Realgar is an orange-red mineral used as a pigment in ancient times. Pararealgar is orange-yellow. They have different colours because they have different crystal structures, despite having the same empirical formula, AsS. The crystal structures of realgar and pararealgar contain differently-connected As4S4 molecules − these are shown above, realgar to the left, pararealgar to the right.
Colour by Design is the eleventh unit in the Salters Advanced Chemistry course.