236 Allen, etc. — Mineral Sulphides of Iron. 



The results of the different investigations are brought into 

 agreement by the recognition of the two forms of pjrrhotite 

 and also of the change in^> with a change in the composition 

 of the mineral. The early view of Streng* that pyrrhotite is 

 orthorhombic and belongs to the chalcocite group holds true 

 for a-pyrrhotite, while /3-pyrrhotite is hexagonal. Further 

 investigation may show that other members of these groups are 

 dimorphic. 



A further study of Table XIII shows that most of the eleven 

 crystals were probably formed as a-pyrrhotite. Crystals 1 to 

 3 have low values for j£> , tabular habits, and crystal 3, at least, 

 has an association which would indicate the high temperature 

 form. Crystal 2, in particular, is described as having a "decided 

 orthorhombic appearance" and the orthorhombic axis is shown 

 by the elongation along it and the development of the form 

 {221 1 but not {041 J, and of {Oil \ but not {112 J. Crystals 4 

 and 5 from Kongsberg might belong to either form, but their 

 tabular habit suggests a-pyrrhotite. Crystal 6 has the tabular 

 habit of a-pyrrhotite. Crystal 10 has a tabular habit and is, 

 in addition, associated with minerals which indicate the high 

 temperature form. The occurrence of crystals 7 and 8 in 

 meteorites places them as a-pyrrhotite. The hexagonal 

 development and the twinning of crystal 9 indicate /5-pyrrho- 

 tite, but the association with feldspar, etc., and the position of 

 the magnetic poles are not consistent with this view. 



Any list of the faces which have been found on a-pyrrhotite 

 will be somewhat uncertain since some of the measured crystals 

 might belong to either form, and, besides, the variable charac- 

 ter of p Q makes it impossible to assign a symbol to some of the 

 measured pyramids. The following list omits doubtful faces 

 and marks with a star those found only on synthetic crystals. 



c (001) / (012) j (061) r (221) 



b (010) 6(023)* rc (201) 5(331) 



a (100) W 01] ) 2( 114 ) w (623) 



m(110) h (021) v (112) 



I (310) i (041) p (111) 



* Loc. cit. 



