Abstract
Multiple linear regression analysis has been applied to the geometric and chemical variables in sodic plagioclases in order to determine their relative effects on individual T-O bond lengths in the Al1+xSi3−xO8 tetrahedral framework. Using data from crystal structure analyses of low and high albite, An16 and An28, and assuming that low albite is completely ordered,
where the Al content of a particular tetrahedral (T) site can be estimated from empirically-derived determinative curves, where Δ Albr is a linkage factor to account for the Al content of adjacent tetrahedral sites, where the formal charge on the (Na1−xCax) atom is q=1+x, and where T-O-T is the inter-tetrahedral angle involving the T-O bond. For sodic plagioclases it is essential to know only the anorthite content and the 2Θ131-2Θ1¯31 spacing (CuK α radiation) in order to determine the independent variables in this equation and thus to evaluate the individual T-O distances.
The 64 individual T-O distances predicted for the four sodic plagioclases by this equation agree well with the observed T-O bond lengths (σ=0.004 Å; r=0.994), and the method has been used by way of example to rationalize the T-O bond lengths in analcime (cf. Ferraris, Jones and Yerkess, 1972).
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Phillips, M.W., Ribbe, P.H. The variation of tetrahedral bond lengths in sodic plagioclase feldspars. Contr. Mineral. and Petrol. 39, 327–339 (1973). https://doi.org/10.1007/BF00376472
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DOI: https://doi.org/10.1007/BF00376472