Abstract
In a cordieritite marking the contact of a granite massif one of the large porphyroblasts of cordierite was found to contain relatively high amounts of BeO (0.93 wt.%) and Na2O (1.25 wt.%), while others are very poor in these components. Na and Be were introduced following the substitution NaBe→Al. Like in other sodian beryllian cordierites the distortion index Δ is low (0.12), and a negative correlation between Δ and Be contents is established using additional data from the literature.
Dehydration of NaBe-cordierites through heating consistently leads to increasing Δ, and subsequent hydrothermal rehydration of the Soto sample reestablished low Δ. This behavior is consistent with that implied by the model of Stout (1975) which, however, does not apply to the pure system Mg-cordierite-water. Nevertheless it is not certain that the unusual behavior of NaBe-cordierites is due to hydrogen bonding of H2O molecules within the structural channels of cordierite to oxygens of the framework, although the Type II water molecules occurring exclusively in alkali-bearing cordierites and having their H-H vectors parallel [010] (Goldman et al. 1977) would seem mechanically fitter to have this effect than the Type I water with H-H parallel [001] as proposed by Stout (1975). An alternative explanation of this dehydration/rehydration behavior implies positional shifts of sodium within the channels depending on whether or not water is available.
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Schreyer, W., Gordillo, C.E. & Werding, G. A new sodian-beryllian cordierite from Soto, Argentina, and the relationship between distortion index, Be content, and state of hydration. Contr. Mineral. and Petrol. 70, 421–428 (1979). https://doi.org/10.1007/BF00371048
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DOI: https://doi.org/10.1007/BF00371048