A study of the system Calcium orthophosphate-calcium chloride in air at atmospheric pressure shows that synthetic chlor-spodiosite melts incongruently at 945° C ± 5° and that a binary eutectic Ca2Cl(PO4) - CaCl2, i.e. chlor-spodiosite-hydrophilite, exists at 760° ±5°. Above 1100° C rapid vapourisation of the calcium chloride and the formation of a chloroxyapatite (chlorvoelckerite) prevent the use of the present techniques. The structure of chloroxyapatite is distinct from that of chlorine-rich apatite and is apparently very similar to that of certain synthetic mixed-apatites described by Chaudron and Wallaeys (1949). A natural voelckerite with a low chlorine content has been recently found. Artificial chlor-oxyapatite readily forms when pure chlorapatite is heated in air at 1400° C. Experiments performed so far indicate that a loss of chlorine from chlorapatite in air takes place slowly at temperatures as low as 980° C.