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Cordierite
Mineral structure: Cyclosilicate
Common mineral in Pelites (Schists & Gneisses), some GranitesAssociated with low pressure metamorphism
Rock-forming minerals associated with Cordierite: Biotite, Muscovite, Quartz
Other index minerals commonly associated with Cordierite: Andalusite, Sillimanite
Cordierite in Hornfels
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Key optical features of cordierite in thin section:
Colourless
Low relief
No cleavage
Low orderinterference colours
Very readily altered, along fractures or at edges of grains
Simple or multiple twinning
Yellow pleochroic haloes
Common minerals that might be confused with cordierite and occur in similar rocks:
Quartz(no twinning, no alteration, few inclusions)
Plagioclase(more regular multiple twinning, dusty brown homogeneous alteration)
K-feldspar(cross-hatched twinning, exsolution, homogeneous alteration)
Cordierite pleochroic haloes around radioactive accessory minerals
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http://web2.ges.gla.ac.uk/~minerals/Accessory_minerals.htm#Tophttp://web2.ges.gla.ac.uk/~minerals/Plagioclase.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Twinninghttp://web2.ges.gla.ac.uk/~minerals/Adv.htmhttp://web2.ges.gla.ac.uk/~minerals/Intro.htmhttp://web2.ges.gla.ac.uk/~minerals/Accessory_minerals.htm#Tophttp://web2.ges.gla.ac.uk/~minerals/K-Feldspar.htmhttp://web2.ges.gla.ac.uk/~minerals/Plagioclase.htmhttp://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Twinninghttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Alterationhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Cleavagehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Reliefhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Colourhttp://web2.ges.gla.ac.uk/~minerals/Sillimanite.htmhttp://web2.ges.gla.ac.uk/~minerals/Andalusite.htmhttp://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/Muscovite.htmhttp://web2.ges.gla.ac.uk/~minerals/Biotite.htmhttp://web2.ges.gla.ac.uk/~minerals/Intro.htmhttp://web2.ges.gla.ac.uk/~minerals/Adv.htm -
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Cordierite twinning
Cordierite may show a variety of different types of twinning, from sector twinning of cordierite porphyroblasts,
simple twinning and multiple twinning of recrystallised matrix porphyroblasts. Twin boundaries often poorly
defined.
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Note: Sector twinned porphyroblast Note: Simple twinning and marginalalteration
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Note: Multiple TwinningNote: Inclusion-rich occasionally twinned cordierite with more
regular multiple-twinned smallerplagioclase
Cordierite alteration to pinite (muscovite and chlorite mix)
Alteration very common, typically yellow-brown, focussed at margins of cordierite or along fractures.
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http://web2.ges.gla.ac.uk/~minerals/Chlorite.htmhttp://web2.ges.gla.ac.uk/~minerals/Muscovite.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Alterationhttp://web2.ges.gla.ac.uk/~minerals/Plagioclase.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Alterationhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Twinning -
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Note: Alteration along fractures and at margins of cordierite
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Note: Marginal alteration of cordierite porphyroblast
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Note: Complete alteration of cordierite porphyroblasts produces fine-grained aggregate of muscovite
and chlorite.
Cordierite shape
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http://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shape -
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Note: At high grade cordierite forms a matrix phase in gneisses
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Note: At medium grade cordierite porphyroblasts are common and mineral inclusions typically far more
abundant than within quartzand feldspar.
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http://web2.ges.gla.ac.uk/~minerals/Intro.htmhttp://web2.ges.gla.ac.uk/~minerals/Adv.htmhttp://web2.ges.gla.ac.uk/~minerals/Cordierite.htm#Tophttp://web2.ges.gla.ac.uk/~minerals/Quartz.htm