Cordierite
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Cordierite or iolite is a magnesium iron aluminium cyclosilicate. Iron is almost always present, and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: 2
Cordierite den 07 06aa
Left: rough specimen showing dichroism; right: cut stone
Facet cut iolite gemstone
Pleochroism
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Pleochroism is an optical phenomenon in which a substance has different colors when positioned at different angles with respect to linearly polarized light.
Cordierite
…iolite" comes from the Greek word for violet. Another old name is dichroite, a Greek word meaning "two-colored rock", a reference to cordierite's strong pleochroism. It has also been called "water-sapphire" and "Vikings' Compass" because of its usefulness in determining the direction of the sun on overcast days…
Pleochroism of cordierite shown by rotating a polarizing filter on the lens of the camera
Pleochroism of tourmaline shown by rotating a polarizing filter on the lens of the camera