Stibnite, Costerfield, Victoria

Mineral Matters #361

This week’s Facebook Mineral of the Week Group’s selected theme is crystals with curved habits.

Curved or bent stibnite is not uncommon. It has a layered orthorhombic crystal structure and within these layers, strong covalent bonds hold antimony (Sb) and sulphur (S) atoms together.

The bending in stibnite crystals is a result of a specific crystallographic phenomenon called translation glide. This works through:

• Weak van der Waals Forces: The layers in stibnite are held together by weak van der Waals forces, significantly weaker than the covalent bonds within the layers.

• Shear Stress: When stress is applied to a stibnite crystal in a specific direction (parallel to the layers), it can cause these weak van der Waals forces to be overcome.

• Layer Movement: Under this shear stress, the entire layers of atoms can essentially slide slightly past each other in a specific direction (translation). This "gliding" movement allows the crystal to bend without breaking the stronger covalent bonds within the layers.

The photo below shows an interesting bent crystal found by me on the mullock heap area near the main shaft of the Costerfield mine. When you look closely, you can also see that the main crystal is also splitting apart.

Below: Stibnite, Costerfield, City of Greater Bendigo, Victoria, Australia. Width of view 3.5mm.