Carbonates, such as chalk and limestone, are rather soft minerals and may lose at least some mechanical integrity after undergoing stimulation. For instance, carbonate minerals may experience proppant embedment or asperity weakening after fracturing or matrix acidizing. Methods for hardening a carbonate mineral may comprise: introducing an aqueous carrier fluid comprising an ammonium phosphate salt into a subterranean formation comprising a carbonate mineral, and interacting the carbonate mineral with the ammonium phosphate salt to convert at least a portion of the carbonate mineral into a hydroxyapatite mineral. The subterranean formation may exhibit an increased hardness after forming the hydroxyapatite mineral. The ammonium phosphate salt may be diammonium hydrogen phosphate in some examples.