232 Allen, Crenshaw, Johnston, and Larsen — 



perature for about three months before opening. A consider- 

 able number of small, twinned crystals similar to the measured 

 crystal and in no case over 0*15 mm in length were formed. 

 The measured crystal was similar in its development to crys- 

 tal 1, except that the pyramids were somewhat striated paral- 

 lel to the base and {16.0.16.7} was the dominant pyramid. 

 The reflections were not quite so good as those of the first 

 crystal. Six measurements of the polar angle of (16.0.16. 7) 

 varied from 66°_0' to 6Q° 31', averaging 66° 18'. The signals 

 for the face (2021) did not stand out sharply from the other 

 signals of its zone, but three measurements of the angle 

 between the points where the strings of signals for the pyra- 

 mid zones of the two individuals intersected gave 52° 17', 

 53°_32', and 52° 31', averaging 52° 58'. The polar angle of 

 (2021) is, therefore, 63° 31'. One good measurement of the 

 angle between the prism faces of the two individuals was 

 90° 11'. The angles in the prism zone may differ as much as 

 30' from 60°. 



The third preparation was formed as was the second but the 

 crystals differed in that the base was rather prominent and the 

 form {5051} was present. The habit of these crystals is 

 shown in fig. 23. The measured ciwstal which was less than 

 0*2 mm in length gave rather poor signals, and many of the 

 faces were very poor or missing. There was a continuous 

 line of signals from the prisms to the pyramids (20^1) and the 

 angle between the intersections of these zones belonging to 

 the two individuals measured 52° 31' and 52° 0\ averaging 

 52° 20'. Therefore the polar angle of (2021) is 63° 50'. The 

 angle between the bases of the two individuals measured 

 90° 7'. Three measurements of the polar angle of (16.0.16.7) 

 averaged 66° 25' with a maximum deviation of 19'. Five 

 measurements of the polar angle of (5051) averaged 78° 55 r 

 with a maximum deviation of 26'. The angles between the 

 prisms could not be measured accurately but tliev differed 

 from 60° by less than 20'. 



The data for the three measured crystals of /3-pyrrhotite are 

 assembled in Table XII. The table shows the principal inter- 

 facial angles as measured on each crystal and as computed 

 from the average value of the constant p Q , the faces observed 

 on each crystal and the value of the vertical axis c. There is 

 a close agreement between the measured and the computed 

 angles and it seems certain that the difference in the value of 

 p for the different crystals represents an actual difference in 

 the crystal constants. Their difference is easily accounted for, 

 as the amount of sulphur in pyrrhotite varies considerably. 



A comparison of the two forms of pyrrhotite. — The follow- 

 ing criteria for distinguishing between the two forms of 



