Dear All,
It seems to me that Eunyoung Jeon is asking a microstructural question
in addition to the one answered by Eric, i.e. why does hornblende grow
around ilmenite?
The answer must be that reaction pathways can be complex, so that a
reaction product does not necessarily appear in contact with the
principal reactants. The overall reaction is likely to involve hydration
of pyroxene. It may be that ilmenite interfaces provide energetically
favoured nucleation sites for hornblende. Having nucleated, growth here
may be favoured if there is a suitable local supply of material (Ca, Al,
Si from plagioclase, Fe and some Ti from ilmenite), if there is adequate
transport of the remaining ingredients (Mg, etc.) from elsewhere, and if
the contributing transport and growth processes have relatively small
activation energy barriers. The rock is telling you what the kinetically
favoured reaction pathways are for the overall balanced reaction.
Im my experience of comparable rocks ilmenite boundaries appear to be
popular growth sites not only for hornblende but also for biotite and
garnet.
yours,
Dave
Eric Essene wrote:
>
> Eunyoung and all,
> Berman's programs could be used to calculate reactions with pargasite,
> hornblende, edenite, tschermakite, diopside, anorthite, quartz and water.
> Several dehydration reactions should fall out for various Ti-free
> amphiboles with cpx-pg-qz. Simple reactions such as Pg + Qz = Hb + Ab, Ed
> + Qz = Tr + Ab can also be balanced
> >Pargasitic horblende appears between ilmenite and plagioclase, also between
> >clinopyroxene and plagioclase. The hornblendes suppose to be results of
> >hydration reaction, but how the former case occurs. The hornblende around
> >ilmenite does not seem to be primarily clinopyroxene.
> > Please inform me what possibility should be considered.
--
Dave Waters - Lecturer in Metamorphic Petrology, Oxford University
Dept of Earth Sciences, Parks Rd, Oxford OX1 3PR, UK
Tel: +44 1865 272000
Direct: +44 1865 272058 Email: [log in to unmask]
Fax: +44 1865 272072 http://www.earth.ox.ac.uk/~davewa/
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