Belay Zeleke Dilnesa - Eawag-Empa Library

CHAPTER 3 SYNTHETIC FE-CONTAINING HYDRATES
The sample prepared at 110 °C was used for the structural determination. Multipattern
Rietveld refinement was performed (Fig. 60). Traces of impurity were identified as
indicated by weak diffraction peaks observed at d = 3.77 Å and 3.47 Å (see Fig. 60,
bottom for 2 = 10.6° and 11.5°). The sample is not single-phased, and two Fe siliceous
hydrogarnet phases have been observed (Fig. 61). The measured sample is composed of
Fe siliceous hydrogarnet phase N°1 (64 weight %), Fe siliceous hydrogarnet phase N°2
(27 wt. %), calcite CaCO3 (8 wt. %) and C3FH6 (1 wt. %).
The two Fe siliceous hydrogarnet phases correspond to the structure already described for
hydroandradite, the Al-free iron-hydrogarnet compound [126]. The two Fe siliceous
hydrogarnet phases with cubic symmetry have different lattice parameters (a = 12.5424
(5) Å and 12.4297 (7) Å, respectively for Fe siliceous hydrogarnet N°1 and Fe siliceous
hydrogarnet N°2) and different silica contents (refined composition are
Ca3Fe2(SiO4)0.95(2)(OH)8.20(2) and Ca3Fe2(SiO4)1.52(4)(OH)5.92(4)). A miscibility gap seems
to appear between these two hydroandradites. Table 30 compiles the refined parameters
for both hydroandradite phases which crystallized in the cubic Ia3 d space group.
131