Abstract. Many natural secondary arsenates contain a small fraction of phosphate. In this work, we investigated the olivenite–libethenite (Cu2(AsO4)(OH)–Cu2(PO4)(OH)) solid solution as model system for P–As substitution in minerals. The synthetic samples spanned entire range from pure olivenite (Xlib=0) to libethenite (Xlib=1). Acid-solution calorimetry determined that excess enthalpies are non...