Hemihedral quartz crystals

In recent years, stereochemistry, dealing with the three-dimensional behavior of chiral molecules, has become a significant area of research in modern organic chemistry. The development of stereochemistry can, however, be traced as far back as the nineteenth century. In 1801, the French mineralogist Haiiy noticed that quartz crystals exhibited hemihedral phenomena, which implied that certain facets of the crystals were disposed as nonsuperimposable species showing a typical relationship between an object and its mirror image. In 1809, the French physicist Malus, who also studied quartz crystals, observed that they could induce the polarization of light. 

In 1822, the British astronomer Sir John Herschel observed that there was a correlation between hemihedralism and optical rotation. He found that all quartz crystals having the odd faces inclined in one direction rotated the plane of polarized light in one direction, while the enantiomorphous crystals rotate the polarized light in the opposite direction. 

The symmetry of the model of a molecule or of a molecular ensemble depends on the conditions of the relevant physical (or chemical) measurement, and may vary for the same system according to time scale of observation and instrumental sensitivity. Whether the model of a chemical system is chiral or achiral may therefore depend on the conditions of observation. There is no ambiguity when chirality properties are observed the hemihedrality of quartz crystals, the optical rotation of hexahelicene, and the enantiospecificity of hog-kidney acylase, for example, are all unmistakable manifestations of an underlying structural chirality. On the other hand, achirality is not so simply implied by the absence of such observations. 

Unlike the clays, owing to its chiral lattice, quartz must display certain selectivities for the adsorption of mainly neutral organic compounds possessing molecular or crystal-like chirality. Glucose, galactose and arabinose selectively adsorb on quartz crystals and quartz dissolves in solutions of these monosaccharides Racemic acids seem not to be resolved on quartz however, it has been shown that d- or /-quartz exert an orienting influence on the epitaxy of hemihedral crystals of Glu, Ala, and Gly. For example, the influence is seen for Glu, Ala, and Gly adsorbed on the surface of quartz crystals 1010, and of Ala on /-quartz 1011. These crystals are like the asymmetric epitaxy of crystals of (+) camphor on biotite and calcite... 

III. Relationship between Macroscopic and Microscopic Structures as Illustrated by Quartz and its Nonsuperimposable Hemihedral Crystal Faces... 

These crystals have a shape known to geometricians as hemihedral, first observed in the mineral quartz. The two forms of quartz rotate the plane of polarization of polarized light in opposite directions. This relation between crystal form and optical activity was perceived by the astronomer Sir John Herschel in 1820. 

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