Tourmaline

Ring silicates

trigonal (+)

Relief - moderate

Colour - brown, blue, pink

RI - 1.610-1.655

Birefringence - 0.021-0.026

Interference colours - 2nd-3rd order

Extinction - straight

Comments


 

brown, colour-zoned tourmaline

PPL

Aplite, Megilligar Rocks

 


 

colour zoned tourmaline with quartz

PPL

St Austell Granite

 


 

colour zoned tourmaline with quartz; the image above under crossed-polars

XPL

St Austell Granite


 


 

Above; brown tourmaline with epitaxial growth of acicular blue tourmaline (left) and the same field of view rotated through 90° to show the extreme pleochroism (right)

< the same crystals under crossed polars

 

St Austell Granite


brown primary tourmal with epitaxial growth of blue secondary tourmaline in PPL (left) and XPL (right)

Luxullianite, St Austell Granite

 


Tourmaline 'suns' in PPL (left) and XPL (right), the latter showing the straight extinction cross

Luxullianite, St Austell Granite


prismatic crystal of tourmaline in PPL (left) and XPL (right).

St Austell Granite

 


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