An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall.

  • Groth, P. (Paul), 1843-1927.
Date:
1906
Catalogue details

Licence: Public Domain Mark

Credit: An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall. Source: Wellcome Collection.

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    different. Such a substance then exhibits a “pseudo- hexagonal” crystalline form, i.e., those faces which together complete an apparently hexagonal form, though not altogether equivalent, are yet, in consequence of the close agreement in the arrangement of the particles in them, so similarly favoured during the formation of the crystal that they regularly appear together and are developed in a similar manner. In this case the crystalline form is apparently hexagonal, and might be imagined as resulting from a really hexagonal crystal by a slight homogeneous deformation, e.ga rhombic crystal by an extension or compression in the direction of one of the lateral axes, effected-uniformly on all parts of the crystal ; a monoclinic one by straining in an oblique direction lying in a plane of the hexagonal prism ; finally, a triclinic pseudo-hexagonal form by a compression or extension in any general direction. Crystal structures of the kind here described possess, as we learn from physical crystallography, three kinds of positions in which they are in equilibrium with one another, as follows from the fact that they regu¬ larly appear as triplet growths, twinned on the faces of the pseudo-hexagonal prism (as so-called mimetic forms). In the usual case of these mimetic forms being composed of thin twinned lamellae, then, the thinner the lamellae (/.£., the more frequently the three positions alternate with one another in the composition of the whole structure), the more will the variations of the angles from those of a truly hexagonal crystal become obliterated. If, finally, this alternation takes place in a regular manner, and at such minute distances that the twin structure is no longer recognisable even under the microscope, there results a structure which cannot by any of its properties be dis¬ tinguished from a simple hexagonal crystal. In that case the substance appears in two apparently different modifica¬ tions, one hexagonal and the other rhombic (or monoclinic, or triclinic) pseudo-hexagonal, with twin structure. Since we have here to do not with a different kind of crystal