476 



W. H. Melville — Diaspore 



Crystals. 



Brachypinacoid. 

 [100]. i-i 



[120], i-2 



Braehydome 

 [101], l-i 



Prisms 

 [HO], I 



[340], i-\ 

 [670], i-\ 



Pyramids 

 [111], 1 _ 

 [122], 1-2 



In the zone circle [111, 122] the mean of the most accurate 

 measurements are given* under I, and another series under II. 

 In almost all cases the signals reflected from the brachypinacoid 

 were duplicated, so that it was often necessary to calculate the 

 angle which this plane made with its adjacent planes from the 

 difference of the sum of the other angles in the zone and 180°. 





Measured. 









I. 



II. 



*Kokscharof. 



100 ^ 111 



63° 7 



63° 20^ 



63° 5£ 



111 ^ 122 



12° 41i 



12° 44 



12° 40 



122/, 122 



28° 23 



28° 23 



28° 29 



122 „ 111 



12° 41£ 



12° 39 



12° 40 



111 ^100 



63° 7 



63° 21 £ 



68° 5i 



180° 180° 28' 



In the zone [100, 101J the measured angles were 100 a 101 

 = 58° 50 / (1) and 58° 54' (2) ; mean 58° 52'. Tor this angle 

 Dana gives 58° 52J' and Marignac 58° 53'. The following are 

 the mean of the most reliable angles obtained in the zone of 

 prisms : 



22 670^340 3° 7£ 



100 

 340 



110 

 120 



46° 54£ 

 10° 20£ 



110 

 120 



670 

 120 



The axial ratio was calculated from the angles 100^110 = 

 46° 54J and 100^101 = 58° 52. 



d : 6 : c = 0-6457 : 1- : 1'0689 found. 

 = 0-64425 : 1 : 1-067 Dana. 



The second type consists of crystals which are white and 

 almost opaque. They are stout and often pre- 

 sent a pyramidal habit, as they are imbedded 

 in the rock, resulting from the combination 

 of the prism (120) and the octahedron (122). 

 The annexed figure illustrates this habit of 

 diaspore, and is drawn from a crystal which 

 was removed from the rock and measured. 

 The dominant forms are (120) and (122), while 

 a macrodome (011), the brachypinacoid (100), 

 and the octahedron (111) appear as small sec- 

 ondary planes. The crystals rarely show more than can be 

 seen in the drawing, because they have grown into each other 

 forming dense aggregates. The following angles were ob- 

 tained from this crystal : 



* The angles between normals are given in all cases. 



