Two intermediate compounds of the system Ca2SiO4-Ca-3(PO4)(2)-CaNaPO4 were synthesized by reaction sintering at 1600 degrees C and analyzed structurally, chemically, and optically. The structure of Ca-7(PO4)(2)(SiO4)(2) nagelschmidtite (space group P6(1), a = 10.7754(1) angstrom, c = 21.4166(3) angstrom) was determined by single crystal X-ray analysis. Its unit cell can be interpreted as a supercell (approximate to 2 x a, 3 x c) of the high-temperature polymorph alpha-Ca2SiO4. Evidence for pseudo-hexagonal symmetry is shown. Using electron microprobe, the solid solution Ca7 - xNax(PO4)(2)+ x(SiO4)(2 - x), (x <= 2), of nagelschmidtite was confirmed. Volume thermal expansion coefficients of Ca6.8Na0.2(PO4)(2.2)(SiO4)(1.8) and Ca5.4Na1.5(PO4)(3.7)(SiO4)(0.3) were determined using high-temperature X-ray powder diffraction, yielding mean alpha(V) = 3.95 and 5.21 [x10 (5)/degrees C], respectively. Ca-15(PO4)(2)(SiO4)(6) is a distinct phase in the binary section Ca2SiO4-Ca-3(PO4)(2) and was found to extend into the ternary space according to Ca15 - xNax(PO4)(2 + x)(SiO4)(6 - x), (x <= 0.1). Quenching experiments of the latter allowed for structural analysis of a strongly disordered, defective high-temperature polymorph of the alpha-Ca2SiO4-alpha-Ca-3(PO4)(2) solid solution. Structural relations between nagelschmidtite, Ca-15(PO4)(2)(SiO4)(6) and the end-member compounds of the system are discussed.