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Volume 20, pages 469-473, 1935 THE SIZE OF CRYSTALS CLIFFORD FRONDEL, Bayside, Long Island, New York. In a recent paper, Palache1 has given the dimensions of a number of large crystals of various minerals and has directed attention to the problem of the genetic factors that influence their growth. It would appear that a growing crystal would increase in size indefinitely if a continuous supply of material were brought to its surface. However, experiment has shown that growth may cease with the attainment of a critical size, regardless of the amount of material still available. Retgers2 concluded that there is a distinct maximum in size for every crystal, varying somewhat with the conditions, beyond which it is incapable of further growth. Von Hauer3 earlier observed that there is a certain limit to growth beyond which the crystals become markedly imperfect, developing a composite and fissured structure with surface excrescences and cavities. This limit was found to vary for different substances and under varying circumstances. A similar observation was made by Ord4 for crystals of calcium oxalate and the phenomenon was likened by him to the property of liquids to form drops, but only drops of a certain limit of size for each kind of liquid. Both Retgers and von Hauer found that upon addition of a small amount of a foreign substance, such as a metallic chloride or an acid, to the crystallizing solution the crystals resulting could grow perfectly to much larger sizes. In addition to the citations of these workers on this effect there may be mentioned the observations of Buchner5 on the property of Mg, Cu and Fe chlorides to perfect and enlarge NaCl, KCl and NH4Cl crystals; of Gibbs and Clayton6 on the similar effect of lead salts on NaCl; of Ehrlich7 on the remarkable action of pectin in inducing such changes in NH4Cl and other salts; and of Yamamoto8 on the effect of a large number of metallic salts on the size and transparency of NaCl, KCl and NH4Cl crystals. Addition of the foreign substance is frequently found to be accompanied by a change in crystal habit; borax, for instance, inducing the development of hemimorphic faces on magnesium sulphate in addition to perfecting the crystals (von Hauer). Zwicky9 has suggested in explanation of the phenomenon of size limitation in ordinary growth that the liberation of heat of crystallization or heat of fusion at a growth surface may cumulatively produce, through thermal stresses, a disordered mosaic surface which, at the attainment of a critical size, may become such as to prevent further growth. The observation of Stober,10 that large perfect crystals can be obtained from a melt by placing the axis of greatest heat conductivity of the crystal in the direction of heat flow, is of significance in this connection. W. H. Bragg has suggested that the regular fitting of molecules into a crystal may begin correctly enough, in the nuclear stage, but that errors of adjustment may creep in until the surface becomes somewhat disordered, and growth ceases because fresh molecules cannot slip into their proper places.11 Von Hauer observed that the heterogeneous surface occurring at the maximum size is marked by a difference of angles in the composite parts, and said that a crystal may bear in itself a tendency to irregularity which becomes more conspicuous with continued growth. Retgers believed that the tendency for a size maximum is a property of the crystal itself and that its cause can not be sought in the properties of the solution. Foreign substances apparently increase the transparency and perfection of crystals, and induce a larger size, by decreasing the tendency for a mosaic structure, as was suggested by Oka,12 for micro-composite NaCl crystals, but in what manner this would be accomplished is not evident. Slow growth, in general, enhances the size and perfection of crystals, as was brought out in this connection by Zwicky, and the effect of adsorbed foreign substances may possibly be by slowing up growth. This view is supported by the frequent occurrence of habit modification in such cases, and is not inconsistent with the fact that under other circumstances addition agents may cause the formation of fine grained aggregates or of skeletal growths, in place of normal crystals, as a result of an extreme hindering of growth. The effect of magma-derived mineralizers in promoting crystal growth in certain geological processes may also include an action on the coarse structure of the crystals, of the nature described, in addition to the viscosity and chemical control that is generally ascribed to them. A number of large crystals that have come to the attention of the writer are cited in the following table. The specimens are contained in the collection of The American Museum of Natural History, unless otherwise stated. Further instances have been mentioned by Spencer.13
NOTES
1 Palache, C., The largest crystal: Am. Mineral., vol. 17, pp. 362-363, 1932. 2 Retgers, J. W., Ueber den Einfluss fremder Substanzen in der Lösung auf die Form, die Reinheit and die Grösse der ausgeschiedenen Krystalle: Zeit. physik, Chem., vol. 9, p. 278, 1892. 3 Von Hauer, K., Krystallogenetische Beobachtungen: Verh. der k. k. Geol. Reichsanst. Wien, pp. 45, 57, 75, 90,162, 296,1877; pp. 185, 315,1878; pp. 20,181, 1880. 4 Ord, W. M., On the influence of colloids upon crystalline form and cohesion. London, 1879, p. 125. 5 Buchner, L. A., Ueber die Bildung durchsichtiger, dem Steinsalze ähnlicher Salzwürfel: Jour. prakt. Chem., vol. 111, pp. 259-266,1871. 6 Gibbs, W. E., and Clayton, W., The production of large, clear cubical crystals of sodium chloride: Nature, vol. 113, pp. 492-493, 1924. 7 Ehrlich, F., Ueber die Beeinflussung des Krystallwachstums von Salmiak durch Pektin: Zeit. anorg. Chem., vol. 203, pp. 26-38, 1931. 8 Yamamoto, T., Bull. Inst. Phys. Chem. Res. Tokyo, vol. 10, pp. 52-60, 679686,1931; vol. 11, pp. 1083-1097, 1932. 9 Zwicky, F., Secondary structure and mosaic structure of crystals: Phys. Rev., vol. 40, p. 74, 1932. 10 Stober, F., Kunstlicher Darstellung grosser, fehlfreier Kristalle: Zeit. Kryst., vol. 61, p. 299, 1925. 11 Bragg, W. H., Cohesion and molecular forces: Alexander's Colloid Chemistry, vol. 1, p. 266, 1926. 12 Oka, S., On the opacity of sodium chloride crystals: Jour. Soc. Chem. Ind. Japan, Supplem. Binding, vol. 35, pp. 178-179, 1932; vol. 36, pp. 141-143, 144145, 1933. 13 Spencer, L. J., Large specimens of spar from the Snailbeach Mine, Shropshire: Nat. Hist. Mag. Brit. Mus., vol. 1, p. 259, 1928. ** {probably Ontario, Canada -DVB}
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1) edge
(Takimoto).