358 



M. E. Bertrand on the Law of Twinning 



Von Lang {Mineral. Mittheil. Tschennak, 1871). I have also 

 published a note on this mineral in the Annates des Mines, 

 vol. iii. 1873 ; but up to the present time two interesting 

 facts concerning it have not been noticed. It is known that 

 leucophane has a very good cleavage parallel to the base, 

 p = 1, and that the acute bisectrix of the optic axes is per- 

 pendicular to this cleavage. With a sufficiently large plate it 

 is easy to obtain with the polarizing microscope two direc- 

 tions apparently at 90° to one another, in which the hyperbolas 

 and the lemniscates are perceived. 



In certain cases one finds two crystals separated by a single 

 plane of junction which have the planes of their optic axes 

 orientated at about 90° to one another. In other cases one 

 observes a -series of very narrow bands corresponding to as 

 many crystals of which the axes are situated in planes making 

 angles of nearly 90° with one another. By an examination 

 of a great number of cleavages of leucophane, I have succeeded 

 in establishing that these twins are very frequent, especially 

 in plates of some magnitude. I shall describe a very good 

 twin crystal of this kind, which, moreover, presents another 

 peculiarity. It consists of two crystals placed at about 90° to 

 one another, the two bases lying in one and the same plane. 

 Each of these crystals shows two well-developed faces, 

 b l = l 12, truncating two edges parallel to the base; the two 

 other edges are not truncated in the large crystal ; but in the 

 smaller one faces b* = l 1 1 are found truncating these edges. 



Is, then, leucophane hemihedral, and indeed doubly so, as 

 is the case with edingtonite ? To prove this, the observation 

 of a large number of crystals will be necessary; and unfortu- 

 nately crystals of leucophane are very rare. I have examined 

 a crystal in the collection of Mr. Adam, which presents the 

 same hemihedrism : two edges parallel to the base are modi- 

 fied by b l =l 1 2, and one of the two others by b =111. 



