One-parameter characterization of the average Al/Si distribution in plagioclase feldspars

By using any one of the related parameters Δ2θ ≡ 2θ131 − 2θ13̅1, Γ ≡ 2θ131 + 2θ220 − 4θ13̅1, 2θ2̅41 − 2θ2̅4̅1, γ, or γ* or the c cell edge, the distribution of Al and Si averaged onto the four nonequivalent tetrahedral sites of albite can be determined for plagioclase of any composition and structural state. There are linear correlations of Δ2θ and ΔAl (where ΔAl is the difference between the Al content of the T1O site and the average of the Al contents of the T1m, T2O, and T2m sites) for the anorthite-low-albite series and the high-albite-low-albite series. Since in all thirteen plagioclase structures presently refined the Al content of T1m ≃ T2O ≃ T2m, only ΔAl and the total Al content of the feldspar need be determined to obtain the average tetrahedral Al/Si distribution.