Titanium is often present, and occasionally the mineral contains magnesium, nickel, &c. It is always strongly magnetic. Magnetite crystallizes in the cubic system, usually in octahedra, less commonly in rhombic dodecahedra, and not infrequently in twins of the " spinel type " (fig.
Lead monoxide is dimorphous, occurring as cubical dodecahedra and as rhombic octahedra.
It crystallizes in the cubic system, usually in cubes, pentagonal dodecahedra or octahedra, often of great beauty and perfection.
The felspathoid minerals, sodalite, haiiyne and nosean, which crystallize in isometric dodecahedra, are very frequent components of the phonolites; their crystals are often corroded or partly dissolved and their outlines may then be very irregular.
The "small stellated dodecahedron," the "great dodecahedron" and the "great stellated dodecahedron" are Kepler-Poinsot solids; and the "truncated" and "snub dodecahedra" are Archimedean solids (see Polyhedron).
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