Hi all,
just another minor comment on common and not-so-common assemblages,
although this is not really important for the triple-point
discussion:
There have been various statements (Charles in particular made a
point here) that Al-rich pelite compositions are extremely rare in
nature. I never looked at "statistics" of pelite compositions (any
good references? I'd be interested), but I have seen quite a few
Al-rich metapelite occurrences (i.e., above the AFM garnet-chlorite
line) in various places, during my own studies and during field
trips. Is this a matter of biased perception, one way or the other?
Apart from straight bauxites, wouldn't a highly evolved sediment that
is largely derived from common quarzto-feldspathic igneous rocks
tend to have quartz+kaolinite+Na/K clay minerals assemblages,
considering that alkalis are easily depleted during weathering of
feldspars, just to give an example? It may not be the most
interesting rock type to study, but that's beside the point. Also, I
seem to remember a previous discussion when the question had been
brought up whether pyrophyllite is perhaps more common in low-grade
rocks than we think as it is difficult to distinguish from white
mica, using optical means only (was it Bob Tracy?).
Cheers, Juergen
J. Reinhardt
Dept. of Geology
University of Natal
Durban, 4041
South Africa
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