I have studied hercynite - cordierite symplectites in LP-HT
metapelites in the Bushveld complex in South Africa. In that case
they appeared to replace chiastolitic andalusite at pressures
probably as low as 2 kbar and temperatures not exceeding 750°C. All
the phases were rather Fe-rich and the matrix was often Qtz-rich.
Just as Dave Waters pointed out, the replacement appeared
non-isochemical with equilibration probably only being achieved at
very small scale, producing namely strong a(SiO2) gradients between
the matrix and the Crd-Spl intergrowths.
This was described in our paper published this May in the Journal of
metamorphic geology (JMG 19, p. 311-325), where you may find a PT
grid calculated with THERMOCALC (containing some of the Spl-Crd
producing equilibria) together with some other potentially
interesting phase diagrams, as well as some discussion and references
to other similar occurrences.
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