Humans have always been fascinated with precious stones, using various forms of crystals since ancient times. These crystalline solids with highly ordered microscopic structures have been at the forefront of fashion and wellness trends. Just recently, a cave was unearthed and found to be home to some of the largest known crystals in the world.
Giant Cave of Crystals
One of the most widely mined crystals is gypsum, a soft sulfate mineral used as fertilizer and a major component of drywall, plaster, blackboard, or sidewalk chalk. It is also used as a food additive with the potential to be a dietary source of calcium.
In 2000, two miners looking for new ore deposits came across an unexpected and awesome sight. They saw towers of massive, milky-white crystals filling a U-shaped cave. The discovery was made after water was pumped out by the mining company they worked for.
The Cave of Crystals, as it became known, is a chamber connected to the Naica Mine at a depth of 980 feet (300 meters) in Chihuahua, Mexico. It takes the form of a tunnel within the limestone host rock of the mine, measuring about 358 feet (109 meters) with a volume of 180,000 to 210,000 cubic feet (5,000 to 6,000 cubic meters).
Also known as the Giant Crystal Cave, the horseshoe-shaped cavern is covered from top to bottom with cloudy-white selenite gypsum crystals of enormous sizes. The tallest of them stands 3.2 feet (1 meter) wide and 37.4 feet (11.4 meters) tall, weighing around 13.2 tons (12 tonnes).
Although the picturesque interior may look like a perfect spot for tourists, authorities closed the Cave of Crystals to the public because the conditions inside the cave are treacherous and deadly. With a stable interior temperature of about 136 degrees Fahrenheit (58 degrees Celsius) and humidity levels between 90 and 99%, spending time inside the cave longer than 10 minutes can cause fluid build-up in a person's lungs and even death.
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How Did the Gigantic Crystals Form?
About 26 million years ago, a pile of magma strained upward beneath the southeastern Chihuahua, Mexico. As the rising magma created the mountain near Naica, it forced hot, mineral-rich water into tunnels and gaps in its limestones. From these waters, the giant crystals of Mexico were born.
The cave was filled with water rich in calcium sulfate. While calcium sulfate can form several forms of crystals, it turns out that gypsum came to be the dominant mineral in the caves. According to crystallographer Alexander Van Driessche from the National Center for Scientific Research's Institute of Earth Science, each mineral has a stability zone. Anhydrite (CaSO4) gets more stable at a temperature above 136.4 degrees Fahrenheit (58 degrees Celsius), while gypsum (CaSO4 ·2H2O) is more stable or less soluble below that temperature.
At first, anhydrite deposits formed in the waters heated by magma, and then the water gradually cooled over thousands of years. As the temperature dropped below 136.4 degrees Fahrenheit, anhydrite started to dissolve, and gypsum crystals began to nucleate and grow.
The dissolving anhydrite provided the subterranean solution with sufficient calcium and sulfate to keep it saturated, creating ideal conditions for slow-growing gypsums. This process was explained in the paper "Formation of natural gypsum megacrystals in Naica, Mexico."
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