Mineral Sulphides of Iron. 



223 



goniometer. The crystals are almost certainly orthorhombic 

 in symmetry, although the angles in the prism zone are very 

 nearly 60°. In the following discussion, the crystals are 

 treated as orthorhombic. The crystals are usually twinned 



Fig. 17. Artificial a-pyrrhotite showing the forms m {110}, b {010}, and 

 c {001}. Twinned after (023). 



Fig. 18. 



Fig. 18. Artificial a-pyrrhotite showing the forms 6 {010}, c {001}, and 

 P {HI}. 



with the twinning plane (023); one crystal is twinned only 

 after (021), and two crystals are twinned after both laws. 

 The twinned crystals are tabular parallel to the base and elon- 



