Anisotropic mean-square displacement parameters, as routinely obtained from single-crystal structure refinements, are used to calculate difference values (ΔU) along the (Si,Al)-O vector in alkali feldspar tetrahedra. ΔU values averaged over a tetrahedron ((ΔU)) provide physical information on (Si,Al) order-disorder.

In the framework tetrahedra of feldspars, mean (Si,Al)-O bond lengths range from l.6l Å to 1.74 Å, mainly depending on the (Si,Al) population of symmetrically equivalent tetrahedral sites. Values of (ΔU) calculated for 153 individual tetrahedra in a sample of 49 (Si,Al)-ordered and (Si,Al)-disordered structures vary as a quadratic function of the mean (Si,Al)-O distance. This correlation may be understood in terms of a distribution of disordered oxygen atoms over two positions that are 0.13 Å apart, leading to an increase of difference displacement parameters ((ΔU)) with increasing (Si,Al) disorder. (ΔU) for a SiO4 tetrahedron is 0.0004 Å2 and 0.0005 Å2 for an AlO4 tetrahedron, whereas a tetrahedron with 50% Al and 50% Si yields (ΔU) ≈ 0.004 Å2.

Difference displacement parameters in feldspars are sensitive to (l) (Si,Al) order-disorder, (2) errors in the model of a structure refinement, and (3) errors in lists of anisotropic displacement parameters

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