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All,
   One can find simple platelets of Stp in phyllitic rocks.  
   Miyano & Klein back-calculated ferro-Stp equilibria based on P-T estimates of an iron formation, an electron microprobe analysis, an assumed formula and an ideal mixing model.  Their inverse approach is interesting and gives a target to test with future data and models. We tried to address the complexity of Stp-in and Stp-out reactions in the Otago schists (Li et al. 2000).  It is very difficult to analyze Stp on EMP without generating excess K during the analysis (Dave Craw, 1981, Lithos).
   The dark platelets in some schiller pyroxenes can be a dark greenish olive or even a kind of violet color when the optical path length through the lozenge is short.  Ilmenite is not opaque when it is thin enough, my impression is less than a few microns thick. The color should depend on whether it has finely exsolved hematite or not.
cheers,
eric


Li, G., Essene, E.J., Peacor, D.R., & Coombs, D.S. (2000)  Reactions leading to the appearance and disappearance of stilpnomelane in the Otago Schist, New Zealand. J. Metam. Geol. 18, 393-407.


On Dec 30, 2009, at 2:48 PM, Bruce Yardley wrote:

Dear Pavel et al.

Best wishes to all for 2010!

I am doubtful about this being stilpnomelane because in my experience it tends to be spiky rather than purely parallel sided. Also, ferri-stilpnomelane is probably not stable at depth and if these really are inclusions I would expect them to be the greenish brown of ferro-stilpnomelane.

Many years ago a student of mine identified similar platelets in an epidote-crossite blueschist as hematite, and hematite does sometimes look like this in ore deposits. However at this distance I have no idea whether his identification was actually supported by evidence or was an inspired guess. We used to work like that quite a lot at one time!

Bruce

Professor Bruce Yardley
School of Earth and Environment
University of Leeds
Leeds LS2 9JT
UK
Tel. +44 (0)113 343 5227
________________________________________
From: Metamorphic Studies Group [[log in to unmask]] On Behalf Of Pavel Pitra [[log in to unmask]]
Sent: 30 December 2009 16:55
To: [log in to unmask]
Subject: mineral to identify

Hello,

studying glaucophane + cpx + grt-bearing rocks
from Syros (Greece) and Ile de Groix (France), I
recently came across a tiny but outstanding
mineral that I am ashamed to admit not to be able
to identify. It occurs in several samples from
both localities, but I never saw it so clearly in
other rock types (at least I don't remember). I'm
attaching three photographs of it and would be
very grateful for a suggestion.

It is brownish-red to orange, slightly pleochroic
with relatively high relief and occurs typically
as very thin lamellae (inclusions?) in white
micas, apparently parallel to the cleavage.
Locally it also ocurs in garnet. It is so thin
that the optical properties interfere strongly
with the mineral hosts. I'm afraid that for the
same reason microprobe analyses wouldn't be
conclusive either. Normally I would neglect it as
a "iron hydroxyde", but its attractive appearance
in these sections (and the fact that it seems
enjoy such mineral assemblages) prevents me from
doing so.

Thanks a lot for any suggestions!

Have a nice end of the year, and a good new one!

Cheers,

Pavel

--
Pavel PITRA
Geosciences Rennes                      email: [log in to unmask]
Université Rennes 1                     tel: (++33) 2.23.23.65.06
Campus de Beaulieu - Bat. 15            fax: (++33) 2.23.23.60.97
F - 35 042 RENNES CEDEX
FRANCE                          http://www.geosciences.univ-rennes1.fr/
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