The Crystal Cave

The city of Naica sits on a fault that is above a magma chamber (shown below)

The origin of the Naica mines is from a volcanic eruption that happened approximately 26 million years ago and is what created the Naica mountain

The mountain was filled with the mineral anhydrite (the form of gypsum that lacks water)

The crystal structure of this mineral is orthorhombic and is made up of CaSO4 

The mineral anhydrite is stable in temperatures above 58 degrees Celsius, going any lower the stable form is gypsum

When the magma below the mountain of Naica cooled, this caused the temperature to decrease

Since anhydrite was no longer stable, it began to dissolve and left the water in the mountain enriched with calcium and sulfate molecules

The elemental make up of the mineral gypsum is CaSO4 2H2O

Over five hundred-thousand years the water solution has dripped to form the crystal in the cave

which are selenite gypsum crystals

Because of this unique situation there is no other place on earth that has crystals like this, so the area is used more for research than actual mining