When ferrous sulphate crystals [FeSO4.7H2O] are heated, the water of crystallisation is lost first. Then the salt decomposes giving red brown ferric oxide [iron[III]] and sulfur dioxide and sulfur trioxide gases.
2FeSO4 ----> Fe2O3 + SO2 + SO3
Ferrous sulphate crystals have the common name Green vitriol and it is normally called iron[II] sulfate crystals. Although this is the 'correct ' modern name, some coutries and teachers are still using the older system.