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FΟRM OF MINERALS.
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The following is a description, with figures, of the different forms above mentioned, beginning with The Holohedric forms.
1.
The hexahedron or cube (fig. 1) is bounded by six equal squares, has
twelve edges, formed by faces meeting at 90°, and eight trigonal
angles. The principal axes join the centre points of any two opposite
faces.—Examples are fluor spar, galena, boracite.
2.
The octahedron (fig. 2), bounded by eight equilateral triangles, has
twelve equal edges, with planes meeting at 109° 28', and six tetragonal
angles. ' The principal axes join the opposite angles, two and
two.—Example, alum, spinel, magnetic iron ore.
3. The rhombic-dodecahedron (fig. 3) is bounded by twelve equal and similar rhombs (diagonals as 1 and
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has
twenty-four equal edges of 120°, and six tetragonal and eight trigonal
angles. The principal axes join two opposite tetragonal angles.—Ex.,
garnet, boracite.
4.
The tetrakishexahedrons (variety of icositetrahedron, fig. 4) are
bounded by twenty-four isosceles triangles, arranged in six groups of
four each. They have twelve longer edges which correspond to those of
the primitive or in-
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