358 Merwin — Simultaneous Crystallization of Calcite, etc. 



of pyrite completely encircling marcasite were found. An 

 intermediate stage is shown in fig. 1, D. The orientation of 

 the two minerals is such that the vertical axis of the marcasite 

 coincides with one of the cubic axes of the pyrite, and the 

 lateral axes are turned counter-clockwise nearly 53° from the 

 other cubic axes so that the prism face 110 nearly coincides 

 with the cube face 010. This orientation has been observed in 



Fig. 2. 

 A B 



9 



o+ 



o © I o*-o d© b 



d + + d 



to o + 



112 112 



m o 



two other occurrences of these minerals.* Either mineral may 

 be the later. Sadebeck* has observed another orientation in 

 which the turning of the lateral axes is 45°. In the first 

 orientation, sections of the two minerals parallel to the faces 

 upon which common growth began, show similar outlines, but 

 crystals of the two minerals having faces of the dominant 

 forms or forms with simple indices can not be made to come 

 very near to coincidence. In the second orientation there 

 would be more nearly coincidence between the crystals. The 

 stereographic projection, fig. 2, B, shows the arrangement of 

 the faces of some forms in the second orientation. The circles, 

 larger letters, and numerals are used for marcasite. The two 

 minerals approach closely to an intermediate tetragonal or 

 orthorhombic mineral having axial ratios about equal to those 

 of the octahedron set up as the tetragonal pyramid (101). 

 Apparently no great change in the distribution of crystallo- 

 graphic forces would be required to cause the transformation 

 from one of these forms of iron disulphide to the other. In 



*A. Sadebeck, Zs. Kryst., iii, 628 (abs.); and C. 0. Trechmann, Min. 

 Mag., ix, 209. 



