A new sodian-beryllian cordierite from Soto,Argentina, and the relationship between distortion index,Be content,and state of hydration

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 NaBeAl. Like in other sodian beryllian cordierites the distortion index is low (0.12), and a negative correlation between and Be contents is established using additional data from the literature.Dehydration of NaBe-cordierites through heating consistently leads to increasing , and subsequent hydrothermal rehydration of the Soto sample reestablished low . 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.