Impurities clock crystal growth rates

Quartz is one of the purest minerals known. Nevertheless, tiny amounts of impurities have an important effect on this crystalline form of silicon dioxide. The delicately hued type of natural quartz known as amethyst, for example, owes its distinctive purple color to traces of iron compounds that are locked into its crystal lattice as it grows.

By focusing on another type of impurity, researchers have now developed a simple method of determining growth rates along different directions in a quartz crystal. In effect, “we can see just what the crystal looked like throughout its growth history,” says geophysicist Phillip D. Ihinger of Yale University.

Ihinger and Yale coworker Stephen I. Zink describe their technique in the April 20 Nature. Such research could provide insights into geological processes such as magma crystallization, they say.

Growing into fluid-filled cavities, natural quartz crystals typically form prismatic structures with flat faces. They also invariably incorporate traces of hydrogen-bearing compounds, such as water, lithium hydroxide, and aluminum hydroxide.

Like a snowflake, every quartz crystal has a unique structure, reflecting details of how it arose, the researchers note.  Moreover, the concentration of impurities can vary widely within a single crystal.

Ihinger and Zink used high-resolution infrared spectroscopy to map impurity concentrations across different slices of a gem-quality quartz crystal. The hydrogen-containing compounds produce defects in the crystal lattice. Faster growth rates generally lead to higher impurity concentrations.

“We discovered that the actual growth rate of individual . . . faces is preserved in the chemical makeup of the crystal,” Ihinger says. “Our technique allows us to read the preserved growth-rate record, much like reading the speedometer in a car.”

In the quartz sample that they studied, for example, the researchers found that two crystal faces growing from the same fluid at the same time actually formed at rates that differed by a factor of 10. From such data, they were able to reconstruct a single crystal’s complete history.

At present, Ihinger and Zink can obtain only relative growth rates. They plan to calibrate their speedometer by measuring the hydrogen content of synthetic quartz crystals grown in controlled environments at known rates.

“Our technique can be applied to crystals from a variety of geological environments to determine their growth history,” the researchers contend. The technique may also prove useful in the electronics industry for monitoring the production of defect-free synthetic quartz crystals.