79'2 



Mineralogy. — "BönUienpaüems! of Boracite, obtained ahove and 

 below its inversion-tent perature". By Prof. H. Haga and Prof. 

 F. M. Jaeger. 



(Communicated in the meeting of January 31, 1914). 



§ 1. Comparison of numerous pliotogriiphic diffraction-patterns, 

 obtained by the method of Laue, Knipping, and Friedrich, i.e. by the 

 transmission of RöNTGEN-rays through planparallel crystalplates, has 

 led with increasing evidence to the conviction, that the symmetry 

 of those patterns agrees with that of the • space-lattice, whicli presents 

 itself as the very fundament of the molecular arrangement of 

 the investigated crystal. On this assumption, the new method of 

 research will be in future a very important manner to elucidate the 

 question, if with polymorphic changes, and principall}' in cases of 

 enantiotropic inversions in the neighbourhood of the critical inversion- 

 temperature, a change of the molecular arrangement takes place, or 

 if the cause of polymorphism must be attributed to a change only 

 of the crystalmolecules themselves. 



This problem seemed to us of high importance, especially in the 

 case of those remarkable reversible inversions, which are found in 

 a class of crystals, whose optical behaviour does not agree with 

 the symmetry of their external form, of their cohesion, etc., or 

 generally speaking : with their total crystallographic character ; so 

 that it has been a custom already from an early date, to discern these 

 cases as thote of "optically anomalous" crystals. Of this class of 

 mimetic crystals the minerals boracite: Mg^ B^^ O^^ Cl^, and leucite : 

 KjAljSi^Oi, may, after the investigations of Mallard, Klein, etc., 

 be considered to be typical representatives. 



The boracite crystallizes in forms, which by no means can be 

 discerned from real hexakistetraëdrical ones ; even by the most 

 accurate goniometrical measurements it appeared to be impossible 

 to iind any deviation of the external torm from (hose possessing the 

 above mentioned symmetry. On the other hand, however, the 

 optical investigations, and also those concerning the corrosion-pheno- 

 mena, have shown with perfect evidence, that the crystals of boracite 

 possess no regular symmetry in the common way ; they must be 

 considered as composed by a very complicated system of birefringent 

 lamellae, which according to their optical properties, cannot have 

 any higher symmetry than that of rhombic crystals ; these lamellae 

 have intergrown in such a way, that their conglomerate corresponds, 

 with respect to its external form, very exactly with a true regular 

 crystal. More particularly the individuals of boracite seem to represent 



