68 Mr Haidinger on the Composition of Crystallized Bodies. 



pound varieties of Copper-pyrites, the striae upon the faces of 

 crystallization yield the means of ascertaining the situation of 

 the individuals. Generally the number of individuals, aggre- 

 gated according to this law, is not confined to three ; but we 

 find, that to every one of those added to the central one, some 

 other individuals are attached. 



Regular composition often also takes place in this species 

 parallel to a plane of P — 1, or perpendicular to the terminal 

 edges of P ; there are particularly two varieties of this case, 

 which, in the present place, deserve our attention. The in- 

 dividuals are either joined in pairs, or one central individual 

 is surrounded by four others, added in the direction of all the 

 edges of P. The product of the first, in the fundamental py- 

 ramid, would be Fig. 30. This has not yet been observed ; 

 but it will serve for explaining Fig. 31, a variety of the form, 



3 3 



P — oo • P — 2 • P • q-Tq-'P • a~j2 P + 1 ' from ^ ie mines m tne 



district of Siesen in Prussia. This and several other inte- 

 resting varieties of forms from the same locality, I have de- 

 scribed, on another occasion,* from specimens in the posses- 

 sion of Mr Sack of Bonn. 



If repeated in all the terminal edges of P in a form consist- 

 ing of P — oo . P — 1. P and P + l, the result is like Fig. 32, in 

 which the re-entering angles produced by the faces of P-f-1, 

 and the stria? upon P, parallel with the edges of combination 

 with P+l, diverging in three directions from the centres of 

 the faces of P demonstrate, that we really observe a compound 

 crystal, while mineralogists have been long deceived by the 

 equal brightness of the faces of P — oo, in directions apparent- 

 ly corresponding to the hexahedron. When the hemi-pyra- 

 midal character of the combinations is more distinctly pro- 

 nounced, a figure is produced resembling a tetrahedron, com- 

 bined with various other forms, but consisting of six indivi- 

 duals, the apices of which are contiguous to the edges of the 

 tetrahedron, or of five at least, if we conceive the central in- 

 dividual to be continued through the centre of the group. 

 Generally the individuals are much striated parallel to the 



* Mem. Warn. Sor. vol. iv. part i. p. 1. 



