Photomicrograph gallery: a kyanite-muscovite-biotite-garnet schist

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Small grains of zircon surrounded by biotite, viewed through a single polarizing filter.  The "halos" around the grains are probably due to minor amounts of radioactivity emitted into the biotite from the zircon.  This radioactivity is in turn the effect of the decay of trace amounts of uranium in the zircon, which in turn is what makes zircon useful for the uranium-lead (U-Pb) method of radiometric dating.  Typically, such zircons date from the pre-metamorphic origin of the rock.
 
Garnet (high relief, just above center), unknown opaque mineral (black, near center), kyanite, muscovite, biotite, and quartz.
 
 
 
Same view as above, with both polarizing filters in place ("crossed nicols")
 
 
 

Garnet (high relief), surrounding quartz grains and opaque minerals, flanked by biotite and muscovite.
 
 

Same view as above, with both polarizing filters in place ("crossed nicols")
 
 
 

"Twinned" kyanite crystals (moderate to low birefringence, in center), surrounded by muscovite and biotite.  Crossed nicols.
 
 

Muscovite defining foliation (lining up of minerals in a metamorphic rock due to directed pressure), with elongated train of garnet and blade of kyanite, both parallel to the foliation.  Crossed nicols.
 
 
 
Kyanite, surrounded by muscovite, biotite, and quartz. Crossed nicols.
 
 
 

Chlorite (bluish tinge) surrounding garnet (black under crossed nicols).  This shows a lower temperature mineral replacing a higher temperature one, evidence of the minerals in the rock continuing to change as the rock cooled after metamorphism.  Biotite at lower right corner.  Crossed nicols.
 
 

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