Alexander,
The term exsolution should not apply unless the structures of the
host and newly formed phases are similar structures, e.g., feldspars,
pyroxenes, amphiboles, carbonates. Some people use the term
exsolution rather carelessly however. The term precipitate comes from
metallurgy.
It is rather difficult to get rutile alone to form from garnet by
a closed system process because rutile is an RO2 phase and garnet is
R2O3. A small amount of reacted TiO2 could be accommodated by
simultaneous oxidation of Fe in the garnet. Pate precipitation should
also involve a halo around the needle, as has been shown by Dave Wark
for rutile needles in some quartz. The amount one sees of needle
rutile in transmitted light gives a mistakenly large overestimate of
how much is really there because it includes needles from depth inside
the host into what seems to be just two dimensions. This mistake has
been made repeatedly by authors describing quartz in eclogitic Cpx,
ilmenite in olivine, etc. The areal percent in reflected light or BSE
images is a much better estimate. Spotting the needles in reflected
light is OK once you learn how to identify the often 1-5 micron wide
cross-sections of needles. They can be oriented on tetrahedral edges
inside the garnet. So could they be epitaxial or topotaxial in
origin, or do they represent a reaction upon cooling? Both mechanisms
involve preferred orientation of the precipitate on the host.
cheers,
eric
On Sep 26, 2008, at 4:45 AM, proyer wrote
:
> Hello everybody,
>
> hope some of you can help me out on two questions related to each
> other:
> 1) Is there a source which clearly defines when to call a mineral
> phase unmixing from a solid solution "exsolution" or "precipitate"?
> 2) Do you know of any article expaining explicitly (reaction
> equations etc.) how rutile unmixes from high-tempertaure garnet?
> I know of three such papers for ultrahigh-pressure garnets (Van
> Roermund, 2000; Zhang et all. 2003 and Yang & Liu, 2004),
> but none for granulite facies rocks etc.
> Please let me know if you have come across or even authored any such
> references.
>
> Sincerely,
> Alexander Proyer
> University of Graz, Austria
>
>
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