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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