8. L. Penfield — Crystalline form of Sperrylite. 71 



Aet. IX. — On the Crystalline form of Sperrylite ; by 

 S. L. Penfield. 



The crystalline form of sperrylite is isometric; pyritohe- 

 dral. Simple cubes are common, octahedrons are exceptional, 

 while the majority of the crystals, which are usually fragmen- 

 tary, show combinations of cube and octahedron. The first 

 crystal which was selected for measurement was a fragment 

 showing the above mentioned combination ; one of its central 

 octahedral faces being imperfect, the best measurements were 

 obtained from a cubic to an adjoining octahedral face. The 

 results, which are given below, are very satisfactory consid- 

 ering the small size of the crystals, and prove that the mineral 

 is isometric ; it may also be said that where the reflections were 

 sharpest and best the values came nearest to the theoretical. 









Calculated. 



a*o 001 ^ 111 



54° 



34' 



54° 44' 



" 001 ^ 111 



54 



46 



" 



" 100 A 111 



54 



35 



" 



" 100^111 



54 



45 



it 



a^a 100^.001 



90 



2 



90° 



At first only the above mentioned forms were detected, but 

 on sifting off the smallest crystals and carefully looking over 

 the largest ones some were detected which suggested pyrite 

 forms. The chemical relation of the mineral PtAs 2 to the 

 minerals of the pyrite group caused me to make a very careful 

 search for pyritohedral forms, which was fortunately successful. 

 Cubes with replacement of the edges are very exceptional ; a 

 number of them were found, however, and in all cases the re- 

 placements, which were necessarily small and frequently failed 

 on some of the edges, had the arrangement required by the 

 combination of cube and pyritohedron. The best crystal se- 

 lected for measurement was the top of a cube measuring 35 X 

 0.4.5mm \ n combination with octahedron and two small but well 

 developed pyritohedral faces ; the latter gave very good reflec- 

 tions. The measured angles are 



Calculated. 

 i-i^i-2 001 ». 102 26" 28' 26° 34' 



" 001 ~ 102 26 31 



Another crystal which w#s carefully measured was an irregu- 

 lar one measuring 0*35 and 0*55 mm in two diameters; this 

 was developed in all directions ; in one zone the four cubic and 

 four pyritohedral faces were all present in their proper order 

 and gave satisfactory measurements, in a second zone four cubic 

 and two pyritohedral faces were found and in the third zone 

 four cubic and one truncating rhombic dodecahedral face 



