[ 354 J 



XLVL Notice of Crystallographical Forms of Glaucodote. By 

 W. J. Lewis, M.A., Fellow of Oriel College, Assistant in the 

 Mineral Department, British Museum* . 

 [Plate L] 



THE mineral from Hakansbo in Sweden is found in large 

 crystals of metallic lustre and dull tin-white colour, 

 imbedded in towanite and pyrites. The crystals are, for the 

 most part, twins, most of them being twinned about the normal 

 to the face (01 1), so far not described, some about the nor- 

 mal to the prism-face (1 10). The specific gravity and the 

 prism-angle agree fairly well with those of Breithaupt's 

 acontite. 



Glaucodote. Acontite (Br.). 

 Sp. g-r. 5-985-6-18. 6-008-(r059.. 



Prism-angle . . . 69 32 69 31 



Brachydome-angle . 100 2 102 



The one discrepancy consists in the value of the angle of 

 the brachydome {101}, which from direct observation and a 

 calculation by the method of least squares involving all the 

 best angles measured, I make out to be 100° 2'. The angle 

 of the prism varies considerably in different crystals — 68° 57', 

 69° 6f, 69° 13', 69° 32', 69° 40' having been obtained on 

 fairly good specimens. The planes of the brachydome are 

 much striated, and do not allow of such precise determination. 

 Differences in the brachydome-angle, such as those found in 

 the prism-angle, have been observed, though much more limited 

 in extent. This variation of the crystallographic elements of 

 the mineral is probably to be accounted for by a variation of 

 the quantity of cobalt present. The angle of the prism ( = 67° 

 24') and the cleavage c, given in Miller's ' Mineralogy,' seem 

 to belong to some other mineral. They were not determined 

 by Professor Miller himself; and I have been unable to find 

 out whence they are taken. On the very large crystals are 

 found the forms {010}, {110}, {101}, {10 2}, {011} ; on 

 smaller crystals have been found the additional forms, {111}, 

 {1 2 2}, {2 1}. Fig. 1 (Plate I.) is a stereographic projection 

 of these forms. Such simple crystals as I have observed were 

 extended considerably in the direction of the edge of the bra- 

 chydome, but were all broken at one end. On one of these, 

 whose prism-angle measured 69° 19^, a second prism {160} 

 was observed. Its faces were small and not well developed. 

 Assuming the measured angle 69° 19^' for the fundamental 



* Communicated by the Crystallological Society, having been read 

 December 15, 1876. 



