Dear Trisrota
JC is completely correct. He may well recall from his time at Kingston University a thin section that we use in our introductory microscopy course where we use a serpentinised dunite. The main aim of this is to explore what is meant by the term "interference colours" which we can then relate to "birefringence". However, the neat part of the slide which I have always enjoyed are the little opaque crystals which sit in with the serpentinite. That means that I can bring solid solution into the discussion as well.
JC is right - the iron in the olivine does not go into the serpentine/talc/brucite. But neither does the Cr. When you analyse your opaque phases please ensure that you interrogate the spectrum for Cr. I would not be suprised to see that your spinel contains a significant chromite component.
Best wishes
Peter
Professor Peter J. Treloar FGS CGeol
Professor of Mineralogy and Petrology
School of Geography, Geology and the Environment
Kingston University
Kingston upon Thames
Surrey
KT1 2EE
UK
Tel: +44 (0) 208 547 7525
Fax: +44 (0) 208 547 7497
e-mail: [log in to unmask]
________________________________________
From: Metamorphic Studies Group [[log in to unmask]] On Behalf Of Clemens, JD, Prof <[log in to unmask]> [[log in to unmask]]
Sent: 17 September 2011 10:10
To: [log in to unmask]
Subject: Re: Texture in olivine cumulate
Hi Trisrota,
This is an almost universal feature of rocks in which mafic minerals have been serpentinised or altered to talc. The cause is simple, as you suspect. The new magnesian hydrous minerals are unable to accomodate all the Fe released by the reaction of the olivine or pyroxene into their crystal lattices, at the conditions of alteration. It thus expresses itself as magnetite-rich spinel oxide.
Cheers,
JC
John D. Clemens
Professor in Geology & Exec. Head
Dept of Earth Sciences, University of Stellenbosch,
Private Bag X1, 7602 Matieland, South Africa
tel: +27 (0)21 808 3159 fax: +27 (0)21 808 3129
e-mail: [log in to unmask]
web page: http://www.sun.ac.za/geo/people/clemens_e.htm
________________________________________
From: Metamorphic Studies Group [[log in to unmask]] On Behalf Of Trisrota Chaudhuri [[log in to unmask]]
Sent: 17 September 2011 10:25
To: [log in to unmask]
Subject: Texture in olivine cumulate
Dear all,
I'm getting a typical texture of olivine cumulate rocks in thin sections (photo attached), in which olivines are extremely altered into serpentine and/or talc. But the interesting thing is that every altered olivine grain has a thin rim of opaques along their grain boundaries. Is it possible that the olivine, due to alteration, released some extra Fe/Mg, which might not been needed to form the altered product, and hence concentrated along olivine grain boundaries? Or something else like grain boundary exsolution has taken place? Please enlighten me.
Regards,
--
Trisrota Chaudhuri,
JRF of Indian Statistical Institute and Jadavpur University,
Kolkata, India.
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