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Chlorite

Mineral structure: Sheet silicate

Common mineral in Pelites (Low grade), Metabasic rocks (low grade), or the alteration products of Fe-

rich minerals

Rock-forming minerals associated with Chlorite: Quartz, Muscovite, Amphibole, Biotite

Chlorite in Schist

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Key optical features of chlorite in thin section:

Green to Palegreen

Pleochroic

Single cleavage and straightextinction

ModerateRelief

Low order (almost isotopic) and anomalous interference colours (see below)

Common minerals that might be confused with chlorite and occur in similar rocks:

Amphibole (typically strongly coloured, higher relief, two cleavages, inclined extinction),

Chlorite replacing garnet (and biotite (low centre of image))

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http://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/Muscovite.htmhttp://web2.ges.gla.ac.uk/~minerals/Muscovite.htmhttp://web2.ges.gla.ac.uk/~minerals/Biotite.htmhttp://web2.ges.gla.ac.uk/~minerals/Garnet.htmhttp://web2.ges.gla.ac.uk/~minerals/Amphibole.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Reliefhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Extinctionhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Cleavagehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Pleochroichttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Colourhttp://web2.ges.gla.ac.uk/~minerals/Biotite.htmhttp://web2.ges.gla.ac.uk/~minerals/Amphibole.htmhttp://web2.ges.gla.ac.uk/~minerals/Muscovite.htmhttp://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/Intro.htm

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Chlorite replacing amphibole

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Note: Chlorite often forms as an alteration product of minerals (biotite,amphibole,pyroxene,garnet)

Chlorite pleochroism and cleavage

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Distinguishing chlorite from amphibole

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http://web2.ges.gla.ac.uk/~minerals/Amphibole.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Cleavagehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Pleochroismhttp://web2.ges.gla.ac.uk/~minerals/Garnet.htmhttp://web2.ges.gla.ac.uk/~minerals/Pyroxene.htmhttp://web2.ges.gla.ac.uk/~minerals/Amphibole.htmhttp://web2.ges.gla.ac.uk/~minerals/Biotite.htmhttp://web2.ges.gla.ac.uk/~minerals/Amphibole.htm

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Note: Chlorite is palergreen, has lowerreliefand lower orderinterference colours

Anomalous interference colours in chlorite

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Note: Chlorite interference colours typically dark, anomalous blue or brown. Chlorite has straight

extinction but due to the dark interference colours this is sometimes difficult to confirm.

Chlorite in slate (fine grained) with colourless muscovite, quartz and plagioclase

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http://web2.ges.gla.ac.uk/~minerals/Plagioclase.htmhttp://web2.ges.gla.ac.uk/~minerals/Quartz.htmhttp://web2.ges.gla.ac.uk/~minerals/Muscovite.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Extinctionhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Reliefhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Colour

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Note: Low order interference colours of chlorite tend to be difficult to pick out in fine grained low grade

rocks. Due to chlorite fine grain size in low grade metamorphic rocks the green colourwith analyser out

may be not obvious!

Chlorite shape

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Note: As a sheet silicate with a single goodcleavage chlorite tends to form well shaped individual

crystals. When chlorite forms as an alterationproduct of other large minerals stable at higher

temperature it often forms clusters or clumps of finer grainsize.

Folded chlorite-rich fabric (with high order interference colour muscovite)

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Note: Cleavage domains with muscovite in similar orientation

Chlorite as a low temperature hydrothermal mineral often fills veins and amygdales

(below)

http://web2.ges.gla.ac.uk/~minerals/Muscovite.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interferencehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Alterationhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Cleavagehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Shapehttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Colour

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Note: anomalous dark blue interference colours with analyser in

Aligned flakes of chlorite in amygdales

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Take the Chlorite test

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http://web2.ges.gla.ac.uk/~minerals/Intro.htmhttp://web2.ges.gla.ac.uk/~minerals/Chlorite.htm#Tophttp://web2.ges.gla.ac.uk/~minerals/Chlorite_assessment.htmhttp://web2.ges.gla.ac.uk/~minerals/Basic%20Properties.htm#Interference