, according to

The Naica crystals grew in a low-salinity solution in temperatures around 54 degrees Celsius, forming through a "self-feeding mechanism," where the evaporite mineral anhydrite—which often forms where large amounts of sea water are evaporated—changed to gypsum after being exposed to water.

The geode of Pulpí.
The geode of Pulpí is thought to have formed between 600,000 and 2 million years ago. Hector Garrido

Exactly when the geode of Pulpí formed is not known, but it was at some point after the Mediterranean Sea dried up 5.6 million years ago. Best estimates currently put the geode of Pulpí at between 600,000 and 2 million years old.

In their study, García-Ruiz and the team discovered the Pulpi megacrystals formed at around 20 degrees Celsius, at a depth where temperature fluctuations were lower than the maximum solubility of gypsum. This, they found, led to the dissolution and recrystallization that would allow the crystals to grow.

The growth is similar to the formation of the Naica crystals, the team notes. They also say the geode of Pulpí formed when it was closer to the surface and would have been more sensitive to changes in global temperatures. Climate change, the study says, "episodically contributed by a ripening process enhanced by temperature oscillations."