How does “watermelon tourmaline” form?
Tourmaline is one of many accessory minerals frequently found in granitic rock. Wikipedia puts it nicely – ‘Tourmaline is a complex silicate of aluminium and boron, but because of isomorphous replacement (solid solution), its composition varies widely with sodium, calcium, iron, magnesium, lithium and other elements entering into the structure.’ After most of the constituent magma has solidified certain gasses and liquids are left behind. These gasses and fluids are often enriched with elements like boron or fluorine as well as the others mentioned above. These are released and escape through fractures in the rock (still at c. 500 C/900 F), and change the minerals they come into contact with on the way. Hot gasses usually result in tourmaline-rich rock. Watermelon tourmaline are lithium-rich. Lithium can produce almost any colour of tourmaline, and twin colours are not uncommon, reflecting variations of fluid/gas chemistry during crystallisation.
