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The case of giant crystals

In 1910 beneath the desert near the Mexican town of Naica miners stumbled upon a cave coated with millions of sword-like crystals. Every cm2 of the walls, floors and ceilings are covered with up to 2m long crystals. The world was amazed, and scientists flocked to see them and ponder how they had come to be. But the best was yet to come. In 2000, some of the largest crystals ever seen in nature – massive columns some more than 11m long and 1m wide – were found in another cave deeper into the same mine. These criss-crossing crystalline beams are as confusing as they are beautiful. Not only are they almost unimaginably huge but – in stark contrast to the shallower cave – there are only a few hundred crystals and the walls, floors and ceilings are mostly bare.

The ongoing quest to explain how and why these giant crystals grew is outlined in the second volume of the Handbook of Crystal Growth, published by Elsevier and available on ScienceDirect.

The mountainous area around the mine is rich in minerals pushed into the soil by the hot magna that formed beneath this region millions of years ago. The area has been mined for its reserves of lead, gold and zinc since the 19th century, but it’s the mineral anhydrite (anhydrous calcium sulfate, CaSO4) − combined with other rare geological conditions at Naica − that is responsible for the formation of the gypsum (calcium sulfate dehydrate, CaSO.2H2O) crystals.

Juan Manuel García-Ruiz at the CSIC/University of Granada, Spain, heads one of the teams investigating the formation of these crystals. “Several geological conditions have been fulfilled to form the giant crystals, among them the existence of both anhydrite and gypsum with reverse solubility versus temperature, a strong temperature gradient and very slow cooling of the mountain for thousands of years,” he explains.

The caves were flooded until the 1950s, when they were pumped empty for mining purposes. As the magna − and therefore the water in the caves − slowly cooled over the millennia, the water was unable to hold as much calcium sulfate (dissolved from anhydrite) meaning some of it crystallized out. García-Ruiz and his team proposed that as this happened the CaSO4 incorporated water molecules to precipitate as CaSO.2H2O. “We used isotopic analysis to prove our hypothesis that anhydrite dissolves to grow the crystals of gypsum,” explains García-Ruiz.

His team then proved that the reason the crystals grew so large in the deeper cave was because they were being continuously fed very small amounts of gypsum salt, meaning their growth was steady and very slow. They collected tiny crystals of gypsum and water from the mine, and reconstructed the temperature conditions of the cave in their lab. We used advanced confocal microscopy to measure the very slow growth rates of the crystals, García-Ruiz says. They found that they grow at a rate of approximately 60μm per century. “These crystals were growing for hundreds of thousands of years.” If the mine was to be re-flooded – a possibility once mining activities in the region finally ceases – the crystals would continue to grow again.

The very small amount of gypsum salt present in the water at any one time in these very low supersaturation conditions also explains why so few crystals formed. In the shallower cave, there are many more crystals because the water cooled far faster – due to mixing with surface water – meaning much more gypsum crystallized out and nucleation occurred more readily.

There are still questions about these giant crystals such as exactly how old they are and why the morphology of the crystals is so different from the form that is theoretically predicted. To answer these many more trips to the caves will be necessary, especially as each visit is very limited in time. “The temperature of the cave is about 46 ºC and the humidity almost 100%,” explains García-Ruiz. “Every time we enter the cave to study the crystals we lose almost 2kg of water during the brief 10 minutes we are able to stay inside.”

The second volume of the Handbook of Crystal Growth, contains more information on the mechanisms and dynamics of bulk crystal growth.

Fuente: Materials Today