Hemihedrism of Cuprite. 129 



and left. With the notation of Miller they may be denoted 

 respectively by <z{k h 1} and a{h k 1}. 



The faces x are therefore to be assigned to the half- form 

 a {9 8 6}, though, as may be seen from the limits of variation 

 in the above table, the numerical values of the indices cannot 

 be assigned with absolute precision. 



Fig. 1 represents the complete form {9 8 6}. 



Fig. 2 represents the half-form a{9 8 6^, obtained by reject- 

 ing the shaded faces, and is the half-form peculiar to cuprite. 

 This is Mohs' left pentagon- icositetrahedr on. 



Fig. 3 represents the half- form <z{8 9 6}, obtained by 

 retaining the shaded faces; this is Mohs' right pentagon- 

 icositetrahedron. 



It may be noticed that this mode of hemihedrism only 

 affects the form of the 48-faced figure {h k 1} ; so that in the 

 case of cuprite, in which such faces are very rare, it is not 

 surprising that the hemihedral character has hitherto escaped 

 attention. Hexakis-octahedron faces have been figured as 

 occurring holohedrally by W. Phillips *, and Kokscharow f , 

 but in neither case have indices been ascribed to the faces. 



Where such forms have been noticed upon specimens in 

 the Museum, they are so uneven and irregular that neither their 

 position nor their number can be satisfactorily determined. 



The cases in which this trapezohedral hemihedrism has 

 been already recorded are the following : — 



(1) TschermakJ observed small faces of a{l 8 5} on 

 artificial crystals of sal-ammoniac, upon edges of the form 

 p={211}. 



Calculated. 

 ^ = (785) p = (896) 



Observed. 



pi (121)= 13 10 

 jp:(211) = 21 



O I O / 



= 13 49-3 

 = 19 49-8 



= 13 197 

 = 20 13-8 



It is thus possible that the hemihedral faces of sal-ammoniac 

 and of cuprite may belong to the same form ; but it is remark- 

 able that the crystals of sal-ammoniac exhibit the right, 

 and those of cuprite the left half- form. 



(2) Ben-Saude§ obtained by corrosion with water upon 

 artificial crystals of mixed potassium and sodium chlorides 

 faces which were obliquely striated in such a way as to 

 suggest the presence of small asymmetric faces. 



These crystals also exhibited " double refraction in sectors." 



Lastly, it has been a question of interest whether this mode 



* Transactions of the Geological Society, vol. i. 1811, p. 37. 

 t Materialen zur Mineralogie Rusdands, i. 1853, p. 84. 

 % Min. und Petr. Mitth. 1881, p. 531. 

 § Bull. Soc. Min. de France, 1883, p. 260. 



Phil. Mag. S. 5. Vol. 18. No. 111. Aug. 1884. K 



