Tourmaline, Quartz, and Piezoelectricity
Smartphones respond almost magically to our requests; simply tapping their touch screens completes a remarkable variety of chores. Smartphones, of course, have no supernatural properties. What they do have are paperthin, transparent, piezoelectrical films that convert the mechanical energy of tapping into electrical energy that activates the selected computer functions.
Piezoelectricity, the electrical current generated when certain crystalline materials are mechanically stressed, has become a big part of our everyday lives. Modern piezoelectrical devices rely on advanced piezoceramic and piezochemical materials. However, our awareness and understanding of piezoelectricity began with natural crystals of tourmaline and quartz.
Piezoelectricity—the name stems from the Greek piezein, meaning “to press”— was first investigated by French researchers Jacques and Pierre Curie in 1880. The Curies found that mechanical pressure applied to tourmaline crystals generated a measurable electrical charge on opposing crystal faces. They also observed an inverse effect when applied electrical current measurably deformed tourmaline crystals.
Certain other natural crystalline materials, notably quartz, also exhibit piezoelectrical properties. However, the exact cause of piezoelectricity was not explained until the 1920s after X-ray diffraction had revealed the nature of crystal-lattice structures.
The tourmaline-group minerals and quartz are silicates, in which silica tetrahedra join to form seven different structural groups. Tourmaline is a ring silicate or cyclosilicate; quartz is a tectosilicate or framework silicate. In both, silica tetrahedra link together in repetitive, closed configurations that provide incredible strength and rigidity. With their exceptional spatial stability, tourmaline and quartz deform only minimally when compressed.
Rather than causing physical distortion, most of the mechanical energy applied to tourmaline and quartz crystals instead displaces ions from their normal lattice positions to produce an electrical potential—piezoelectricity.
Piezoelectricity remained a laboratory curiosity until the early 1900s. It was then that thin, precisely cut quartz wafers mounted in hydrophone-like devices were found to generate a measurable microcurrent in response to the subtle pressure variations of underwater sound waves. Shortly after that, the sinking of HMS Titanic in 1912 spurred interest in adapting quartz hydrophones to locate icebergs by detecting underwater echoes from artificially generated sound waves. When World War I brought an urgent need to detect submarines, researchers quickly developed sonar (Sound Navigation Ranging) as the first successful application of piezoelectricity. Tourmaline, quartz, and piezoelectricity played significant roles in submarine warfare during World War II. Anti-submarine vessels employed sonars that used quartz-based hydrophones. Piezoelectrical applications required electronic-grade crystals of tourmaline and quartz with high chemical purity and virtually no structural distortion. But such crystals were rare and found only in a few localities. During World War II, the electronic grades of quartz, also used in chronometers, radios, radars, and bombsights, and tourmaline, were classified as strategic materials. Most electronic-grade crystals came from Minas Gerais, Brazil, the tourmaline from granite pegmatites, and the quartz from massive shale formations. Smaller amounts of electronic-grade quartz were also mined from hydrothermal veins in Arkansas, while some electronic-grade tourmaline came from pegmatites in Maine and California. Since World War II, the piezoelectrical properties of tourmaline and quartz have found many additional applications. In addition to smartphones, other devices include sound pick-ups on electric guitars, fitness monitors that provide real-time respiration and pulse rates, gas-flame lighters, medical ultrasound imagers, retail check-out scanners, and musical greeting cards.