COMPOUND MINERALS. 



85 



Twin-crystals of this kind, are common among the forms of Octa- 

 hedral Iron Ore and Spinelle. 



2. Another kind of twin crystal occurs among the crystals of 

 Hornblende, which may be conceived of in the following manner. 

 If we suppose two crystals of the form of an oblique rhombic Prism, 

 (Fig. 44,) whose lateral edges are replaced by the faces x, and whose 

 terminal edges are truncated by the planes rr and r f r lf , are made to 

 penetrate each other, so that the vertical plane abed, (which passes 

 through .the middle of the planes x, and which is a diagonal plane of 

 the rhombic Prism,) in each, shall coincide, the result will of course 

 be similar to Fig. 140. If now, the anterior portion, separated by the 

 section abed, undergoes a semi-revolution, or passes through the an- 

 gle of revolution, upon an axis perpendicular to the sectional plane, 

 the faces r 1 and r' r will come to the upper base, Fig. 141 , with a small 

 triangular portion of the lower base P, which will form a re-entering 

 angle with the remaining portion of the upper base P. The re- 



Fig. 140. 



Fig. 141. 



semblage of the two Octahedrons cannot be con- 

 sidered as a twin crystal, because the correspon- 

 ding faces of the two forms, for example ace and 

 a' c 1 e', are exactly parallel to each other. It will 

 become obvious, on inspection, how different is 

 the case with the faces of the twin crystal above 

 described, in which it is seen that the respective- 

 ly similar parts, in the composing crystals, have 

 assumed that different, though fixed situation, 

 with respect to each other, which is the peculiar 

 character of the regular composition. 



8 



Fig. 139. 



