The CaO?MgO?P2O5 System at 1000°C for P2O5 ⩽ 33.3 mole %

The quasi ternary phase diagram CaO-MgO-P₂O₅ at 1000°C for P₂O₅ ? 33.3 mole-% is determined by heating some 140 compositions prepared from the appropriate ones of 4 starting chemicals (reagent grade CaHPO₄, CaCO₃, MgO, and MgHPO₄ · 3 aq.). Products were investigated with X-ray diffraction and heating was continued until no more changes in phase composition and peak positions in the X-ray diffraction pattern were observed, leading occasionally to heating times of over 1000 hours. There are three major regions of solid solution: for the whitlockite phase, and for the ternary compounds on the pyro- and the ortho-phosphate join. The extent of solid solution of the ternary pyrophosphate compound was determined by the change in peak position in the X-ray diffraction pattern with composition and was found to be 15.9–27.5 mole-% MgO. Two and three phase areas were determined using the change in peak position data for the whitlockite and ternary pyrophosphate compound. The experiments were carried out in air. However additional experiments in a CO₂ atmosphere revealed no changes in the X-ray diffraction patterns, apart from those in which apatite was involved. The apatite phase did not incorporate detectable amounts of Mg, not on heating in air nor on heating in a carbondioxide atmosphere.