New insights into the crystal chemistry of epididymite and eudidymite from Malosa, Malawi : a single-crystal neutron diffraction study

The crystal chemistry of two dimorphic hydrated sodium beryllium silicates, epididymite (a =12.7334(4), b=13.6298(5), c=7.3467(3)Å, V=1275.04 Å3, space group Pnma) and eudidymite (a=12.6188(10), b=7.3781(5), c=13.9940(9)Å, β=103.762(5)°, V=1265.47Å3, space group C2/c) from Malosa, Malawi, has been reinvestigated by means of energy dispersive X-ray spectroscopy, thermo-gravimetric analysis, inductively coupled plasma-optical emission spectroscopy and single-crystal neutron diffraction. Two anisotropic structural refinements have been performed with final agreement index R1=0.0317 for 137 refined parameters and 2261 unique reflections with Fo>4σ(Fo) for epididymite and R1=0.0478 for 136 refined parameters and 1732 unique reflections with Fo>4σ(Fo) for eudidymite. The analysis of the difference-Fourier maps of the nuclear density of the two dimorphs confirms the presence of extra-framework water molecules in both the dimorphs, and not hydroxyl groups as wrongly reported in previous studies and in several crystal structure databases. The real chemical formula of edipidymite and eudidymite is Na2Be2Si6O15•H2O (Z=4). The configuration of the water molecules and the hydrogen bonds are fully described for both the dimorphs. The chemical analysis shows that a low, but significant, amount of Al and Fe (most likely substituting Si in the tetrahedral sites) and K (substituting Na as extra-framework cation) occurs in both the two dimorphs.