Mineral Sulphides of Iron. 



233 



pyrrhotite applied to all of the artificial crystals which were 

 examined and should serve as more or less reliable means of 

 distinguishing between the two forms in both natural and syn- 

 thetic crystals : 



1. The habit of /3-pyrrhotite is hexagonal and the dominant 

 forms are the prism, and a steep pyramid, and sometimes also 

 the base (figs. 22 and 23). a-pyrrhotite, on the other hand, is 

 always tabular parallel to the base ; a few of the crystals 

 appear to be hexagonal, but most of them are much elongated 

 in the direction of the a axis and the orthorhombic symmetry 

 is further shown by the common association of the forms { 001 \ , 

 1 100}, and {111} (figs. 17, 18 and 20). 



2. The low temperature form (/3) is almost invariably devel- 

 oped as cruciform twins with an angle of about 90° between 

 the two individuals (twinning plane 1011), while the high tem- 

 perature form (a) is usually twinned after (023) with the two 

 individuals at about 65° to each other, and sometimes also 

 after (021) with the two individuals at about 55° to each other. 



3. The constant p Q for the measured crystals of /3-pyrrho- 

 tite varied from 0*9967 to 1*0100, while for a-pyrrhotite it 

 varied from 0*9267 to 0*9927. While crystals for which the 

 value of p Q was approximately 1*0000 were found in both 



Table XII. 

 Crystal data of /3-Pyrrhotite. 



Polar angle 



of (2021) ._. 

 Angle between 



twins 



Polar angle 



of (5051) _. . 

 Polar angle_ 



of 16.0.16.7). 



Faces 



c . 



Crystal 1 



Observed 



63° 19' 

 89° 45' 



Computed 

 p - 0-9967 



63° 19' 

 89° 42' 



(1010) (2021) 



0*8632 



Crystal 2 



Observed 



63° 31' 



90° 14' 



Computed 

 po=l*004 



63° 32' 

 90° 14' 



66° 18' 66° 27' 



(1010) (2021) 

 (16.0.16.7) 



0*8695 



Crystal 3 



Observed 



63° 50' 



90° 7' 

 78° 55' 

 66° 25' 



Computed 



w=i-oi 



63° 40' 



90° 34' 

 78° 48' 

 66° 35 ; 



(1010) (2021) 

 (16.0.16.7) (0001) 

 (5051) 

 0*8742 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 195.- 

 16 



-March, 1912. 



