Abstract

The structure of low cordierite, (Na0.05K0.02Ca0.02) (Mn0.01Mg1.91Fe0.08)(Si5.01Al3.95O18.01)(H2O)0.56; a = 17.079(3), b = 9.730(2), c = 9.356(2) Å; Cccm, from White Well, Australia (Pryce, 1973), was refined by least-squares methods using X-ray and neutron diffraction data. A site-refinement with the neutron data indicates an ordered arrangement of A1 and Si in the tetrahedral framework. Neutron Δρ maps calculated around the point at (0,0,1/4) indicate that the H2O molecules in the cavity at z = 1/4 are disordered into four different orientations with the H-O-H plane of each nearly parallel to (001). This result disagrees with the possible orientation parallel to (100) proposed from infrared absorption spectra and confirmed by an NMR study. A neutron site refinement showed no evidence for substitution of H+ for Al and Si in the tetrahedral framework, and a Ap map calculated over the whole unit cell shows no residual negative peaks that may be ascribed to H+. The alkaline atoms are centered about (0,0,0). Mulliken population analyses calculated for the tetrahedral framework using constant bond lengths and the observed angles within and between the tetrahedral ions indicate that the bond length variations in the framework may be ordered in terms of bonding and antibonding Mulliken overlap populations. As expected, the bond length variations in the framework correlate linearly (r = 0.9) with the valence angles within and between the tetrahedra. Calculations for 4-membered Al2Si2O12 rings like those in cordierite result in a lower electronic energy when Al− and Si−containing tetrahedra alternate than when “the aluminum avoidance rule” is violated. Calculations for the 6-membered Al2Si4O18 rings like those in cordierite predict a higher energy when AlO4 tetrahedra are adjacent, but predict identical energies when the two Al04 tetrahedra are separated by one or two SiO4 tetrahedra.

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