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