what product is made, other than carbon dioxide, when magnesium carbonate undergoes thermal decomposition?

The other product formed is magnesium oxide (MgO).

When magnesium carbonate is heated strongly, it undergoes thermal decomposition according to the word and symbol equations:

• Magnesium carbonate → magnesium oxide + carbon dioxide

• MgCO3→MgO+CO2\text{MgCO}_3\rightarrow \text{MgO}+\text{CO}_2MgCO3​→MgO+CO2​

So, apart from carbon dioxide, the solid product made is magnesium oxide.