142 



Twinning is tolerably common in leucosphenite. The law 

 according to which the twins are formed is as follows: the 

 twinning plane is the third pinacoid с {OOl}; the 

 twinning axis is a normal tothis; one of the indi- 

 viduals is revolved 180° about the twinning axis. 

 As the salient and re-entering angles between the faces of the 

 form n on the sub-individuals deviate very little from 180°, 

 these twins are not easily distinguished from simple crystals 

 (Pig. 8, Plate Vil). The vertical axes of the two subindividuals 

 make Avith each other an angle = 2{ß — 90°) = 7° 16'. On 

 one of the twins (cryst. No. 4) the following determinations of 

 angles have been made. 



Measured 



Calculated 



d:d = ir 20' 



73° 22' 



n : n = 3° 24' 



3° 30' 



In colour leucosphenite is white inclining to greyish blue. 

 This shade does not, however, occur on crystal No. 8. Pure 

 crystals that have no cracks are often quite clear. Often, 

 however, the leucosphenite crystals are traversed by cracks, or 

 they contain inclusions of extraneous matter; in such cases they 

 ai'e somewhat opaque. The crystalline planes as well as the 

 fracture show a marked vitreous lustre. Certain faces, as those 

 of the forms b and n, often exhibit a pearly lustre. 



In accordance with the monoclinic nature of the mineral 

 sections oriented parallel to the first and the third pinacoids 

 show extinction parallel to the second pinacoid. But also a 

 section cut parallel to the last-mentioned pinacoid shows ex- 

 tinction parallel to the direction of the a-axis. At all events the 

 deviation, if such there be, is so small that it has not been 

 possible to ascertain it. On the other hand, one of the extinc- 

 tion-directions makes, of course, with the vertical axis an angle 

 = /9 — 1)0° = 3°21' in the plane of symmetry. This direction 

 lies in the obtuse angle ß. 



