Mineral Sulphides of Iron. 



225 



yield of pyrrhotite crystals. They are strongly magnetic and 

 show polarity with the poles along the a axis. Many of them 

 are about 0'3 ram in length and about a fifth as thick and broad. 

 Twinning is rare and the base resembles a hexagon much elon- 

 gated along one diameter and delicately striated parallel to the 

 six sides. There are a very few hexagonal tablets and twinned 

 crystals similar to those of preparations 1 and 2. The six 

 measured crystals, I to !N, are all similar in habit to the crystal 



Fig. 20. 



Fig. 20. Artificial a-pyrrhotite showing the forms c {001}, b (010), p {111}, 

 h {021}, and g {011}. Twinned after (021). 



shown in hV. 18, and the dominant forms are J001 



,{010}, 



and 



| 111 J. One crystal showed also the form {021} poorly devel- 

 oped, and another crystal showed {091}. Fig. 19 is the stereo- 

 graphic projection of crystal I. The crystals were mounted 

 along the a axis. They did not, in general, give sharp signals 

 and measured angles between (001) and (010) were in some 

 cases out as much as 2° ; while the measurements for the faces 

 {111} were only a little better. 



A fourth preparation was formed by the action of H 2 S on a 

 solution of acid FeCl at a maximum temperature of 210°. 

 Nearly all of the crystals are tabular parallel to the base and 



