Multiple linear regression analysis has been applied to the geometric and chemical variables in sodie plagioclases in order to determine their relative effects on individual T~O bond lengths in the Al~+xSi3_xO s tetrahedral framework. Using data from crystal structure analyses of low and high albite, Anl~ and An2s, and assuming that low albite is completely ordered
where the A1 content of a particular tetrahedral (T) site can be estimated from empiricallyderived determinative curves, where A Albr is a linkage factor to account for the A1 content of adjacent tetrahedral sites, where the formal charge on the (Nal_xCax) atom is q= l%x, and where T--O--T is the inter-tetrahedral angle involving the T--O bond. For sodie plagioelases it is essential to know only the anorthite content and the 2 O131--2 O1~1 spacing (CuK~ 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 sodie plagioclases by this equation agree well with the observed T--O bond lengths (~ = 0.004 A; 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).