Wednesday, April 11, 2007

Gypsum megacrystals

Although this isn't exactly a chemistry article, it is most certainly chemistry related, and I hope that you will agree that these pictures are too awesome to believe. The gigantic crystals pictured above made the cover of this month's Geology. Almost 80 years ago, the excavation of caves and tunnels at the Naica mine (112km Southeast of Chihuahua, Mexico) led to the discovery of meter-sized single crystals of selenite, which is one of the four crystal forms of gypsum. (The other three forms are satin spar, desert rose, and gypsum flower. As a side note, when I was younger I had a great collection of rocks and minerals that included a very nice sample of desert rose). Often these crystals of calcium sulfate dihydrate are found coated in calcite (calcium carbonate), celestite (strontium sulfate), or trace amounts of iron oxide, which give the crystals either a white or slightly red hue; selenite is colorless/transparent in its pure form. Amazingly, the Cueva de los Cristales (Cave of Crystals) contains selenite crystals up to 11 meters in length and 1 meter thick, with minimal contamination from other minerals.

While several have made conjectures as to how these crystals formed, none had been investigated carefully until now. Garcia-Ruiz and coworkers set out to explain the formation and growth of the Naica megacrystals after closely considering several factors. First, gypsum is slightly soluble in water, with a maximal solubility observed at 58 degrees C; conveniently, water samples from the Naica mines have temperatures ranging from 48-59 degrees C. Thus, the water found in the area is slightly supersaturated for gypsum and slightly undersaturated for the anhydrite form of calcium sulfate, suggesting a self-feeding mechanism. In other words, crystal growth might have been driven by a solution controlled anhydrite-gypsum phase transition. Calculation of the nucleation rate indicated that this suggested mechanism is a probable one, but only within a very narrow range of temperatures--46 to 60 degrees C. Such calculations indicate that these crystals have been growing in the caves at Naica for over one million years!

For more information:

The Largest Crystals on Earth

More pictures



12 comments:

Ψ*Ψ said...

Your post gives me crystal envy. I suck at growing crystals!

Christoph said...

Those pictures really look like from a science fiction movie, like computer manipulated. And another thought: Imagine to search for a crystal small enough for the diffractometer ;-)

Liquidcarbon said...

How's that - gypsum-anhydrite equilibrium in solution? anhydrite is anhydrous calcium sulfate? Ok, Wikipedia says " when the solution contains an excess of sodium or potassium chloride anhydrite is deposited if temperature is above 40°C", but there is no gypsum or anhydrite in solution, it's dissociated calcium sulfate!

Amanda said...

Liquidcarbon--thanks for that point. Neither my post nor the original article are very clear on that fact. Both gypsum and the anhydrite form are dissociated calcium sulfate in solution. Maybe it makes more sense to say that the close-to-equilibrium dissolution of anhydrite coupled with the close-to equilibrium crystallization of gypsum fueled the growth of the Naica mine crystals.

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