In a paper published in the journal Geology, Dr. Juan Manuel García-Ruiz from the Universidad de Granada and colleagues reveal the geological history that ended with the formation of the famous Geode of Pulpí in Spain.
Discovered in in 1999, the Geode of Pulpí is an 11 m3 ovoid in the abandoned Mina Rica (the Rich Mine) in Almería, southeast Spain.
Its walls are covered with large (about 0.5 m), blocky gypsum crystals of great transparency.
“To reveal their formation has been a very tough task because unlike in the case of the giant crystals of Naica in Mexico, where the hydrothermal system is still active, the large Geode of Pulpí is a fossilized environment,” Dr. García-Ruiz said.
He and his co-authors performed a study of the geology and geochemistry of the abandoned Mina Rica mine, including a detailed mapping of the underground mining works, which has been used to allow the tourist visits in the mine.
They found that the crystals of Pulpi formed at around 20 degrees Celsius (68 dgrees Fahrenheit), 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 degrees Celsius, or 104 degrees Fahrenheit), led to the dissolution and recrystallization amplifying a maturation process that is known as Ostwald maturation.
“This is somewhat like the temperature cycles in crystal quality control in industrial processes,” Dr. García-Ruiz said.
“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 Naica crystals.”
Because of their purity, the crystals forming the geode cannot be dated precisely, but indirect constraint can be done.
“They grew for sure after the desiccation of the Mediterranean Sea that occurred 5.6 million years ago,” Dr. García-Ruiz said.
“They are most probably younger than two million years but older than 60.000 years because this is the age of the carbonate crust coating one of the large gypsum crystal.”
A. Canals et al. The origin of large gypsum crystals in the Geode of Pulpí (Almería, Spain). Geology, published online October 15, 2019; doi: 10.1130/G46734.1
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