Structural and Crystal Chemical Investigation of Intermediate Phases in the System Ca₂SiO₄–Ca₃(PO₄)₂–CaNaPO₄

Abstract

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.

New minerals tsangpoite Ca5(PO4)2(SiO4) and matyhite Ca9(Ca0.5□0.5)Fe(PO4)7 from the D'Orbigny angrite

“…Nagelschmidtite Ca 7 □(PO 4 ) 2 (SiO 4 ) 2 with the assigned P 6 1 space group and unit-cell correspondence a nag ≈ 2 a a , c nag ≈ c a (where a nag = 10.78 Å, c nag =21.42 Å, P6 1 and Z = 6) (see Fig. S5) has one MMM … chain (6 1 axis), three equivalent MMM … chains (2 1 axis), six equivalent TMTM … chains (3 1 axis), and other two equivalent TMTM … chains (Widmer et al ., 2015). Given the M 42 T 24 O 96 stoichiometry deviating from 48 M sites and 24 T sites in the unit cell, nagelschmidtite requires six vacant M sites at different heights of the individual six equivalent TMTM … chains to yield six T □ TMTMT □ T … chains in accordance with the 6 1 and 3 1 symmetry of the P 6 1 space group (see projection in fig.…”

“…Given the M 42 T 24 O 96 stoichiometry deviating from 48 M sites and 24 T sites in the unit cell, nagelschmidtite requires six vacant M sites at different heights of the individual six equivalent TMTM … chains to yield six T □ TMTMT □ T … chains in accordance with the 6 1 and 3 1 symmetry of the P 6 1 space group (see projection in fig. 8 of Widmer et al ., 2015). Furthermore, following the screw axis 6 1 rather than 6 3 of tsangpoite, nagelschmidtite is expected to have a c axis unit repetition distance three times that of tsangpoite when derived from α-Ca 2 SiO 4 .…”

“…Instead, the four vacancies have to be placed on the four individual equivalent TMTM … chains on the {010} mirrors, thereby yielding four T □ T □ T … chains along the a axis (see structural refinement in Galuskin et al ., 2016). In contrast to the above cases with vacancies at the specific M sites, the vacancies in the synthetic flamite-like phase Ca 15 □(PO 4 ) 2 (SiO 4 ) 6 by the unit-cell correspondence a fla ≈ √3 c a , b fla ≈ 4 a a , c fla ≈ c a (where a fla = 9.40 Å, b fla = 21.71 Å, c fla =6.83 Å, Pnm 2 1 and Z = 2) were considered most likely to be placed randomly at M2 and M3 along TMTM …columns, such as Na in flamite Ca 14 Na 2 (PO 4 ) 2 (SiO 4 ) 6 (Saalfeld and Klaska, 1981, Gfeller et al ., 2015; Widmer et al ., 2015).…”

“…This is further supported by X-ray diffraction (XRD) of the exceptionally large tsangpoite crystals (a few hundreds of μm) with a high Fe 3+ /ΣFe ratio (~0.8; X-ray absorption near edge structure (XANES) analysis) showing an apatite-like unit cell and space group (Mikouchi et al ., 2011). Further, in this present investigation we show that tsangpoite, with a hexagonal structure, a silicocarnotite-like stoichiometry, and abundant structure vacancies due to aliovalent cation substitution, is most probably structurally related to the α-Ca 2 SiO 4 high- T polymorph, as are flamite, nagelschmidtite and silicocarnotite (Saalfeld and Klaska,1981; Gfeller et al ., 2015; Widmer et al ., 2015; Galuskin et al ., 2016). This indicates a mineralogical origin for tsangpoite at T > 1200°C, similar to the high-temperature hexagonal phase(s) in the α-Ca 2 SiO 4 –-Ca 3 (PO 4 ) 2 binary system, although an apatite-like structure with fully empty anion channels for tsangpoite cannot be excluded completely pending future single-crystal refinements.…”

Synthesis, crystal structure and spectroscopic characterization of a new cadmium phosphate, Na2Cd5(PO4)4

“…Na 2 Ca 5 (PO 4 ) 4 is a member of the nagelschmidtite compounds family which crystallizes in the hexagonal system with space group P6 1 . Its structure involves NaO 9 , CaO x and (Na,Ca)O x (x 6e8) polyhedra linked together through the monophosphate groups [9]. Na 2 Zn 5 (PO 4 ) 4 crystallizes in the orthorhombic system with space group Pbcn.…”