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Biotite Mica

 

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Biotite Mica  (Click PPL image to show labels)

Olivine

 

Biotite in a schist.

If it is brown, has a single cleveage direction, and near parallel extinction, then it is most likely biotite mica. Darker spots caused by radioactive elements "burning" the biotite are very common. Biotite cut parallel to the clevage direction has a slightly different appearance. Curved cleveage common. One of the easier minerals to identify. Strong pleochroism, generally.

 

Width of field of view: 7mm.

 

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Olivine XPL view

 

Biotite in Schist. Body colour can mask or subdue the interference colours. Brighter colours in this section may be muscovite. Most micas show a "speckled" appearance near extinction, which is a very useful identicication aid.

 

Width of field of view: 7mm.

 

 

   

 

   

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Olivine rollover

 

Biotite in a granite. Biotite is the most common mafic mineral in acid plutonic rocks. Some atleration to green chlorite not uncommon.

 

Height of central crystal: 2mm

 

 

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Pleochroism

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Biotite in granite. Roughly rectangular shapes common.

 

Height of central crystal: 2mm

 

 

 

   

 

   

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Olivine rollover

 

Biotite in a kyanite schist. All mid brown minerals are biotite. Note radioactive inclusion spotting and curved cleveages.

 

Width of field of view: 9mm.

 

 

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Biotite in kyanite schist. Muscovite shows bright colours, biotite more pastel shades, or orange bodycolour.

 

Width of field of view: 9mm.

 

 

 

   

 

   

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