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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…

This figure shows a crystal of Rhodonite (manganese meta-silicate), whose forms are: (c), (b), (a), (n), (h), (o), and (s).

Rhodonite

This figure shows a crystal of Rhodonite (manganese meta-silicate), whose forms are: (c), (b), (a),…

An alternate combination of manganese meta-silicate (Rhodonite), showing forms: (c), (b), (a), (n), (h), (o), and (s).

Rhodonite

An alternate combination of manganese meta-silicate (Rhodonite), showing forms: (c), (b), (a), (n),…

This form shows a rhombic dodecahedron (d) combined with the trigonal tristetrahedron (q). It sometimes occurs on Zincblende.

Rhombic Dodecahedron Combined With the Trigonal Tristetrahedron

This form shows a rhombic dodecahedron (d) combined with the trigonal tristetrahedron (q). It sometimes…

Crystal; If the alternate planes of the hexoctahedron extend until the intersect, a new for will result. This form, the Pentagonal Icositetrahedron, is bounded by twenty-four similar but unsymmetrical sides. Naumann's symbol for this form is (m0n/2)r, y{lkh}; Miller's is (m0n/2)l, y{klh}; h>k>l. This form is known as 'right-handed' because it contains the right, top plane of the front, upper octant.

Right-Handed Pentagonal Icositetrahedron

Crystal; If the alternate planes of the hexoctahedron extend until the intersect, a new for will result.…

The tetragonal rutile grows upon the rhombohedral titanic iron, so that its prism of the second order, ∞ P ∞ (100), coincides with the basal pinacoid of the latter mineral, while its vertical axis has the direction of one of the intermediate lateral axes of the iron ore.

Rutile Growing Upon Titanic Iron

The tetragonal rutile grows upon the rhombohedral titanic iron, so that its prism of the second order,…

This figure shows a combination of forms sometimes observed on the silicate Scapolite, with the unit pyramid, (o); the prisms of the first and second orders, (M) and (b) respectively; and the pyramid of the third order, (s).

Scapolite

This figure shows a combination of forms sometimes observed on the silicate Scapolite, with the unit…

This is a crystal of Sodium Chlorate, exhibiting the tetrahedron (-o) and the pentagonal dodecahedron (p).

Sodium Chlorate

This is a crystal of Sodium Chlorate, exhibiting the tetrahedron (-o) and the pentagonal dodecahedron…

This figure represents a solid state where there is no regularity of molecular orientation, as in the case of amorphous substances.

Solid State Showing No Regularity of Molecular Orientation

This figure represents a solid state where there is no regularity of molecular orientation, as in the…

An example of sphenoidal crystallization is Iron-Copper-Sulfide (FeS_2_Cu), also known as chalcopyrite.

Sphenoidal Crystallization

An example of sphenoidal crystallization is Iron-Copper-Sulfide (FeS_2_Cu), also known as chalcopyrite.

This figure shows a contact twin of Spinel, one of the three simplest isometric holohedrons according to the "spinel law".

Spinel Contact Twin

This figure shows a contact twin of Spinel, one of the three simplest isometric holohedrons according…

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…

This figure shows an orthorhombic crystal of sulphur, showing the forms: (p), (s), (c), and (n).

Sulphur

This figure shows an orthorhombic crystal of sulphur, showing the forms: (p), (s), (c), and (n).

This figure shows a case of a supplementary twin, formed from two hemimorphic crystals, placed symmetrically to their basal pinacoid (calamine).

Supplementary Twin

This figure shows a case of a supplementary twin, formed from two hemimorphic crystals, placed symmetrically…

This figure shows two tetrahedrons with parallel axes, and symmetrically placed with reference to the faces of the cube. The example given is a tetrahedrite crystal.

Tetrahedrite

This figure shows two tetrahedrons with parallel axes, and symmetrically placed with reference to the…

This figure shows a penetration twin of two tetrahedrons, symmetrical to the octohedral face (tetrahedrite).

Tetrahedrite Penetration Twin

This figure shows a penetration twin of two tetrahedrons, symmetrical to the octohedral face (tetrahedrite).

This form shows a tetrahedron (o), cube (h), and dodecahedron (d) in combination.

Tetrahedron, Cube, and Dodecahedron in Combination

This form shows a tetrahedron (o), cube (h), and dodecahedron (d) in combination.

This crystal of Tourmaline exhibits hemimorphism in the direction of the vertical axis. Its forms are the prisms, (g), which as a result of hemimorphism, appear as a trigonal prism, and (a); the scalenohedron, (s), and the two rhombohedrons, (R) and (r), at the antilogue pole, while only (R) occurs at the analogue pole.

Tourmaline

This crystal of Tourmaline exhibits hemimorphism in the direction of the vertical axis. Its forms are…

This figure shows two twin rhombohedrons, symmetrical with reference to a prism of the first order, but without any composition face. There is a complete interpenetration of their substance, and the space common to both is very irregularly distributed between them, as may be shown by an examination in polarized light.

Twin Rhombohedrons

This figure shows two twin rhombohedrons, symmetrical with reference to a prism of the first order,…

This illustration shows the union of a pyramid and a prism of different orders.

Union of a Pyramid and a Prism of Different Orders

This illustration shows the union of a pyramid and a prism of different orders.

This illustration shows the union of a pyramid and a prism of the same order.

Union of a Pyramid and a Prism of the Same Order

This illustration shows the union of a pyramid and a prism of the same order.

This form is a cube with its edges unsymmetrically truncated by the faces of the pentagonal dodecahedron.

Unsymmetrically Truncated Cube

This form is a cube with its edges unsymmetrically truncated by the faces of the pentagonal dodecahedron.

A Urea (CH_4_N_2_O) crystal exhibiting Sphenoidal Crystallization.

Urea

A Urea (CH_4_N_2_O) crystal exhibiting Sphenoidal Crystallization.

This form shows a very complex combination of thirteen tetragonal forms occuring on the mineral Vesuvianite: the prism of the first order, (d); prism of the second order, (M); basal pinacoid, (s); three pyramids of the first order, (c), (b), and (u); the ditetragonal prism (f); and the four ditetragonal pyramids (z), (x), (e), and (a).

Vesuvianite

This form shows a very complex combination of thirteen tetragonal forms occuring on the mineral Vesuvianite:…

Also known as Fergusonite, this crystal shows the basal pinacoid, (c); the unit pyramid, (s); and the pyramid and prism of the third order, (z) and (r) respectively.

Yttrium Niobate

Also known as Fergusonite, this crystal shows the basal pinacoid, (c); the unit pyramid, (s); and the…

This figure shows a contact twin of Zinc blende, one of the three simplest isometric holohedrons according to the "spinel law".

Zinc blende Contact Twin

This figure shows a contact twin of Zinc blende, one of the three simplest isometric holohedrons according…

In the tetragonal system, the unit pyramid of the second order is the most common twinning plane. This is represented here as it occurs on crystals of Zircon, bounded by the forms (m), (u), (p), and (o).

Zircon (Twinning)

In the tetragonal system, the unit pyramid of the second order is the most common twinning plane. This…