For natural cordierites (except for indialites), the cell-edge variations which cause variations in the distortion index Δ more likely represent differences in composition than in Al/Si ordering. Back-reflection Weissenberg data from 11 cordierite grains representing atomic ratios (Fe + Mn)/(Fe + Mn + Mg) from 0.046 to 0.898 reveal that, with increase in Ra (the average radius of the ions occupying the octahedral sites), cell edges a and b increase linearly whereas c decreases until exceeds ca. 0.765, then apparently increases. Statistical analyses on data for unheated grains reveal Δ to decrease markedly as x (mole fraction Na) increases, increase moderately as Ra increases, but seem relatively independent of H2O, probably because the strong correlation between Na and H2O content in unheated cordierites partially masked the effect of H2O on Δ. In contrast, Δ increased significantly when four cordierites were heated at 800°C with H2O being driven off. Inclusion of these data in the regression analysis revealed H2O content, as suggested by Stout (1975), to have a significant effect on Δ. For a natural Na,Be cordierite, Schreyer (1979) confirmed this by changing its Д value from 0.12° to 0.26° by dehydration, then returning it to 0.11° by rehydration.

The optical data, remeasured for 10 grains after their partial dehydration at 800°C for six hours in a slowly flushing H2 atmosphere, showed reduction of all refractive indices and yielded remarkably regular curves relating the indices α, β, and γ to (Fe + Mn)/(Fe + Mn + Mg) ratio. Exceptions were two specimens high in Na and Be whose indices, even after heating, exceed the general trends. As for Δ, heating increased 2 Vx (λ = 666 nm) except for the two optically (+) grains which show slight decreases. Prior to heating, 2Vx correlates inversely with H2O whereas, after partial dehydration at 800°C, 2Vx correlates inversely with Na content and, less certainly, with Ra.

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