76 3 MINERALOGY. 
a small quantity of oxyde of iron and lime. It occurs in the Tyrol and 
Rhineland, being not very rare, especially in basaltic and volcanic rocks. 
24. Leucite (disilicate of potassa and alumina). 
This mineral is known by the form of its crystals, the shape shown in 
jig. 64, deriving from it the name of leucitohedron. It occurs in volcanic 
masses, as in the neighborhood of Rome and Vesuvius. It is generally of a 
grey color, and its crystals frequently exhibit considerable size and 
perfection. 
25. Analcime (disilicate of soda and alumina). 
Analcime is a hydrated silicate of soda and alumina. Its crystals are 
shaped like those of the preceding, and occur generally in hemispherical 
white crystals, as in the Tyrol, or in small transparent crystals of one or two 
lines in diameter, as shown in fig. 15, from the clay slate of Andreasberg in 
the Hartz. : 
26. Chabazite (silicate of soda, lime, potassa, and alumina) pl, 34 fig. 18. 
Chabazite is a hydrated silicate whose crystals belong to the hexagonal 
system (figs. 65 and 66.) It is semi-transparent, or translucent, of 
conchoidal fracture, and brittle. Stilbzte and heulandite are minerals allied 
to chabazite in chemical composition, consisting of lime, alumina, silica, 
and water. The crystals are highly pearly rectangular prisms, belonging to 
the trimetric system. Fig. 14, pl. 34, is taken from an Andreasberg 
specimen. Mesotype (fig. 16) and epistilbite (fig. 63) are allied minerals. 
27. Tourmaline. 
Common tourmaline, as it occurs in granite in the Hartz, is represented 
in fig. 4, pl. 34. This mineral is of great interest, owing to the optical and 
electrical properties of many of its varieties. The primary form of the 
tourmaline is an obtuse rhombohedron (fig. 34), under modifications of 
which the tourmalines occur of considerable size. The constitution of 
tourmaline is frequently very complex; it generally contains soda, lime, 
manganese, iron, alumina, silica, and boracic acid. 
28. Harmotome. 
This mineral exhibits very interesting forms of twin crystals. PJ. 35, 
fig. 5, illustrates a specimen from Andreasberg. The crystals are right 
rectangular prisms, terminated by square octahedron faces ; of these crystals, 
four unite at right angles, forming a cross. These sometimes unite at 
angles of 90°. This rare mineral consists of silicates of alumina and 
baryta, with water, potash, and lime. 
29. Albite (disilicate of soda and alumina). 
Albite or soda feldspar forms a component of many rocks, occurring in 
Siberia, in very large crystals. Their primary form is an _ oblique 
rhomboidal prism (p/. 34, fig. 65). Albite is generally of a white or grey, 
yellow, and red color; lustre vitreous ; iridescent on the cleavage surface ; 
lateral faces frequently striated. | 
30. Heldspar. 
Of all silicates, feldspar is the most important, as well as most generally 
diffused. It forms a constituent of many rocks, and consists of silicates of 
potassa and alumina; it crystallizes in forms of the monoclinic system 
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