Dear all,
Interesting to hear that some of the classic triple point localities
may not be so "classic" after all. These triple point occurrences
always worried me a bit. It would only make sense, if the
aluminosilicates actually crystallized at or near the triple point,
which of course requires an aluminosilicate-producing reaction. The
polymorphic reactions may seem to be obvious candidates, but really
don't make sense here, because of the relatively large reaction
overstepping required, particularly for andalusite-sillimanite. From
that alone, it seems extremely unlikely to find sillimanite in a
metamorphic rock that just about "touched" the sillimanite stability
field, considering polymorphic reactions only. Also, the coexistence
of two (And-Sil or Kya-Sil) or three polymorphs would commonly
indicate temperatures well above that of the triple point. I am not
sure whether fluids and deformation can affect that system enough to
get rid of unstable phases more efficiently, and help to nucleate the
stable ones.
Considering common metapelite reactions producing Al2SiO5 on their
high-T side, at P conditions around the triple point, normally proceed
at higher temperatures than the triple point itself, as Bruce pointed
out. Perhaps there are situations where aH2O is low enough to achieve
a significant shift to lower T. But even if these possibilities are
considered, the expected result would be metastable coexistence of
aluminosilicate polymorphs if P-T paths crossed the triple point (or
one of the three reactions emanating from it). Regional-metamorphic
P-T paths will have crossed two or three of the polymorph's stability
fields in the process.
The bottom line is that it is difficult enough to get the right
conditions for Al2SiO5 polymorphs to crystallize at the triple point,
and perhaps even more difficult, if not impossible, to verify triple
point conditions from Al2SiO5 polymorphs alone.
Cheers, Juergen
J. Reinhardt
Dept. of Geology
University of Natal
Durban, 4041
South Africa
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