| Abstrakt: |
Two intermediate compounds of the system Ca2SiO4-Ca3(PO4)2-CaNaPO4 were synthesized by reaction sintering at 1600°C and analyzed structurally, chemically, and optically. The structure of Ca7(PO4)2(SiO4)2 nagelschmidtite (space group P61, 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-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 αV = 3.95 and 5.21 [×10−5/°C], respectively. Ca15(PO4)2(SiO4)6 is a distinct phase in the binary section Ca2SiO4-Ca3(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 α-Ca2SiO4-α-Ca3(PO4)2 solid solution. Structural relations between nagelschmidtite, Ca15(PO4)2(SiO4)6 and the end-member compounds of the system are discussed. [ABSTRACT FROM AUTHOR] |
