The Three Allotropic Forms of Sulfur

The Three Allotropic Forms of Sulfur Sulfur occurs in three allotropic forms:

Top view of the S₈ ring	Side view of the S₈ ring

orthorhombic,
- The orthorhombic form is the most common crystalline form because it is the most stable form below 96^oC. It is a yellow, brittle solid which occurs in S₈ rings. It can be found in Sicily, Mexico, and some southern states of the USA.
monoclinic,
- Monoclinic sulfur is another crystalline form of sulfur. It has the same type of S₈ rings, packed in different ways to form their crystals. Monoclinic sulfur is stable only between 96^oC and 119^oC. It forms naturally as molten sulfur and gradually cools through that temperature range. Below 90^oC, monoclinic sulfur quickly returns to the orthorhombic form.
amorphous
- When heated to above 120^oC, sulfur becomes a liquid. When heated above 200^oC it becomes dark-coloured and viscous. This change is due to the breaking open of the S₈ rings to form chains and the atoms at the ends of the chains have electrons that are easily excited by absorption of light and cause the dark colour. The S₈ chains join up to form long chains which coil around one another giving the increase in viscosity.
  When the dark viscous liquid is quenched in cold water, the coiled molecules do not have time to re-form the S₈ molecules and to arrange themselves in orthorhombic crystals. The product is a soft, elastic material and is an example of a super-cooled liquid. It is difficult to maintain in its amorphous form because it gradually reverts to the more stable orthorhombic form.