• journal_title：American Mineralogist
• Contributor：G. Diego Gatta ; N. Rotiroti ; G.J. McIntyre ; A. Guastoni ; F. Nestola
• Publisher：Mineralogical Society of America
• Date：2008-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2008.2965
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：93
• issue：7
• firstpage：1158
• section：Articles

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 ų, space group Pnma)] and eudidymite [a = 12.6188(10), b = 7.3781(5), c = 13.9940(9) Å, β = 103.762(5)°, V = 1265.47 ų, 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 structure refinements have been performed with final agreement index R₁ = 0.0317 for 137 refined parameters and 2261 unique reflections with F_o > 4σ (F_o) for epididymite, and R₁ = 0.0478 for 136 refined parameters and 1732 unique reflections with F_o > 4σ (F_o) 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, and not hydroxyl groups as wrongly reported in previous studies and in several crystal-structure databases. The correct 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 analyses show that a small, but significant, amount of Al and Fe (most likely substituting for Si in the tetrahedral sites) and K (substituting for Na as an extra-framework cation) occurs in both dimorphs.