The giant geode of Pulpi

The geode of Pulpi is an 11-meter hollow ovoid with crystal-paneled walls. It is like those familiar couplets of stone interiors covered with bright crystallites, but so large that several people can fit inside. The crystals, of up to two meters in size, are so transparent that they look like ice crystals. In this paper for Geology, Juan Manuel Garcia-Ruiz and colleagues reveal the geological history that ended with the formation of the Pulpi geode.

The geode of Pulpi [Credit: Hector Garrido]

Like the giant crystals of Naica in Mexico (see the 2007 Geology article by Garcia-Ruiz and colleagues), the crystals of Pulpi are gypsum (calcium sulfate with two water molecules). Garcia-Ruiz says, "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 Pulpi is a fossilized environment."

The team performed a study of the geology and geochemistry of the abandoned mine where the geode was found, including a detailed mapping of the underground mining works, which has been used to allow the tourist visits in the mine.

The Mina Rica in the Sierra del Aguilon, Pulpi, Almeria
[Credit: Javier Trueba]

They found that the crystals of Pulpi formed at around 20 °C, at a shallow depth where the temperature fluctuations of the climate are still perceptible. These temperature fluctuations, being below the maximum solubility of gypsum (40 °C), led to the dissolution and recrystallization amplifying a maturation process that is known as Ostwald maturation.

Says Garcia-Ruiz, "This is somewhat like the temperature cycles in crystal quality control in industrial processes." A continuous supply of salt for the formation of the crystals was provided by the dissolution of anhydrite (the anhydrous form of calcium sulfate), the mechanism accounting for the formation of the large crystals of Naica.