In the chemical equation \( \text{MgCl}_2 + \text{Li}_2\text{CO}_3 \rightarrow \text{MgCO}_3 + 2\text{LiCl} \), the precipitate is \( \text{MgCO}_3 \) (magnesium carbonate).
Magnesium carbonate is typically insoluble in water, which means it will form a solid precipitate when the solution contains both magnesium ions \( (\text{Mg}^{2+}) \) and carbonate ions \( (\text{CO}_3^{2-}) \). In contrast, lithium chloride \( ( \text{LiCl} ) \) remains soluble in the solution.