Mr. L. Fletcher's Crystalloyrapluc Note*. 289 



octahedron (/{2Qo}, although present on the crystal, are not 

 shown in the figure. 



A chemical examination made by Dr. Flight shows that this 

 specimen has a composition very nearly represented by the 

 typical formula CuFeS 2 . The following results were ob- 



talned: ~ Observed Calculated, 



Ubseived. CuFeS 2 . 



Copper 34-37 34'45 



Iron 30-03 30-57 



Sulphur 31-92 34-98 



Quartz 4-19 



100-51 100-00 



Up to this point, for the sake of simplicity, a very important 

 property of copper pyrites, its hemihedral structure, has been 

 left as much as possible out of sight. It is found, however, 

 that the faces of the octahedron {111} of this mineral are not 

 all similar, but must be regarded as belonging to two di- 

 stinct tetrahedra : in the case of the first tetrahedron, for con- 

 venience of distinction termed the positive or o tetrahedron, the 

 faces are rough or striated, and are sometimes coated with oxide 

 of iron; on the other hand, the faces of the second or negative 

 or co tetrahedron are smooth, bright, free frdm this coating, and 

 in general smaller than the former. From this it follows 

 that the set of faces denoted above by the italic letters 

 abed, though similar to each other, are distinct in physical 

 character from those denoted by the greek letters a J3 y 8 ; 

 whence we infer that in such a growth as would be represented 

 by fig. 7, where there has been a simple rotation of one indi- 

 vidual through two right angles from a position of identical 

 orientation with the other, and adjacent faces of the two indi- 

 viduals thus bear respectively italic and greek letters, the com- 

 position-plane will be a plane of geometrical, but not of physical 

 symmetry. As, however, the correlative tetrahedra and also 

 the correlative hemiscalenohedra are independent of each 

 other, not only in surface-characteristics, but also in their pre- 

 sence on the crystal, even this geometrical symmetry could 

 scarcely be expected in the actual twin-growth. 



Now Sadebeck states that in the actual twin-growth the 

 composition-plane is really a plane of symmetry not only to 

 the geometrical, but to the physical peculiarities — the regular 

 composition thus belonging to the class called by Grroth 

 "symmetric twins;" that instead of the faces of the octahe- 

 dron which are parallel, or nearly so, in the two individuals 

 belonging in one to the positive, and in the other to the nega- 

 tive, they really belong either both to the positive or both 

 to the negative tetrahedron. To pass, therefore, to the actual 

 Phil. Mag. S. 5. Vol. 14. No. 88. Oct. 1882. U 



