Eric,
Rt certainly does form granules in Grt in many cases of prograde HT
growth, as was nicely documented in a recent paper by Luvizotto &
Zack in Chem Geol (doi:10.1016/j.chemgeo.2008.07.023).
However, do we actually know what the controlling mechanism on the
shape of the volumetrically minor phase is? Rutile in hydrothermal
quartz, for example, also forms oriented needles and is certainly not
exsolved or precipitated from the host crystal.
I agree that a TEM or nano-SIMS study may deliver some insight.
horst
On 28 Sep 2008, at 16:31, Eric Essene wrote:
> Horst,
> But why would a prograde reaction at high T not produce granules
> of rutile? Whatever the mechanism, the oriented needles suggest
> epitaxy or topotaxy, or an internal reaction process. Garnet in
> granulite facies rocks is commonly of meta-igneous rocks where Ti-
> rich biotite is not present.
> cheers
>
>
> On Sep 28, 2008, at 5:20 AM, Horst Marschall wrote:
>
>> Sumit, Eric and others,
>>
>> I seem to remember that we had a discussion on the origin of Rt
>> needles in Grt a few years ago on this list. One important point
>> at the time was that (at least in the case of granulites) the Ti
>> may have never been in solution in the Grt, but that the oriented
>> needles may be a product of cogenetic growth of Grt + Rt during
>> the breakdown of a Ti-bearing precursor phase, such as Bt.
>>
>> any thoughts?
>> horst
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