Crystals and crystallography

A crystal is a patterned three-dimensional assembly of atoms that is a repetitive (periodic) array of atoms. Crystals contain repeating arrays or atoms arranged in unit cells. Crystallography is the study of the formation processes that produce crystals, and of the structural and identifying details of crystals.

In the ancient and medieval world, crystals were considered a strange union of the animal and mineral kingdoms, growing into predetermined shapes like living things but seemingly without life. Many mineralogists hypothesized that their growth was the result of astrological forces. It was not until Robert Boyle and Robert Hooke began experimenting with microscopes that the true nature of crystals began to be understood. During the course of the last three centuries an entire field of study, crystallography, developed to further the understanding of crystals.

All solid matter is either amorphous (without definite shape) or crystalline (from the Greek word for clear ice ). Crystals are defined by a regular, well-ordered molecular structure called a lattice, consisting of stacked planes of molecules. Because the molecules of the crystal fit together and contain strong electrical attractions between the atoms, a crystal is typically very strong.

There are many shapes in which crystals may be found, depending upon the type of atomic bond that is most dominant

within their molecules (e.g., ionic, covalent, or metallic). Crystals high in ionic bonds are often cubic; these include salt and sugar, as well as iron pyrite. Covalent bonds are very strong, producing an extremely durable crystal such as a diamond . Metallic bonds are typified by a cloud of free-roaming electrons, giving the compound's shape less definition but allowing for great electrical and thermal conductivity; most metals are technically crystals very high in metallic bonds.

All crystals are formed from tiny atomic building blocks called unit cells. By changing the way the unit cells are stacked together, seven different crystal structures can be formed: triclinic, monoclinic, orthorhombic, trigonal, tetragonal, hexagonal, and cubic. In addition to their structure, crystals are classified by their symmetry—that is, the ability of the crystal to look the same when rotated. Some crystals are symmetrical along two axes, some along three axes, and some along four axes; some display no symmetry at all.

Throughout history, many crystalline materials, including most gemstones , have been prized for their ability to be cut along flat planes called cleavage faces. This is accomplished by separating one lattice plane from the next, producing a surface that is almost perfectly flat. However, not all crystals allow such clean cuts. Many substances such as metal, stone, and brick behave like crystals but are very difficult to cut along a cleavage face.

Because of the many industrial and scientific applications of crystals, the demand for clear, perfectly formed samples is very high. Unfortunately, nature rarely produces such crystals; more often the crystal has faults or impurities; and it is for this reason that large, perfect gemstones are valued so highly. In order to meet the demand for pure crystals, scientists have developed methods for "growing" crystals. One common method is simply to melt a large supply of unrefined crystal and allow it to reform; while in a liquid state, the molten crystal is often sifted of impurities, in order to yield crystals of higher quality. Another method for growing crystals is called seeding. Here, a small sample of crystal is placed in a vapor or solution; material is allowed to accumulate on the seed until the system reaches equilibrium. Often in crystal seeding a seed of a different material than that of the crystal is used. This is the case in the natural crystal formation called cloud seeding , wherein seeds of silver iodide are dropped into clouds ; the silver iodide accumulates ice crystals which eventually fall in the form of rain or snow.

Scientists began to investigate the nature of crystals as early as the seventeenth century, when the Danish geologist Nicolaus Steno (1638-1686) began his experiments with common crystals. He found that all crystals of a certain compound have characteristic angles at which the faces will meet. This means that every piece of salt will be cubic in shape, and that smashing a piece of salt will yield smaller and smaller cubes. Thus began the science of crystallography, and Steno's observation became its first law.

The next great crystallographer was the French mineralogist René-Just Haüy (1743–1822), who became involved in the science quite accidentally. While browsing through a friend's mineral collection, he dropped a large sample of calcite. He was surprised to note that the sample shattered along straight planes. Although Steno had pointed this out more than a century before, it was Haüy who hypothesized the existence of unit cells, showing how basic cells could be combined to create the different crystal shapes.

By the early 1800s many physicists were experimenting with crystals; in particular, they were fascinated by their ability to bend light and separate it into its component colors. Because of their varying molecular structures, different crystal types would affect light differently. Among the most influential member of the emerging field of optical mineralogy was the British scientist David Brewster; by 1819, Brewster had succeeded in classifying most known crystals according to their optical properties.

During the mid-l800s the preeminent French chemist Louis Pasteur examined tartrate crystals under a microscope; these crystals were known to twist the path of light sometimes one direction and sometimes the opposite. He found that the tartrate crystals were not all identical and that some were mirror images of the others. When combined, the two shapes within the whole tartrate would bend light in two possible directions. By using tweezers, Pasteur painstakingly separated the crystals into two piles, which were melted and then reformed into two distinct crystals. Once separated, each new crystal would twist light in only one direction, one clockwise, the other counterclockwise.

Pasteur's work became the foundation for crystal polarimetry, a method by which light is polarized, or aligned to a single plane. It was soon discovered that other crystals were also capable of polarizing light. Today, crystal polarimetry is used extensively in physics and optics.

Another phenomenon displayed by certain crystals is piezoelectricity. From a Greek work meaning "to press," piezoelectricity is the creation of an electrical potential by squeezing certain crystals. This strange effect was first discovered by Pierre Curie and his brother, Jacques, in 1880, who were surprised to detect a voltage across the face of compressed Rochelle salt.

The piezoelectric effect also works in reverse: when an electrical current is applied to a crystal such as quartz , it will contract; if the direction of the current is reversed, the crystal will expand. If an alternating current is used, the piezoelectric crystal will expand and contract rapidly, producing a vibration whose frequency can be regulated. Because of their precise vibrations, piezoelectric crystals are used in radio transmitters and quartz timepieces.

Perhaps the most important application of crystals is in the science of x-ray crystallography. Experiments in this field were first conducted by the German physicist Max von Laue. While an instructor at the University of Munich, Laue had done extensive work with diffraction gratings (metal meshes used to separate light into its component colors). His goal was to apply these gratings to the study of x-ray radiation because, at this time the true nature of x rays was yet to be fully understood. However, the wavelengths of x rays were far too short for diffraction gratings to be used, the x rays would pass through the holes unaffected. What was needed was a grating with microscopically tiny holes; unfortunately, the technology did not exist to construct such a grating.

In 1912, Laue perceived that the regular stacked-plane structure of a crystal would act like a very small diffraction grating; this hypothesis was successfully tested with a crystal of zinc sulfide, and x-ray crystallography was born. Using a crystal, scientists could now measure the wavelength of any x ray as long as they knew the internal structure of their crystal. Also, if an x ray of a known wavelength was used, the molecular structure of unknown crystals could be determined.

X-ray crystallography was perfected just a few years later by the father-son team of William Henry Bragg (1862–1942) and William Lawrence Bragg (1890–1971), who were awarded the 1915 Nobel Prize in physics for their work. Since that time, x-ray crystallography has been used to examine the molecular structure of thousands of crystalline substances and was instrumental in the analysis of DNA. Crystallography remains an important branch of earth science because the analysis and study of crystals often yields important information concerning the type and rate of geological processes.

See also Atomic theory; Cave minerals and speleothems; Chemical bonds and physical properties; Minerals; Mineralogy