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

Abstract

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 Å 3 , 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 Å 3 , 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 R 1 = 0.0317 for 137 refined parameters and 2261 unique reflections with F o > 4σ(F o ) for epididymite, and R 1 = 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 Na 2 Be 2 Si 6 O 15 ·H 2 O (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.

Molecular H2O in armenite, BaCa2Al6Si9O30·2H2O, and epididymite, Na2Be2Si6O15·H2O: Heat capacity, entropy and local-bonding behavior of confined H2O in microporous silicates

“…5 and 6) were checked against the structure models available in the literature (Armbruster, 1999;Gatta et al, 2008). The residual factors (i.e., R p = 0.055 and 0.064 and wR p = 0.078 and 0.079 for armenite and epididymite, respectively) and the observed vs. calculated patterns confirm the purity of the phases.…”

“…More information on the armenite sample can be found in Senn (1990) and Armbruster (1999) and that for the epididymite sample in Gatta et al (2008).…”

“…The crystal structure of epididymite was most recently investigated by neutron diffraction by Gatta et al (2008), who gives all atomic positions including those for the H atoms. The structure of orthorhombic symmetry Pnma contains [Si 6 O 15 ] 1 double chains of corner-linked SiO 4 tetrahedra that run parallel to [0 0 1].…”

“…7c). Here, the "extraframework" Na cations are found as well as the single crystallographic H 2 O molecule (Gatta et al, 2008, whose atom labels and bond lengths and angles are cited below). The oxygen atom of the H 2 O molecule is bonded to a Na atom, which is further bonded to six oxygen atoms of the "framework".…”

“…It has been reported from various localities in a number of works (e.g., Brøgger, 1890;Flink, 1894;Lévy, 1961;Petersen, 1966;Č erný, 1963;Shilin and Semenov, 1957;Mandarino and Harris, 1968). Its crystal structure was studied and recently clarified by Gatta et al (2008). We are not aware of any calorimetric or thermodynamic studies that have been made on either phase.…”