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"Triclinic. Twinning very common, according to the albite law and evidenced by fine striation lines on the better cleavage surface." — Ford, 1912

Albite

"Triclinic. Twinning very common, according to the albite law and evidenced by fine striation lines…

"Simple orthoclase crystal showing cleavage lines and the position of the twinning plane in a Carlsbad twin." -Johannsen, 1908

Orthoclase Crystal

"Simple orthoclase crystal showing cleavage lines and the position of the twinning plane in a Carlsbad…

"Albite and Carlsbad twinning combined." -Johannsen, 1908

Twinning

"Albite and Carlsbad twinning combined." -Johannsen, 1908

This image represents a twin of the monoclinic mineral Gypsum. Its twinning plane is the orthopinacoid.

Gypsum (Twinning)

This image represents a twin of the monoclinic mineral Gypsum. Its twinning plane is the orthopinacoid.

This image represents a twin of the monoclinic mineral Orthoclase. Its twinning plane is the orthopinacoid.

Orthoclase (Twinning)

This image represents a twin of the monoclinic mineral Orthoclase. Its twinning plane is the orthopinacoid.

This crystal of Aragonite shows a method of twinning where the twinning plane may remain parallel to itself, so that the alternate individuals are in parallel position. This is called Polysynthetic Twinning.

Polysynthetic Twins

This crystal of Aragonite shows a method of twinning where the twinning plane may remain parallel to…

This crystal of Aragonite shows a method of twinning where the twinning plane may change its direction, as when the two symmetrical faces of a prism successively serve as twinning planes.

Cyclic Twinning

This crystal of Aragonite shows a method of twinning where the twinning plane may change its direction,…

This figure shows an example of cyclic twinning on a crystal of Rutile.

Cyclic Twinning on Rutile

This figure shows an example of cyclic twinning on a crystal of Rutile.

This figure shows an example of Compound Twinning, where two orthoclase twins are twinned in parallel to their basal pinacoid. Compound Twins are produced by the presence of two or more twinning laws in the same group.

Compound Twinning

This figure shows an example of Compound Twinning, where two orthoclase twins are twinned in parallel…

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

Copper Contact Twin

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

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…

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…

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).

A penetration twin of two pentagonal dodecahedrons (known as the iron cross), whose twinning plane is the rhombic dodecahedron, is shown in this figure (Pyrite).

Penetration Twin of Pyrite

A penetration twin of two pentagonal dodecahedrons (known as the iron cross), whose twinning plane is…

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…

The Copper Sulphide, Chalcocite, forms three kinds of twins symmetrical to ∞P, {110}, to 4/3P∞̆, {043}, and to 1/2P, {112} respectively. The latter is shown in this figure.

Chalcocite

The Copper Sulphide, Chalcocite, forms three kinds of twins symmetrical to ∞P, {110}, to 4/3P∞̆,…

Orthorhombic Iron Disulphide (Marcasite) sometimes shows cyclic groups of five individuals, bounded by ∞ P, {110} (M); P∞̆, {011} (l); and 0P, {001}, and united into a pentagonal figure by (M) as twinning plane.

Marcasite

Orthorhombic Iron Disulphide (Marcasite) sometimes shows cyclic groups of five individuals, bounded…

In the orthorhombic system, tabular crystals of lead carbonite (cerussite), bounded by the forms ∞P∞̆, {010} (b); ∞P, {110} (m); and P, {111} (p), form groups in which both (m) and ∞P3̆, {130} act as twinning planes.

Cerussite

In the orthorhombic system, tabular crystals of lead carbonite (cerussite), bounded by the forms ∞P∞̆,…

Iron Sulpharsenide (Arsenopyrite), bounded by ∞P, {110} (M), and 1/3P∞̆, {013} (r), forms penetration twins of two individuals, where P∞̄, {101} acts as twinning plane.

Arsenopyrite

Iron Sulpharsenide (Arsenopyrite), bounded by ∞P, {110} (M), and 1/3P∞̆, {013} (r), forms penetration…

The most common twinning plane is the orthopinacoid ∞P∞̄ , {100}, as may be seen in this example of malachite.

Malachite

The most common twinning plane is the orthopinacoid ∞P∞̄ , {100}, as may be seen in this example…

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…

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 illustration shows a solid state where the orientation is only partial, as in the case of twin crystals. Here, the molecules of each horizontal row have two of their axes parallel with those above and below them, but not the third.

Partial Orientation of Molecules in a Solid

This illustration shows a solid state where the orientation is only partial, as in the case of twin…