page_title: René Just Haüy
section_title: Crystallography
hierarchical_section_title: René Just Haüy / Biography / Crystallography
caption_attribution_description: English: Rhomboid Rock Crystal (Quartz hyalin rhombifère). One of the 597 pearwood crystal models made by Abbot R.J. Haüy of Paris, which Martinus van Marum purchased for Teylers Museum, Haarlem (the Netherlands) between 1802 and 1804.
context_page_description: René Just Haüy FRS MWS FRSE was a French priest and mineralogist, commonly styled the Abbé Haüy after he was made an honorary canon of Notre Dame. Due to his innovative work on crystal structure and his four-volume Traité de Minéralogie,
  he is often referred to as the "Father of Modern Crystallography". During the French revolution he also helped to establish the metric system.
context_section_description: An accident apparently directed René-Just Haüy's attention to what became a new field in natural history, crystallography. Haüy was examining a broken specimen of calcareous spar in the collection of Jacques de France de Croisset. (According to some accounts, Haüy dropped the specimen and caused it to break.) He became intrigued by the perfectly smooth plane of the fracture.
  "The observation I have just noted is that which has served to develop my ideas on the structure of crystals. It presented itself in the case of a crystal that the citizen Defrance was kind enough to give me just after it had broken off from a group this enlightened amateur was showing me, and which formed part of his mineralogical collection. The prism had a single fracture along one of the edges of the base, by which it had been attached to the rest of the group. Instead of placing it in the collection I was then forming, I tried to divide it in other directions, and I succeeded, after several attempts, in extracting its rhomboid nucleus." René-Just Haüy, Traité de minéralogie (1801)
  Studying the fragments inspired Haüy to make further experiments in crystal cutting. Breaking down crystals to the smallest pieces possible, Haüy concluded that each type of crystal has a fundamental primitive, nucleus or “integrant molecule” of a particular shape, that could not be broken further without destroying both the physical and chemical nature of the crystal. He further argued that crystal structures are made up of orderly arrangements of these integrant molecules in successive layers, according to geometrical laws of crystallization. 
  Crystals that had been classed together previously were identified as being of separate mineral species if their fundamental structure differed. Heavyspar, for example, was differentiated into specimens containing barium and strontium. 
  The value of Haüy's discovery was immediately recognized.
  Haüy and his contemporaries worked with limited evidence.  They could observe a crystal's habit and  cleavage planes and measure interfacial angles with an instrument called a goniometer. The internal structure underlying the crystal's integrant molecule would not be determinable until the development of X-Ray diffraction technology many years later, in 1902. 
  Haüy was not the only researcher to observe that calcite crystals could be composed of smaller rhombohedra, but it was he who introduced the idea of triple periodicity in crystals.
  
  This idea was fundamental to later developments in the field on crystal lattices.
  Between 1784 and 1822, Haüy published more than 100 reports discussing his theories and their application to the structure of crystalline substances.
  Haüy first stated his laws of decrement in Essai d'une théorie sur la structure des crystaux (1784). It was a radical departure from his previous works, introducing his theory of molé constituantes or constituent molecules.
  By 1792, he had identified a number  of parallelepipeds as possible primitive crystal forms.
  Haüy worked out the mathematical theory of his work in his Traité de minéralogie (1801), which became a classic in the field. By then, Haüy had applied his ideas to the differentiation of different species. He systematically described all the known minerals, sorting them into classes, and giving their chemical and geometrical properties. His work, in four volumes, including an atlas of plates, was accounted 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".  A second updated edition appeared as Traité de cristallographie in 1822.
  Haüy created comprehensive collections containing hundreds of pear-wood models of crystal models for education and demonstrations. One such set was acquired by Martin van Marum, curator of the Teylers Museum and a director of the Hollandsche Maatschappij der Wetenschappen.
  Haüy is also known for his observations on pyroelectricity.  He detected pyroelectricity in calamine, an oxide of zinc, as early as 1785. 
  He studied pyroelectricity in a
caption: Pearwood model of rock crystal rhomboid, made by René-Just Haüy, Teylers Museum
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