RIETVELD REFINEMENT OF Na2CaMg(PO4)2, NH4Al3(SO4)2(OH)6. J. Maixner, H. Hejdová, Central Laboratories, Institute of Chemical Technology, Technická 5, 166 28 Praha 6, Czech Republic
The structure analysisof Na2CaMg(PO4)2 and NH4Al3(SO4)2(OH)6 has been made by means of X-ray powder diffraction using Rietveld method.
The first compound is known as the mineral brianite(1) and it is structurally isomorphous with the mineral merwinite Ca2CaMg(SiO4)2 (2). The other compound is structurally isomorphous with KAl3(SO4)2(OH)6 (3). None of these compounds has got an entry in ICSD or PDF-2 databases. Therefore, we have refined the structures of these minerals and prepared powder cards for them.The initial cell parameters were evaluated with TREOR 90(4) and the framework atom positions with temperature factors were taken from the literature. The theoretical powder patterns were calculated and their comparison with measured data undoubtedly confirmed isomorphism of solved structures.
The X-ray data were collected on DRON-UM-1 powder diffractometer with CuK[[alpha]] radiation (7 - 120deg. 2[[phi]] range, 0.02deg. step size, 5 seconds per step). The Howard's version DBWS 4.1(5) was used for the Rietveld refinement.The table 1. summarises final cell parameters and R factors for both structures.
Table 1.: Final cell parameters and R factors for both structures.
a(Å) b(Å) c(Å) [[beta]](deg.) V(A3) Rw%
13.385(6) 5.204(1) 9.119(1) 90.78(2) 635.1(5) 8.12
7.007(1) 7.007(1) 17.616(1) 120.0 749.3(1) 10.55
(1) L. Fuchs et al., Geochim. Cosmoch. Acta 31, 1711 (1967)
(2) P. B.Moore & T. Araki, Am. Mineralogist, 57, 1355 (1972)
(3) S. Manchetti, Neus Jahrbuch fur Mineralogie, 406 (1976)
(4) P. E. Werner, Z. Krist., 120, 375(1964)
(5) D. L. Bish & S. A. Howard, J. Appl. Cryst.,21, 86 (1988)