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Tourmaline group

 

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Tourmaline  (Click PPL image to show labels)

Olivine

 

Tourmaline in a granite from Botallack, Cornwall. Bicoloured brown and blue, colour zoned and strongly pleochroic (see movie). Anhedral in this sample, and probably a secondary mineral replacing alkali feldspar. Cracking (random directions) and holes are very common in tourmaline thin sections, cleveage is not usually noticeable

 

Width of field of view: 11mm.

 

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Pleochroism

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Olivine XPL view

 

Tourmaline in granite. Bright interference colours are common, but complex and variable chemistry in tourmaline means that optical characteristics are variable.

 

Width of field of view: 11mm

 

 

   

 

   

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Olivine rollover

 

Tourmaline in a granite pegmatite from Portsoy, Aberdeenshire. Some edges and terminations visible. Note strong pleochroism. Body colour in thin section is commonly blues or brown, sometimes green or colourless.

 

Width of field of view: 7mm

 

 

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Tourmaline in pegmatite. Typical vivid colours

 

Width of field of view: 7mm

 

 

 

   

 

   

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Olivine rollover

 

Tourmaline in Luxulyanite. Very special rock from Cornwall. A few large areas of tourmaline and many acicular radiating 'stars'. Colour of the needles depend on the orientation relative to the single polariser, and change when the polariser or slide is rotated. see movie. Note very altered feldspars.

 

Width of field of view: 8mm

 

 

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Tourmaline in Luxulyanite. Bright colour in larger tourmaline, needles show variable intereference colour. Fine needles in a section may be thinner than the standard 30µm of the section, and will show lower interference colours than expected.

 

Width of field of view: 8mm

 

 

 

   

 

   

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Olivine rollover

 

Tourmaline in a mica schist from Wissahickon, Philladelphia, USA. Colours vary with orientation. Lightest in longitudinal, but these will go dark if rotated 90° with respect to the polariser. Basal sections are rounded triangles, very characteristic shape for tourmaline, and usually dark and show little pleochroism. Cracks in the tourmaline show some direction in this specimen, across the length of the crystal.

 

Width of field of view: 8mm

 

 

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Tourmaline in schist. Body colour mutes the interference colours in this example.

 

Width of field of view: 8mm

 

 

 

   

 

   

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