NEW INSIGHTS INTO THE CRYSTAL-CHEMISTRY OF EPIDIDYMITE AND EUDIDYMITE: A SINGLE-CRYSTAL NEUTRON DIFFRACTION STUDY

Epididymite and eudidymite are two dimorph open-framework silicates minerals, with a poorly defined ideal chemical formula (i.e. Na2Be2Si6O15•H2O [1]; NaBeSi3O7OH [2]). Both of these minerals occur as late-stage minerals in the cavities of alkaline pegmatites generally associated with aegirine, Na-feldspars, zeolites and many other rare exotic minerals including Na-Be-Zr-Y silicates, Nb-Ta oxides and REE-carbonates. With bertrandite (Be4Si2O7(OH)2), beryl (Al2Be3Si6O18), chrysoberyl (Al2BeO4), and phenakite (Be2SiO4), epididymite and eudidymite represent some of the richest minerals in beryllium. The crystal chemistry of two natural samples of 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) collected in a miarolitic granitic pegmatite hosted by A-type peralkaline granites of the Zomba-Malosa pluton in southern 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 starting from the structural models previously published [1,2]. The final agreement index were 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. The unusually high temperature of dehydration observed for epididymite and eudidymite observed in this and previous studies [1,2]: 1) the peculiar configuration of the water molecule, which is bonded to two Na-sites, 2) to the strong hydrogen bonds to the framework oxygens, and 3) to the small “free diameters” of the channels in the tetrahedral framework, which hinder the migration of the water molecules toward the surface of the crystal. ------------------------------------------ References. [1] J.H. Fang, P.D. Robinson, Y. Ohya, American Mineralogist, 57, 1345-1354, 1972; [2] P.D. Robinson, J.H. Fang, American Mineralogist, 55, 1541-1549, 1970