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Twinning
is a phenomenon that occurs during crystal growth
where an error in the stacking of the atomic layers
causes an irregularity in the normal sequence. Normally,
the atomic layers are stacked like ABC-ABC-ABC-ABC etc.
But when the twinning iregularity occurs, a C layer
instead of a B layer is placed next to an A layer resulting
in ABC-ABC-ACB-ACB-ACB stacking.
Where this mistake occurs, a mirror plane is produced.
Some
examples of minerals that can form twins along with
their typical twinning styles:
Elements:
- Diamond
(Spinel Law and penetration twins)
Sulfides:
- Arsenopyrite
(cross-shaped twins)
- Bournonite
("Cog Wheel" twins)
- Chalcocite
(pseudohexagonal cyclic twins)
- Chalcopyrite
(penetration twins)
- Cinnabar (pentration twins)
- Dyscrasite
(pentration twins)
- Enargite
(star shaped cyclic twins)
- Galena (Spinel
Law twins)
- Loellingite
(penetration twins)
- Marcasite
(polysynthetic twining and "Cockscomb" twins)
- Pyrite ("Iron
Cross" twins)
- Sphalerite
(complex; involving penetration and contact twinning)
- Stibnite
(bent angles on elongated crystals)
- Tetrahedrite
(penetration twins)
- Wurtzite
(fourling twins)
Halides:
Oxides:
- Cassiterite
(classic trillings)
- Chrysoberyl
(hexagonal trillings)
- Rutile
(classic
eightlings and sixlings, "sagenite" lattice twinning and "elbow" twins)
- The Spinel
Group
minerals (Spinel Law twins)
Carbonates:
- Aragonite
(pseudohexagonal trillings)
- Calcite
(four
different contact twins)
- Cerussite
(pseudohexagonal trillings and chevron twins; the very best reticulated twin
structures)
- Leadhillite
(pseudohexagonal trillings and Artini Law twins)
- Witherite
(pseudohexagonal pyramidal trillings)
Sulfates:
- Gypsum
(fishtail and dove-tail twins)
- Spangolite
("hatchet" twins)
Phosphates:
Silicates:
- Chabazite
(simple and complex penetration twins)
- Epididymite
(six sided tabular trillings and fish-tail twins)
- Eudidymite
(tabular star shaped twins)
- The Feldspar Group
Group of Minerals:
- Albite
(Manebach, Carlsbad, Baveno and of course Albite Law lamellar
twins)
- Andesine
(Manebach, Carlsbad and Albite Law lamellar twins)
- Labradorite
(Albite Law lamellar twins)
- Microline
(Amazonite) (Manebach, Carlsbad and Albite Law lamellar twins)
- Oligoclase (Manebach, Carlsbad and Albite Law lamellar twins)
- Orthoclase (Manebach, Carlsbad and Albite Law lamellar twins)
- Sanidine (Manebach, Carlsbad and Albite Law lamellar twins)
- Harmotome
(Stempel, Perier and Marburg Law twins forming complex cross-like penetration
twins)
- Muscovite
("star" twins)
- Neptunite
(penetration twins)
- Phenakite (penetration twins)
- Phillipsite
(Harmotome-like
twins)
- Quartz (Japan
Law, Dauphine Law, Brazil Law and others)
- Sphene (Titanite)
(contact twins)
- Staurolite
(cross-shaped twins gave it its name)
- Tridymite
(six rayed trillings)
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