Mineral Sulphides of Iron. 



231 



sealed glass tube which contained a dilute solution of FeS0 4 

 saturated with H 3 S at 0° C. The yield of /3-pyrrhotite was 

 small and consisted of the measured crystal which was about 

 l mm in length, a crystal about half as large and a few crystals 

 less than 0'2 mm in length. All were twinned and otherwise 



Fig. 22. 



Fig. 23. 



Fig. 22. Artificial #-pyrrhotite showing the forms m {1010} and z {2021}. 

 Twinned after (1011). 



Fig. 23. Artificial /3-pyrrhotite showing the forms m {1010}, z {2021}, and 

 c{0001}. Twinned after (1011). 



similar to the measured crystal. This crystal, which was 

 mounted on the goniometer along the c-axis of one of the 

 individuals, gave blurred and multiple signals, but it was not 

 striated, and most of the faces could be located to within about 

 ten_minutes. The only forms developed were the prisms 

 { 1010} and the steep pyramids {2021}. Figure 22 is the cli- 

 nographic projection of the crystal. Six fairly good measure- 

 ments of the ancrle (2021U (2021) gave values of 53° 29', 53° 30', 

 53° 31', 53° 6',"53° 15', 53° 41', averaging 53° 22'; hence the 

 polar angle of (2021) is 63° 19'. Two good measurements of 

 the angle between (1010) and (1010) were 89° 41' and 89° 49', 



averaging 89° 45'. The twinning plane is therefore 1011. 

 The angles measured between the prisms were never over 30' 

 from 60°. 



The conditions of formation of the second crystal were simi- 

 lar to those of the first but the maximum temperature was 

 about 100°* and the tube was allowed to stand at room tem- 



* We do not know the inversion temperature of FeS with an excess of 

 sulphur and can not, therefore, be certain that this and the following crys- 

 tals were formed below that temperature. Their habit, however, is evidence 

 that they belong to the low temperature form. 



