History of crystallography before X-rays

The history of crystallography before X-rays describes how crystallography developed as a science up to the discovery of X-rays by Wilhelm Conrad Röntgen in 1895. The scientific approach to the study of crystals began in the 17th century with the work of Kepler on the structure of snowflakes and Nicolas Steno's discovery that the angles between corresponding faces in a crystalline substance are always the same. The work René Just Haüy published in 1801 and 1802 marked the point where crystallography split from mineralogy to become a science of its own. Some sources state that the history of crystallography started with the investigation of X-ray diffraction by Max von Laue in 1912 but that ignores over a century of previous scientific work in the field. In the period before X-rays, crystallography can be divided into three broad areas: geometrical crystallography culminating in the discovery of the 230 space groups in 1891–4, physical crystallography, and chemical crystallography. Up to 1912, crystallography had been largely based on mineralogy. It was the study of minerals in the 18th and 19th centuries that led to a progressive understanding of the relationships between chemical composition, crystal habit and crystal structure. During the 19th century crystallography was progressively transformed into an empirical and mathematical science by the adoption of symmetry concepts.

Origins

16th century The scientific study of the properties of crystals began in the 16th century. In the first half of the 16th century Paracelsus proposed a theory of mineral formation as an analogy to fruit-bearing plants. In 1546 Georgius Agricola published a study of mineralogy in which morphology, or geometrical shape, was one of the characteristics used to classify minerals. In 1550 Gerolamo Cardano made an early attempt to explain the shape of crystals as the result of a close packing of spheres. In 1591 Thomas Harriot studied the close packing of cannonballs (spheres). In 1597 Andreas Libavius recognised the geometrical characteristics of crystals and identified salts by their crystal shape.

17th century In 1611 Johannes Kepler published Strena Seu de Nive Sexangula (A New Year's Gift of Hexagonal Snow) which is considered the first treatise on geometrical and atomistic crystallography. Kepler studied the packing of spheres, in order to explain the hexagonal symmetry of snow crystals. He demonstrated that in a compact packing each sphere has six neighbours in the same plane, three in the plane above, and three in the plane below, for a total of twelve touching spheres. Kepler concluded that 0.74084 is the maximum possible density amongst any arrangement of spheres — this became known as the Kepler conjecture. The conjecture was finally proved by Thomas Hales in 1998. In 1665 Robert Hooke attempted to explain crystal morphology based on the stacking of atoms. In his work Micrographia he reported on the regularity of quartz crystals observed with the recently invented microscope, and proposed that they are formed by spherules. Nicolas Steno rejected Paracelsus's proposed organic origin for crystals. Steno first observed the law of constancy of interfacial angles in 1669 when studying quartz crystals and noted that, although the crystals of a substance differed in appearance from one to another, the angles between corresponding faces were always the same. Steno's work can be considered as the beginning of crystallography as an independent discipline. In 1678 Christiaan Huygens proposed a structural explanation of the double refraction of calcite based on ellipsoidal atoms. Huygens published his results in his Traité de la Lumière. A geometrical theory of crystal structure based on polyhedra was proposed by Domenico Guglielmini. Guglielmini's publications of 1688 and 1705 concluded that basic forms (cube, rhombohedron, hexagonal prism, and octahedron) of various salt crystals are characteristic of each substance, are identical in form, indivisible, and have faces with identical inclinations to each other. By the second half of the 17th century the ideas of Paracelsus had been displaced by a more scientific approach to chemistry, geology, mineralogy, and the emerging field of crystallography. In his book The Sceptical Chymist of 1661, Robert Boyle criticised the traditional composition of materials, as represented by the teaching of Aristotle and Paracelsus, and initiated the modern understanding of chemical elements using the words "perfectly unmingled bodies". Boyle argued that matter's basic elements consisted of various types of particles, termed "corpuscles", which were capable of arranging themselves into groups (molecules). Boyle was one of the earliest researchers to use the term crystal for crystalline substances apart from quartz.

Geometrical crystallography

18th century In 1723 Moritz Anton Cappeller published Prodromus Crystallographiae, the first treatise on crystal shapes. The introduction of the term crystallography is attributed to Cappeller. In 1773 Torbern Bergman, a leader in the field of chemical analysis, described the crystal forms of calcite and stated that all the forms could be built up from the cleavage rhombohedron. Bergman, building on the previous work of Carl Linnaeus, developed a classification of minerals based on chemical characteristics, with subclasses organised by their external shapes, and defined seven primary crystal forms. In 1774 Abraham Gottlob Werner published his classification of minerals. Werner postulated seven primary forms, and showed that some geometrical forms could be derived from one another by truncation. With Jean-Baptiste L. Romé de l'Isle's Essai de cristallographie published in 1772 and Cristallographie published in 1783 the scientific approach to crystal structure began. Romé de l'Isle described over 500 crystal forms and accurately measured the interfacial angles of a great variety of crystals, using the goniometer designed by his student Arnould Carangeot. He noted that the angles are characteristic of a substance, thus generalising the law of constancy of angles postulated by Nicolas Steno. Romé de l'Isle considered that the shape of a crystal is a consequence of the packing of elemental particles, and defined six primitive forms. However, he criticised René Just Haüy and Torbern Bergman for speculating on the internal structure of crystals without sufficient observational data.

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