Science Scoops: Expansive Crystals
by Stephen James O'Meara
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.
- What is the state of a natrolite crystal at standard atmospheric pressure? What is the state of the crystal at 8,000 to 15,000 times atmospheric pressure?
[anno: The crystal is in a solid state at both standard atmospheric pressure and at higher pressures.]
- A natrolite crystal is put under pressure that is 10,000 times atmospheric pressure. The crystal is not put in a water-alcohol mixture. What do you think happens? Why? Explain your answer in a sentence or two.
[anno: The natrolite crystal probably condenses or gets smaller because there is nothing, such as water, to enter the pores of the crystal and expand it.]
- What do you think might happen if a natrolite crystal were put in water and then the water was frozen? Explain your answer in a sentence or two.
[anno: Answers will vary but could include that the natrolite crystal does not freeze or expand when the water freezes, as there is not enough pressure to cause the water to enter the crystal.]