124 Mr HAIDINGER'S Mlneralogical Account of 



masses. When broken or cleaved in the direction of Pr + <*, 

 and exposed to the light of the sun, minute splinters are often 

 observed, which, by transmitted light, appear of a bright brown 

 colour, so that the mineral cannot be said to be absolutely 

 opaque. 



Brittle. Hardness =. 4.0... 4.25, a little higher than fluor. 

 Specific gravity 4.328, of a number of fragments of crystals ; 

 4.312, in another experiment, of a single crystal of consider- 

 able size. 



Compound Varieties. Twin-crystals, formed in two different 

 manners. In the first of them, the axes of the two individuals 

 are parallel, dependant on the hemi-prismatic character of the 

 combinations of the species ; in the second, they are inclined. 

 1 . Face of composition parallel to Pr -f- oo ; axis of revolution 

 perpendicular to it. Fig. 9. If we did not give attention to the 

 compound state of this variety, shewn in the present instance by 

 the groove along the place of junction, which is not always vi- 

 sible, we might be induced to believe that it possesses a hemi- 

 prismatic character, referred to an axis inclined upon the base 

 of the fundamental pyramid, which is not the case. One can 

 generally trace the peculiar disposition of the crystalline faces 

 upon each of the individuals. A repetition of this law pro- 

 duces thick prisms, terminating perpendicularly upon their axis 

 by a rough face, which consists of the apices of numerous indi- 

 viduals, or rather of numerous particles of two individuals, alter- 

 nating with each other. Such faces are not uncommon in the 

 prismatoidal manganese-ore. 2. Axis of revolution perpendicu- 

 lar, face of composition parallel to a plane of Pr. Fig. 10. The 

 disposition of the faces marked c, upon which the hemi-prisma- 

 tic character of the species depends, is such, that a mere revo- 

 lution of 180 is not sufficient to bring the two individuals into 

 the position required for joining in a regular twin ; though the 

 general disposition takes place also in the present instance, the 



