Low albite: an X-ray and neutron diffraction study Available to Purchase

Least-squares refinements of the structure of low albite from Amelia, Virginia with threedimensional neutron and X-ray diffraction intensity data sets yield weighted R factors of 0.024 and 0.035, respectively, with anisotropic thermal models. The two methods result in essentially identical positional parameters, though slight differences in thermal parameters may be due to the variation in relative scattering powers. Direct refinement of Al/Si occupancy of tetrahedral sites was possible for the neutron data but not for the X-ray data, resulting in the following values of 〈T–O〉 (in Å) and Al occupancy (associated errors in parentheses): T₁O 1.743(1)Å, 0.97(2) Al; T₁m 1.609(1)Å, 0.04(2) Al; T₂O 1.614(1)Å, 0.0 Al; T₂m 1.616(1)Å, 0.0 Al. The difference between the refined T₁O) occupancy and an Al-filled T₁O site is significant at a 90% confidence. When these data are combined with other neutron diffraction estimates of order/disorder for a low sanidine and Himalaya orthoclase, a non-linear plot of 〈T–O〉 vs. Al content results, which is best fit with a 3rd-order polynomial. This curve differs from the linear plot by as much as 0.016Å in the high-Al region. However, direct comparison of average T–O bond lengths between ordered and disordered tetrahedra includes an apparent shortening in disordered feldspars (comparable to a thermal motion effect) which may amount to 0.003Å. Analysis of the apparent-thermal-motion anisotropy of the Na site with displacive split-site models yields a splitting distance of 0.39Å for both data sets, but a single-Na, anisotropic thermal model is preferred.