Mr Haidinger on the Socialite of Vesuvius. 223 
spar, and augite ; the latter of various tints of colour, and often 
even perfectly white and transparent. They possess the form of 
dodecahedrons in combination with the hexahedron, elongated in 
the direction of one of the rhombohedral axes, as represented 
in Plate VI. Fig. 3. Very often, as in Fig. 4, two of them 
are joined in a regular composition, parallel to a plane, which is 
perpendicular to one of the longitudinal faces of the dodecahe- 
dron, and at the same time parallel to the lengthened axis. lie- 
entering angles are thus produced, contiguous to the apices of 
the lengthened groups. This kind of composition is not un- 
common in red silver and other rhombohedral species, but its 
appearance is somewhat different from what is generally observed 
in the tessular forms, in which the junction of the individuals more 
frequently takes place in the direction of a face of the octahe- 
dron, as in fluor, in blende, in spinelle, and others. Those 
crystals, which I could detach, were too imperfect to allow of 
being measured to the last degree of accuracy, in order to ascer- 
tain whether all the angles of the two forms be really = 90° 
and 120°; nor could any thing more to that effect be inferred 
from any particular symmetrical character in the distribution of 
additional facets. Fig, 5. shews the most complicated of these 
crystals, but its greater portion is engaged partly in another in- 
dividual, partly in the surrounding matrix. If completed on all 
sides, it would produce Fig. 6., which represents the combina- 
tion of the hexahedron, of the dodecahedron, and of a tetrago- 
nal icosi tetrahedron. It is the more probable that these forms 
belong to the tessular system, as there are also crystals which 
shew this kind of composition in the perfectly white sodalite 
from Vesuvius, a variety which I long ago saw in the public 
collection of the lohanneum at Gratz, but which is likewise in 
the cabinet of Mr Allan. In some of these white crystals the 
composition is repeated parallel to all the faces, which are per- 
pendicular to the lateral planes of the dodecahedron, and pass 
through the axis, and then the appearance of Fig. 7 . is pro- 
duced. 
Cleavage takes place pretty distinctly parallel to the faces of 
the dodecahedron, but it is much interrupted by conchoidal 
. fracture. The surface of some of the crystals is rather smooth, 
but does not possess much lustre, which is vitreous; more gene- 
o 2 
