Posted on December 14 2016
Crystals must have baffled early humans. They’re some of the few things in nature that look man-made. That is to say that they often come with the kind of flat, smooth surfaces and arrow-straight lines that make them look like they were processed on a machine. Pre-machine Stone Agers would have found them strangely unique and mysterious. Where could they have come from? What unknown power had formed them? No surprise they were held in respect as items of spiritual power and magic. It took boring old geologists and mineralogists and crystallographers to figure out the chemistry and enormous time periods needed to produce most crystals. For my money, the actual facts of their origins make them all the more special and mind-boggling.
We’ve all been to the mineral hall at our local natural history museum to marvel at the highest-quality examples of some of the thousands of different kinds of crystals nature has to offer. For beauty and color, they can’t be beat, and some are surprisingly large. But “large” is a relative term, and fairly recently it took on a whole new level in the world of crystals. The locus of these extra big examples is in the southern central part of Chihuahua state, in Mexico. The town of Naica there is a mining center. Lead, gold, silver, zinc, and other metals have been coming out of Naica Mountain for over two hundred years. In fact, it was in 1910 that miners first made the discovery of what came to be called the Cueva de las Espadas, or the Cave of Swords. The cave took on that moniker because its walls were covered with long, pointed crystals of selenite, up to two meters in length. This cave was only accessible because miners had pumped out the groundwater as part of a process of extending mining operations to a deeper level. The water table normally sits at about 110 m below the surface, and the Cave of Swords is 120 m deep. They discovered the cavity more or less by accident while digging deeper into the limestone mountain, and the world was amazed the size of the crystals they found.
Ninety years later, miners had gone deeper, continuously dewatering to allow progress underground. At 290 m below, they discovered something that was nearly unbelievable. The crystals in this cave, named simply the Cave of Crystals or Cueva de los Cristales, were enormous—commonly 6 m in length. The largest were up to 11m (36 ft!) long and a meter thick, weighing an estimated 55 tons. Miners standing next to them looked like miniature people. Crystals in the cave grow from the floor or ceiling at random angles, in places criss-crossing the whole space with gleaming white pillars. The cave was astounding, but incredibly inhospitable. Temperatures at this depth, fueled by a magma body near the surface, are about 125 degrees F, with up to 100 percent humidity. In these conditions, water vapor will quickly cool and condense upon the coolest surface available, which is your lungs. You could drown just standing there for more than a few minutes. Brief excursions into the chamber, which is about 30 by 90 feet long, could be made by the miners, but greater study required better equipment. Special cold suits packed with ice gels, and respirators that cut the humidity allowed for stays of around a half hour, but the equipment was heavy and cumbersome, and the surfaces within the cave uneven and sharp. The Cave of Crystals, despite its enthralling beauty, is an altogether forbidding place.
But how did it come to be? Since the discovery of the Cave of Swords nearly a century ago, geologists had pondered the conditions that must have existed to allow for such immense crystal growth. Obviously, the caverns had been filled with a mineral-rich solution just as any geode is, with layers of atoms accreting on a surface to grow crystals. But in the case of the caves in Naica Mountain, the anhydrite-saturated waters had to maintain a very narrow temperature range for an enormously long time for the crystals to grow to their gigantic proportions. Anhydrite is a mineral which is stable at temperatures above 136 degrees F. As the heat source below Naica Mountain cooled, the anhydrite dissolved, and its chemicals combined with water molecules and redeposited as selenite gypsum, which is what the giant crystals are made of. For them to have grown so large, the temperature must have remained close to the mineral transition threshold for hundreds of thousands of years. The Cave of Swords had similar conditions, but not for so long, as evidenced by the smaller crystals.
The caves are very delicate in their condition. Now that they are exposed to air, the crystals have begun to deteriorate. Knowing this would happen, researchers went to extraordinary lengths to devise unique equipment to photograph and document this fabulous world. Mechanical devices such as cameras do not do well in high heat and humidity, and many efforts failed. But eventually a film was made (by way of cobbling together thousands of still images) that captures the beauty and rarity of this impossible place. The Cueva de los Cristales is like a dream. It can’t last as an accessible setting. Though it is closed off from the ventilated mining operations, the explorers have noted that crystals in the cave are beginning to react to the air, with some falling from the ceiling and breaking, even as documentary photographers struggle to capture all the beauty. The vibrations caused by the explosions of the mine work don’t help either. This crumbling will continue until the mining operations are done, and the groundwater is no longer being actively pumped out. The cavern will flood again, reintroducing the water that feeds the crystals’ growth. Utterly beyond human observation at that point, the giant slabs of mineral will once again begin their ponderously slow process of becoming even bigger.