Which one of the following is not an example of calcination?
\( \mathrm{CaCO_3 \cdot MgCO_3 \xrightarrow{\Delta} CaO + MgO + 2CO_2} \)
\( \mathrm{Fe_2O_3 \cdot xH_2O \xrightarrow{\Delta} Fe_2O_3 + xH_2O} \)
\( \mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2} \)
\( \mathrm{2PbS + 3O_2 \xrightarrow{\Delta} 2PbO + 2SO_2} \)