Yes, I too reckon that is alteration after cordierite, pretty classic!
Simon
Simon Harley FRSE
Professor of Lower Crustal Processes
School of Geosciences, University of Edinburgh
James Hutton Road
Edinburgh EH9 3FE
ph: (44) 0131 6504839
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From: Metamorphic Studies Group <[log in to unmask]> on behalf of David Waters <[log in to unmask]>
Sent: 01 March 2018 20:18:58
To: [log in to unmask]
Subject: Re: [geo-metamorphism] identifying altered cordierite
Yes, that's cordierite OK. What I used to call 'yellow patch'.
Yours appear to be a sheet silicate aggregate with a bit of birefringence. This material can be amorphous, or nearly so.
It can have a random assortment of alkalis and Ca, but your analyses have mainly K.
You can try seeing if the atomic Al:Si ratio is 4:5, these being arguably the least mobile cations; (it nearly is - you seem to have ratios between 3.74:5 and 3.9:5).
You could do an isocon plot after JM Grant 1986 Econ Geol, just plot wt% oxide in each of your analyses versus the wt% oxide in an 'ideal' cordierite with the same molar Mg/(Mg+Fe) ratio as you have (~0.56). This could tell you what's been gained and lost in the alteration.
Dave
Dave Waters, Metamorphic Petrologist
(1) Department of Earth Sciences, South Parks Rd, Oxford OX1 3AN
(2) University Museum of Natural History, Parks Rd, Oxford OX1 3PW
Email: [log in to unmask] Tel: +44 1865 272000
http://www.earth.ox.ac.uk/~davewa/ Direct: +44 1865 282457
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-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]] On Behalf Of Carl Guilmette
Sent: 01 March 2018 19:57
To: [log in to unmask]
Subject: [geo-metamorphism] identifying altered cordierite
Dear metamorphic community,
We're working on a suite of archean paragneiss (Opinaca subprovince, Superior Craton) that display a barrovian-type sequence of isograds (bt-gt-st-sill). In the field, we have noticed a blueish-greenish mineral in the pressure shadows of early kinematic staurolite, and thought it's either chlorite or cordierite. However, in thin section, the mineral has clearly been altered and shows a heterogeneous chemical composition (see attached document for pictures and probe analyses).
We are looking for ways to confirm or infirm that the altered mineral was once cordierite.
Any comments/advice/references will be greatly appreciated.
Best regards
Carl and Myriam
____________________________________________
Carl Guilmette, PhD. Eng.
Professeur Adjoint
Chaire de Leadership en Enseignement Virginia-Gaumond Département de Géologie et Génie Géologique Université Laval
1-418-656-2131 poste 3137
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