Structural and Crystal Chemical Investigation of Intermediate Phases in the System Ca 2 SiO 4 – Ca 3 ( PO 4 ) 2 –CaNa PO 4

Two intermediate compounds of the system Ca 2 SiO 4 – Ca 3 ( PO 4 ) 2 – CaNa PO 4 were synthesized by reaction sintering at 1600°C and analyzed structurally, chemically, and optically. The structure of Ca 7 ( PO 4 ) 2 (SiO 4 ) 2 nagelschmidtite (space group P 6 1 , a =  10.7754(1) Å, c =  21.4166(3) Å) was determined by single crystal X‐ray analysis. Its unit cell can be interpreted as a supercell ( ≈  2  ×   a , 3  ×   c ) of the high‐temperature polymorph α‐Ca 2 SiO 4 . Evidence for pseudo‐hexagonal symmetry is shown. Using electron microprobe, the solid solution Ca 7− x Na x ( PO 4 ) 2+ x (SiO 4 ) 2− x , ( x ≤  2), of nagelschmidtite was confirmed. Volume thermal expansion coefficients of Ca 6.8 Na 0.2 ( PO 4 ) 2.2 (SiO 4 ) 1.8 and Ca 5.4 Na 1.5 ( PO 4 ) 3.7 (SiO 4 ) 0.3 were determined using high‐temperature X‐ray powder diffraction, yielding mean α V   = 3.95 and 5.21 [ × 10 − 5 /°C], respectively. Ca 15 ( PO 4 ) 2 (SiO 4 ) 6 is a distinct phase in the binary section Ca 2 SiO 4 – Ca 3 ( PO 4 ) 2 and was found to extend into the ternary space according to Ca 15− x N a x ( PO 4 ) 2+ x (SiO 4 ) 6− x , ( x ≤  0.1). Quenching experiments of the latter allowed for structural analysis of a strongly disordered, defective high‐temperature polymorph of the α‐Ca 2 SiO 4 – α‐Ca 3 ( PO 4 ) 2 solid solution. Structural relations between nagelschmidtite, Ca 15 ( PO 4 ) 2 (SiO 4 ) 6 and the end‐member compounds of the system are discussed.