BASTITE— DIALLAGE— DIOPSIDE 453 



BASTITE. 



Bastite or schiller-spar is niineralogically identical with hronzite and differs from this 

 latter only in external appearance. In colour it is of a pale greyish-green, and the sheen, 

 which varies between the metallic and the pearly type, is also green. The principal locality 

 is Baste in the Radauthal, near Harzburg, in the Harz. Single grains of the mineral, not 

 infrequently rather large in size, occur here embedded in a serpentine, which varies in colour 

 between dark green and black. The paler-coloured bastite with its metallic to pearly sheen 

 contrasts very well with the dark serpentine in which it is embedded. The latter, flecked 

 here and there with patches of sheeny bastite, forms a more decorative material than bastite 

 or hypersthene alone, for when a fragment of serpentine containing bastite is moved about, 

 the sheen does not disappear simultaneously from every patch of bastite but disappears from 

 some only to reappear on others. The mineral is usually cut with a slightly convex or 

 scutiform surface ; its application as an ornamental stone is very limited, however, and it is 

 used for the most part in the manufacture of small articles, such as snuff-boxes, letter- 

 weights, &c. 



DIALLAGE. 



Diallage is rather less closely related mineralogically to hypersthene than either bronzite 

 or bastite. In addition to the constituents of hypersthene it contains a large amount of 

 calcium, and it crystallises, not in the rhombic, but in the monoclinic system. It resembles 

 the other members of the pyroxene group so far considered in that it occurs most frequently 

 in iiTegular masses and possesses a definite plane of separation, which displays a shining 

 metallic sheen. In colour diallage may be dark brown, various shades of green including a 

 very pale greenish tint and a pale greyish shade ; it therefore shows more range of colour 

 than other members of the group. The sheen is usually of the same colour as the stone 

 itself ; on the darker stones it is more metallic in character, and on the lighter it is more 

 pearly. The rock known to petrologists as gabbro consists of diallage and felspar, and at 

 many localities it occurs so coarsely grained that the diallage individuals are large enough 

 for cutting ; the application of the mineral as an ornamental stone is, however, extremely 

 limited. Such coarse-grained gabbros are found, among other places, at Volpersdorf, near 

 Neurode, in Silesia, at Le Prese in Veltlin, and at many other places in the Western Alps ; 

 at Prate, near Florence, and elsewhere in the Apennines ; and in the island of Skye. 



DIOPSIDE. 



Among the minerals of the pyroxene group diopside is remarkable for its transparenc}- 

 and for the beauty of its colour, and on this account it is sometimes cut as a gem. It is a 

 silicate of calcium and magnesium, the composition of which is represented by the 

 chemical formula CaMg (Si03)2, and in which a portion of the magnesia is replaced by 

 ferrous oxide. 



Diopside crystallises in the monoclinic system, and the crystals usually have the form of 

 rather long prisms with an oblong cross section and with the edges replaced by narrow faces 

 of a rhombic prism. They always occur attached at one end, and may be terminated quite 

 irregularly at the free end, or there may be a greater or less number of obliquely placed 

 terminal faces. Twin-crystals are not infrequent, and iiTegular, columnar aggregates of 

 crystals are often met with. 



