Chemical crystallography before X-rays describes how chemical crystallography developed as a science up to the discovery of X-rays by Wilhelm Conrad Röntgen in 1895. 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 1891–1894, physical crystallography and chemical crystallography.
Up until 1800 neither crystallography nor chemistry were established sciences in the modern sense; as the 19th century progressed both sciences developed in parallel. In the 18th century chemistry was in a transitional period as it moved from the mystical and philosophical approach of the alchemists, to the experimental and logical approach of the scientific chemists such as Antoine Lavoisier, Humphry Davy and John Dalton.
Before X-rays, chemical crystallographic research involved observation using a goniometer, a microscope, and reference to crystal classes, tables of crystal angles, axial ratios, and the ratio between molecular weight and density (M/ρ). In this period crystallography was a science supported by empirical laws (law of constancy of interfacial angles, law of rational indices, law of symmetry) based on observations rather than theory.
The history of chemical crystallography covers a broad range of topics including isomorphism, polymorphism, molecular chirality and the interaction with mineralogy, structural chemistry and solid-state physics.
Symmetry
During the 19th century crystallography was progressively transformed into an empirical and mathematical science by the adoption of symmetry concepts. In 1832 Franz Ernst Neumann used symmetry considerations when studying double refraction in crystals. Woldemar Voigt, who was a student of Neumann, in 1885 formalized Neumann's principle as "if a crystal is invariant with respect to certain symmetry operations, any of its physical properties must also be invariant with respect to the same symmetry operations". Neumann's principle is sometimes referred to as the Neumann–Minnigerode–Curie principle based on later work by Bernhard Minnigerode (another student of Neumann) and Pierre Curie. Curie's principle "the symmetries of the causes are to be found in the effects" is a generalization of Neumann's principle.
The relations between symmetry and physical and chemical properties were established throughout the 19th century: the notion of hemihedry (Weiss, 1819; Delafosse, 1840), the 7 crystal systems (Mohs, 1822), the notion of point lattice (Seeber, 1824), the 32 crystal classes (Frankenheim, 1826; Hessel, 1830; Gadolin, 1869), molecular chirality (Pasteur, 1848), the 14 Bravais lattices (Bravais, 1850), the 65 chiral groups that contain only proper symmetry operations – rotations, translations and roto-translations (Sohncke 1879), and, finally, the 230 space groups (Fedorov, 1891; Schoenflies, 1891; Barlow, 1894).
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 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 recognized the geometrical characteristics of crystals and identified salts from their crystal shape.
17th century
In 1611 Johannes Kepler studied the packing of spheres, in order to explain the hexagonal symmetry of snow crystals. Kepler 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 /(3) = 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.
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 criticized 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. 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 when studying quartz crystals (De solido intra solidum naturaliter contento, Florence, 1669), 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 discovered the polarization of light by Iceland spar, a transparent form of calcite, and published his results in his Traité de la Lumière.
Domenico Guglielmini's publications of 1688 (Riflessioni filosofiche dedotte dalle figure de Sali) and 1705 (De salibus dissertatio epistolaris physico-medico-mechanica) concluded that the earliest forms (he noted cube, rhombohedral parallelepiped, 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.
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 1735 Carl Linnaeus, who is known for his system of classification of biological species in his Systema Naturae, also classified minerals and stated that "their transparency is derived from their atomical construction". In 1745 Guillaume-François Rouelle carried out a microscopic analysis of sea salt and proposed that the crystals were composed of cubic particles. In 1758 Roger Joseph Boscovich published his atomic theory which stated that particles of matter were linked by attractive and repulsive forces and that the solid so formed was compressible rather than rigid; this would become relevant in the 19th century when Haüy theorised that crystals were constructed from identical units stacked up without spaces.
The idea of a polyhedral molecular unit of crystal structure was promoted by Pierre-Joseph Macquer in his handbook Dictionnaire de chymie of 1766. In 1767 wrote that calcite crystals could be built from rhombohedra. 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 developed a classification of minerals based on chemical characteristics (extending the work of Linnaeus), with subclasses organized by their external shapes, and defined seven primary crystal forms.
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. Romé de l'Isle noted that the angles are characteristic of a substance, thus generalizing the law of constancy of angles postulated by 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.
In 1781 René Just Haüy (often termed the "Father of Modern Crystallography") discovered that crystals always cleave along crystallographic planes. Based on this observation, and the fact that the inter-facial angles in each crystal species always have the same value, Haüy concluded that crystals must be periodic and composed of regularly arranged layers of tiny polyhedra (molécules intégrantes). This theory explained why all crystal planes are related by small rational numbers (the law of rational indices). In 1784 René-Just Haüy published Essai d'une théorie sur la structure des cristaux, appliquée à plusieurs genres de substances cristallisées in which he stated his law of decrements: a crystal is composed of molecules arranged periodically in three dimensions without leaving any gaps. Haüy's molecular crystal structure theory assumed that molécules intégrantes were specific in shape and composition for every compound. Haüy developed a mathematical theory of crystal structure that turned out to be remarkably accurate and gave crystallography a legitimate place among the sciences.
Early 19th century
From the late 18th century it became apparent that a crystal of a substance was composed of units, whether thought of as atoms, ions, molecules, or polyhedra, in a regular spatial arrangement, termed its crystal structure.
The most notable early theory for crystal structures was that of René Just Haüy. In 1801 Haüy, published his Traité de Minéralogie in four volumes, the last of which was an atlas of plates which was considered "among the most wonderful of the 19th century". It has been described as "a work of comprehensive insight, and much of it, written with literary fluency". In this work Haüy described how the law of rational indices establishes relationships between the orientations of the crystal faces, and explains that crystalline solids are formed by replicas of what would now be considered a unit cell. Haüy's theory called for fixed mineral species (based on their molécule intégrante), fixed crystal morphology, and constant chemical composition. This was a mineralogical equivalent to the law of definite proportions in chemistry.
John G. Burke (1966) and (1984) proposed Haüy's crystal structure theory as an example of a paradigm in the sense of The Structure of Scientific Revolutions by Thomas S. Kuhn (1962). In 1822 Haüy published Traité de Cristallographie an updated version of his work of 1801. Haüy postulated, "to each specific substance with a well defined chemical composition, capable of existence in a crystalline form, there corresponds a shape that is specific and characteristic of that substance."
In 1808 John Dalton published his atomic theory of matter. In Dalton's theory, there were four key assertions: "matter is made up of roughly spherical atoms, which were indivisible and indestructible; all atoms of a given element are identical in mass and properties; compounds are formed by a combination of two or more different kinds of atoms; and chemical reactions involve the rearrangement of atoms". In his book A New System of Chemical Philosophy, crystals are considered as a periodic arrangement of spherical atoms. However, Dalton stated that it was premature to form any theory of crystallization. Kuhn proposed Dalton's atomic theory as an example of a paradigm in which Dalton asserted that atoms can only combine in simple, whole-number ratios (law of multiple proportions). Under this new paradigm, any reaction which did not occur in fixed proportion could not be a chemical process.