Material of the month – Quartz

Materials World magazine
,
1 Dec 2015

It is one of the best-known minerals on Earth and is fundamental in many rocks. This month, Anna Ploszajski examines the use of quartz. 

In the 17th Century, Nicolas Steno, a scientist from Denmark was using quartz to lay the foundations of crystallography. He noticed that although different crystals of quartz sometimes look strikingly different, they all have the same angle of 120° between faces. He developed this observation into Steno’s Law – that the angles between the faces of a crystal are constant and characteristic of the species. 

Given that jewellers have treasured quartz crystals since Roman times, the word quartz came about surprisingly late in the English language. It emerged in the mid-18th Century after the German quarz, which derives from the Polish kwardy and the Czech tvrdý, meaning hard. Pliny the Elder – a Roman scholar who published a pioneering encyclopaedia on natural history nearly 2,000 years ago, believed that quartz was actually ice, formed over long timescales. He supported this hypothesis by pointing out that quartz can be found near Alpine glaciers, but not on warmer volcanic mountains. Ironically, it was a volcanic mountain that brought Pliny to an untimely end – he died trying to escape with a friend from the erupting Vesuvius. 

Quartz’s chemical formula is SiO2– silicon dioxide, or silica, as it is also known. Like zeolites (see Material of the Month October 2015), quartz crystals are made up of interconnecting silicon-oxygen tetrahedra, usually arranged in a trigonal crystal system – although hexagonal quartz also exists.

Quartz is the second most abundant mineral in the Earth’s crust, behind feldspar, but is the most common on the continental crust. It is found in most types of rocks, including igneous, metamorphic and sedimentary. Quartz naturally forms either from molten magma, or precipitates from hot watery solutions in hydrothermal veins. Physically and chemically resistant to weathering, grains of quartz find themselves concentrated in soil, rivers and beaches, after their erodible host rock has long since weathered away.

As romantic as rock hunting sounds, nature remains an unruly, unregulated fabricator of quartz. Naturally formed quartz crystals rarely grow as a single crystal, with the structure often twinned or impure. Scientists sought pure crystals, so set out to make them themselves. The synthesis of quartz in the laboratory was achieved in 1845 by a German geologist named Karl Emil von Schafhautl with the help of a pressure cooker. Although the crystals were admittedly microscopic in scale, it was still an excellent start as a proof of concept. Another German, Richard Nacken, developed these hydrothermal methods during the 1930s and 40s, when more sophisticated equipment was available, and successfully synthesised quartz crystals one-inch in diameter. Meanwhile, across the pond, the American military contracted Bell Laboratories and the Brush Development Company to build on Nacken’s work, and achieved a 1.5-inch crystal in 1948 – the largest of its kind at the time. Development of these hydrothermal synthesis techniques soon saw successful production of synthetic quartz crystals at the industrial scale.

Colour varieties 

Anyone who has spent as much time in the Rock Room of London’s Natural History Museum as I have will be well aware of the versatility of humble silica. Rocks in different shapes, textures and of every colour of the rainbow are all based on the basic SiO2 formula. Some of these are single crystals and are usually transparent, whereas others are made up of many microscopic crystals, and the grain boundaries tend to scatter incoming light rendering the overall crystal translucent or opaque. Pure quartz crystals are colourless, transparent and pencil-shaped, and are also known as rock crystals. They most commonly occur as milky quartz, whose white colour is a result of trapped liquid or gases introduced during formation. Metallic inclusions give quartz colour – iron, makes, for example purple amethyst. Incidentally, the name of this gemstone comes from the Ancient Greek to mean not-intoxicated (a-méthystos) – they believed that carrying the protective purple stone would prevent insobriety. 

Other colourful gems made from quartz with inclusions include citrine (yellow to brown), rose quartz (pink to red), smoky quartz (gray to black), prasiolite (mint green) and carnelian (red to orange). Chalcedony is made up of a mixture of quartz and moganite, a quartz polymorph, and forms gems such as agate, onyx, jasper, aventurine and tiger’s eye. Some of these gems exhibit what is called the chatoyancy, cat’s eye effect, arising from a fibrous internal structure.

Quartz is a material with more than a pretty face – it exhibits the remarkable property of piezoelectricity. The word piezoelectric was first derived from the Greek piezein, which means to squeeze or press, and piezo, which is Greek for push. Discovered by Pierre and Jacques Curie (husband and brother-in-law respectively to Marie) in 1880, a piezoelectric charge accumulates in a quartz crystal in response to a mechanical stress, and conversely a potential difference across the crystal is set up when the crystal is subject to stress. 

Piezoelectric properties

But how does a lump of rock generate electricity? It’s all to do with the trigonal unit cell. Ordinarily, the local electronic charges in the unit cell are overall balanced – a positive charge here will be negated by a negative charge there. This is partly what makes the crystal structure stable in the first place. But squeezing or stretching the crystal away from this happy equilibrium perturbs the balance of charge, and a potential difference is set up across the whole crystal. Conversely, applying a voltage across the crystal in a particular direction will make the atoms reshuffle and the whole crystal physically deform. 

Early efforts to make use of this extraordinary characteristic concentrated on oscillators and resonators during the early 20th Century. Paul Langevin investigated quartz resonators for sonar during the First World War, and American electrical engineer Walter Guyton Cady produced the first quartz crystal oscillator in 1921, patented by George Washington Pierce two years later. This work culminated in the first quartz oscillator clock, produced by Warren Marrison in 1927. This technology really took off, most notably in the development of the radio, which depended on piezoelectric crystals to control frequency, and the demand for quartz crystals grew hugely during the 1920s and 30s. It was this explosion in demand, together with the disruption in natural quartz supplies from Brazil during the Second World War, that really encouraged efforts to develop industrial-scale synthesis methods.

The use of quartz as an oscillator remains one of the most common uses of the material today, with more than two billion crystals manufactured for this purpose every year. But how do you get from piezoelectricity to telling the time? Well, all solid objects have a resonant frequency. When that object is made to vibrate at that particular preferential frequency, the object oscillates with very large amplitude. So at the resonant frequencies, even small driving forces can produce large amplitude oscillations. Anyone who has ever pushed someone on a playground swing will be subconsciously aware of the system’s resonant frequency – pushing in time with the natural interval of the swing will make the swing go higher without much effort, whereas trying to push the swing faster or slower will be much harder. Another example is sympathetic resonance observed by players of string instruments.

Tick tock

If a quartz crystal is resonating, its mechanical distortions will set up a piezoelectric signal with a very precise frequency. This signal can be interpreted by simple electronics and a tiny motor into the familiar seconds, minutes and hours that we use to tell the amount of time we’ve spent in the Rock Room. Our wristwatches and clocks both use quartz crystals to keep us on time and in sync. Marrison’s quartz crystal clock boasted accuracies of one second in 30 years, and replaced the pendulum clocks which had up until that time been the world’s most accurate timekeepers. This accolade was short-lived, however, since atomic clocks took the title in the 1950s.

Nowadays if you have a baby, the chances are that your first glimpse of them will have been courtesy of a quartz crystal. That’s because the ultrasound part of an ultrasound scan is produced and received by a quartz crystal. A rapid, known frequency electric current is applied to the quartz crystal, and the piezoelectric effect causes the crystal to vibrate, producing very high frequency sound waves greater than those audible to humans. These waves travel through the mother and the amniotic fluid and reflect off the baby, bouncing back to hit the crystal. The reflected sound waves hitting the crystal produce an electric current, which is reconstructed by a computer into an image of your little one.