230 



Allen, Crenshaw, Johnston, and Larsen — 



Table XI. — Computed interfacial angles of a-pyrrhotite for different 

 values of the axial ratios. 





c = 0-9267 



c = 0-9927 



(001) (Oil) 



42° 49' 



44° 47' 



(112) 



42° 42' 



44° 40' 



(021) 



61° 39' 



63° 16' 



(111) 



61° 36' 



63° 13' 



(010) 



90° 0' 



90° 0' 



(110) 



90° 0' 



90° 0' 



(100) 



90° 0' 



90° 0' 



(023) 



31° 42' 



33° 30' 



(221) 



74° 52' 



75° 50' 



(010) (110) 



59° 55' 



59° 55' 



(100) (100) 



90° 0' 



90° 0' 



Angle between 







twins after (023) 



63° 24' 



67° 0' 



Angle between 







twins after (021) 



56° 42' 



53° 28' 



preparations were made up largely of crystals of high-temper- 

 ature pyrrhotite, and, although no measurable crystals were 

 found, the microscopic study showed that they were very simi- 

 lar in habit to the measured crystals. On the other hand, the 

 crystals from the three preparations which were formed at 

 100° C. and below, invariably had a very different habit (p. 23 1 ). 



p- Pyrrhotite. 



Three crystals of pyrrhotite formed below the inversion 

 temperature were measured. They were each from a different 

 preparation and should give a fair idea of the crystal habit and 

 crystal constants of /3-pyrrhotite. Their habit and the measured 

 angles afford good reasons for supposing that /3-pyrrhotite is 

 hexagonal. The most prominent faces recognized were those 

 of m'{1010} and z {202lj, but those of c |0001( were rather 

 prominent on one crystal, and the vicinal form z a {16.0.16.7 \ 

 on two crystals; the faces of the form x {5051} were subordi- 

 nate on _one crystal. All three of the crystals were twinned 

 with (1011) as the twinning plane, giving cruciform twins with 

 an angle of approximately 90° between the two individuals. 

 Figures 22 and 23 are clinographic projections of the two habits 

 of the crystals. 



The best crystal came from a preparation which was formed 

 by heating for 8 days at a maximum temperature of 80° C. a 



