"The key point is that anhydrite is stable above 58 degrees Celsius (136 degrees F), while below that temperature gypsum is the stable form," said García-Ruiz. "Therefore until the temperature was kept above 58 degrees C anhydrite was stable."
Below that temperature, the anhydrite dissolves and the gypsum crystals started to form. That that’s not the entire recipe for giant crystals, he said.
"To grow very large crystals it is required to form only very few nuclei (‘seed’ crystals) and to feed them continuously with very small amount of calcium sulfate for extremely long times," said García-Ruiz, "something that is provided by the small but continuous dissolution of anhydrite in the mine."
So the trick is having just enough — but not too much — calcium sulfate in warm water that’s cooling — but not cooling too fast.
"For these huge crystals to form, it is mandatory that the cave environment never cool below 45 degrees (113 degrees F), according to our calculation," García-Ruiz told Discovery News.
"Wow! These are essentially single crystals," remarked cave geologist Laurence Davis of the University of New Haven, in Connecticut. He had not heard of the giant crystals until reading the Geology paper.
As for the science behind García-Ruiz’s recipe, "This is pretty standard cave stuff," Davis said. "It sounds pretty good to me."
