The aureoles of the Donegal granites frequently contain combinations of the following minerals: kyanite, andalusite, fibrolite, sillimanite. Textural and spatial evidence suggests these minerals did not grow simultaneously but sequentially in the order above, as the temperature increased in the aureoles. Reactions are not complete and overstepping is common. Early porphyroblast phases do not take part in later reactions, they remain closed systems. Later reactions take place essentially from the groundmass layered minerals. Polymorphic change from one aluminium silicate to another is rare. The presence of more than one polymorph in a rock is therefore due to the metastable persistence of the lower temperature phase(s) which grew stably. Kyanite is confined to rocks with values of MgO/(MgO/FeO) (molecular ratios) greater than about 0-50, so that its absence in some aureoles is due to a lack of rocks of the appropriate composition rather than inadequate pressure during crystallization. Sillimanite, fibrolite, and andalusite show no restriction to rocks of a given composition. The aureoles in Donegal were probably formed over a relatively narrow pressure band and a wide temperature range.
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Naggar et al. (1970) studied this question.