435 



axes and of the horizontal sj/minclrii-plane, is reallij chdrncteristic of 

 the vioh'cidar arrangenieut of the silicate. Foniuliiig our stateiiiciit on 

 these expei'iments, we must therefore draw I he conclusion, that 

 eordierite is an hemiinor/thic mineral, belonging to the rhouihic-pj/ra- 

 midal class (rhombic-hemimorphic class) of the rhombic system, just 

 like calamine and strurite, etc. The threefold twinning-nggregations 

 of the eordierite must thus be considered to be real pseudo-//^.m(/()?za/, 

 and no \)senóo-triqo)/al mimetic forms. 



Because all possible spacedattices of the rliombic system, as deduced 

 by Bravais, possess vertical and horizontal planes of symmetiy, the 

 molecular arrangement of eordierite can therefore by no means 

 correspond to such a Bravais' space-lattice. Howevoi' the pseudo- 

 hexagonal symmetry of the mineral, just as its prismatic twinforma- 

 tions, seem to indicate with strong emphasis a structure-unit, which 

 must be considered derived from tlie i-ectangular prism with rhombic 

 base, whose angles will differ only slightly (ca. 25') from 60° or 

 120°. The choice between the possible structures is hardly to be 

 expected : after Schoknfi.iks' tlieory e.g., there will be no less than 

 22 arrangements, which correspond to the hemimoiph}' of the rhombic 

 system. (Schoenfliks, Krystallsysteme und Krystallstruktur, 1891, 

 S. 433). 



§ 5. A second peculiarity of the obtained R()NTGEN-pat terns is this, 

 that notwithstanding their agreement with respect to their general 

 symmetry, yet appreciable differences in the distribution of the black 

 spots show themselves, if analogous crystal plates, but of difFerent 

 localities are compared. Even a superficial comparison of the figures 

 3, 6. and 8 of plate I to the one side, ajid of fig. 4, 7, and 9 to 

 the other side, is able to manifest the great differences immediately. 

 Doubtless all analogous images show- a number of common spots ; 

 but in every radiogram there are moreover new ones, while even 

 homologous spots in the different photographs appear with such 

 different relative intensities, that the total aspect of the figure becomes 

 a quite different one by it. 



As these photogi-aphs were made all under preciseli/ the same 

 circumstances, we must conclude from this, that the symmetry of 

 a species of minerals being evidently always the same, the number 

 and the arrangement of its molecular reticular planes, just as their 

 molecular densities, are however variable with the special conditions, 

 which were prevailing during the formation of the crystals. With 

 respect to the e.rterntl form of the crystals, this is a fact which has 

 long been known, and which can moreover readily be explained 



