43S 



needs to be reserved in future to the method of RöNTGENograms 

 among all other crvstallograpiiical methods. At the same time however 

 it is proved by the results obtained with minerals of different 

 localities, how strictlj' necessary it properly must be considered, to 

 build up the whole systematical mineralogy starting from this new 

 point of view, and what surprising ref.ults are surely to be expected 

 therefrom. 



We will now describe here the analogous experiments, made 

 with apophi/Ilite. 



§ 7. InveMigations rehtinfi to the Symmetry of Apophyllite. 



For our investigations of the symmetry of apophyllite, we had 

 material at our disposal from the following localities : a. from 

 Paterson (U.S.A.); from Bergen Hill, Erie Railroad JSf.J.; c. from 

 Guanajato, in Mexico; d. from Benifjord in fcelaml. The apophyl- 

 lites of American origin we will place opposite to tiiat of Iceland 

 as a typical group, because they manifest, as will seen below, some 

 peculiarities in their molecular structure, which are not present in 

 the [ceJand-m'mQY?i\, and are substituted in it by other (puilities. 



Apophyllite, a minei'al with the chemical composition: 

 KH,Ca,Si,(),,-\-MH,0 

 belongs to the important group of the remarkable neolithic silicates; 

 they all contain water, and as was proved for many of them already, 

 their vapourtension at constant temperature appears to be continually 

 variable with their momentaneous content of water, — a behaviour 

 quite opi)Osite to that of hydrated salts in general. The explanation 

 of this phenomenon is- commonl}' given in this way, — which is 

 confirmed completely moreover by the physical properties of these 

 silicates, — that the water is not combined with the silicate like 

 the water of crystallisation, but that it is present, at least partially, 

 either in solid solution or hold in the silicate-skeleton by absorption. 



Apophyllite is a typical representative of an optically anomalous 

 or mimetic crystal: Brewster in 1819 already discovered the partition 

 of the crystal-sections in numerous fields, and since that time the 

 pseudo-tetragonal crystals of this mineral have often been the subject 

 of research. For the explaiiation of this anomalous behaviour, two 

 theories have been started : in 1877 by Mallard, who supposed the 

 crystals of apophyllite to be polysynthetic twinnings of perpendicularly 

 crossed and penetrating monosym metric lamellae, — the dimensions 

 of the monosymmetric molecular-arrangement differing only slightly 

 tVom those of a tetragonal structure. The second view, chiefly defended 

 b} (J. Klklm. explains tlio ojttical abnormalities as caused by internal 



