Science Scoops: Expansive Crystals

Who would argue? When you place a solid in a vise and increase the pressure, it should compress, right? Well, an international team of scientists claims that they have discovered a crystal formation that appears to do just the opposite: It appears to expand under pressure.

Meet natrolite—a crystal whose three-dimensional structure contains regularly spaced pores. Yongjae Lee at Brookhaven National Laboratory in Upton, NY, and his colleagues discovered this counterintuitive behavior. They placed a natrolite crystal in a cell filled with a water-alcohol mixture and then squeezed it between two diamond anvils to pressures up to 50,000 times normal atmospheric pressure.

Actually, the crystal initially compressed as expected. But when the pressure ranged between 8,000 and 15,000 times atmospheric pressure, the crystal appeared to expand. As the pressure increased further, however, the material compressed again.

Was the expansion some kind of illusion? No. An X-ray analysis of the expansion suggests that the material expanded because water molecules from the water-alcohol solution were squeezed into the pores within the natrolite, causing it to temporarily expand.

The researchers have already come up with a use for natrolite, namely as a crystal sponge for chemical cleanups. You see, when natrolite expands, so too do its pores. So natrolite can absorb pollutants when it's under pressure. By simply releasing the pressure, its pores will get smaller and trap the pollutants inside.

atmospheric pressure:
Pressure caused by the weight of the atmosphere. At sea level it has a mean value of one atmosphere but reduces with increasing altitude.

A tiny opening, as in the skin or on the surface of a plant, through which liquids or gases may pass.

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  1. What is unusual about natrolite crystals?
  2. What do scientists want to use natrolite crystals to do?
  3. Invent another use for natrolite crystals. What would the crystals do? Why would this be useful? Explain your answer in a few sentences.