ClassicGems.net

Twinning in Minerals

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:

• Boleite  (pseudo-cubic twins)
• Fluorite  (classic penetration twins)


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)
• Leadhillite  (pseudohexagonal trillings and Artini Law twins)
• Witherite  (pseudohexagonal pyramidal trillings)


Sulfates:

• Gypsum   (fishtail and dove-tail twins)
• Spangolite  ("hatchet" twins)


Phosphates:

• Amblygonite  (flattened elongated twins)
• Herderite  (fishtail twins)
• Monazite  (penetration twins)


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 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)