Mineral Sulphides of Iron. 



219 



The common habit of the synthetic marcasite is shown in 

 clinographic projection in fig. 15. The crystals are twinned 

 with the face m (110)* as twinning plane, they are tabular par- 

 allel to the twinning plane and elongated along the vertical 

 axis. The prisms m {110}, especially the large faces parallel 

 to the twinning plane, are deeply striated parallel to the base. 

 The domes e \ 101 \ and I { Oil \ are also prominent. Crystals of 

 a second habit, which are common in some preparations, are 

 more symmetrical in their development and less often twinned ; 

 in them the domes, e {101} and I { Oil}, are characteristic forms 



Fig. 15. 



Fig. 16. 



Fig. 15. Artificial marcasite showing the forms m{110}, e {101}, and 

 2 {Oil}. Twinned after (110). 



Fig. 16. Artificial marcasite showing the forms m {110}, e {101}, Z {011}, 

 and s{lll}. 



and the striated prisms, m {110}, are usually prominent. The 

 crystal represented in fig. 16 is of this habit, but the prisms 

 and pyramids s { 111 } are more prominent than usual. Several 

 crystals were seen under the microscope which resemble the 

 fiveling pictured by Dana.f A preparation formed as usual, 

 but at room temperature, consists of strings of minute crystals 

 which have a rhombic outline under the microscope, and some- 

 times the obtuse angle of the rhombs is truncated by a face 

 which is probably the prism, m (110). 



*Throughout this paper parentheses are used to indicate crystal faces ; 

 brackets, to indicate the entire crystal form, 

 f System of Mineralogy, 6th edition, p. 95. 



