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The form 3O3, {311} on Ammonium Chloride (Sal Ammoniac) may have only six of its twenty-four planes developed, and in this way give rise to a form resembling a tetragonal trapezohedron.

Ammonium Chloride

The form 3O3, {311} on Ammonium Chloride (Sal Ammoniac) may have only six of its twenty-four planes…

This figure represents a crystal of iron pyrite showing representatives of all the forms of the isometric system except the rhombic dodecahedron: ∞O∞ {100} (P); O, {111} (d); [∞O2/2], π{102} (e); [4O2/2], π{214} (s); 2O2, {211} (o); and 3O, {133} (t). It is shortened in the direction of one of the principal axes, which, together with the fact that only two of the three faces belonging to the forms s, o, and t are developed in each octant, gives to it a decidedly orthorhombic habit.

Crystal of Iron Pyrite Showing Representatives of All the Forms of the Isometric System

This figure represents a crystal of iron pyrite showing representatives of all the forms of the isometric…

Cubes of Iron Pyrites frequently show a striation of their planes in one direction, which is perpendicular to the striation on all contiguous faces, owing to oscillatory combination with the pentagonal dodecahedron. This would not be possible in a holohedral cube.

Cube of Iron Pyrite

Cubes of Iron Pyrites frequently show a striation of their planes in one direction, which is perpendicular…

Curvature of crystal planes is quite a constant property of some substances. It may be due to a very fine oscillatory combinations or to an irregularity of growth and a want of perfect parallelism between sub-individuals. A common instance is the Diamond, whose faces, except those of the octahedron, are almost always curved.

Curvature of Crystal Planes (Diamond)

Curvature of crystal planes is quite a constant property of some substances. It may be due to a very…

Gold crystals of the form 3O3, {311}, sometimes resemble combinations of the rhombohedron and scalenohedron.

Gold

Gold crystals of the form 3O3, {311}, sometimes resemble combinations of the rhombohedron and scalenohedron.

Certain crystals of green fluorspar from Saxony, showing the form ∞ 03, {310}, have one half of their planes developed at the expense of the other half, so as to produce a hexagonal scalenohedron.

Green Fluorspar from Saxony

Certain crystals of green fluorspar from Saxony, showing the form ∞ 03, {310}, have one half of their…

As shown on this crystal of Magnetite, the octohedron, by the crowding out of two of its opposite faces, may become a rhombohedron.

Magnetite

As shown on this crystal of Magnetite, the octohedron, by the crowding out of two of its opposite faces,…

The icositetrahedron, 4O4, {411}, on potassium chloride sometimes produces an apparent rhombohedron by the survival of only one fourth of its planes. Two such rhombohedrons, in apparent twinning position, may be derived from the planes of the same icositetrahedron.

Potassium Chloride

The icositetrahedron, 4O4, {411}, on potassium chloride sometimes produces an apparent rhombohedron…

It has been observed that the pentagonal dodecahedron, [∞o2/2], π {201}, on pyrite may have but six of its twelve planes developed in such a manner a to produce an apparent rhombohedron.

Pyrite

It has been observed that the pentagonal dodecahedron, [∞o2/2], π {201}, on pyrite may have but six…

Striation of crystal planes may be produced by repeated polysynthetic twinning. This is well illustrated in the case of the triclinic feldspar, Albite. The twinning plane here is the brachypinacoid, and the contact of a large number of fine lamellæ, alternately in twinning position, would evidently produce a striation on the basal plane parallel to the brachydiagonal axis

Striation of Crystal Planes

Striation of crystal planes may be produced by repeated polysynthetic twinning. This is well illustrated…