|Cordierite single crystal 2.2 mm thick. The
pleochroism is strong from colorless to violet as shown on this section
perpendicular to γ thus showing α
(colorless) and β (violet).
Absorbance in the NIR region is higher than in the
visible wavelength range. The 2 peaks at 950 nm and 1200 nm arise from
crystal field transitions of some Fe2+ ions replacing Mg2+
and the absorption structure in the visible region is a charge
transfer band between Fe2+ and Fe3+. As the
replacement of Mg by Fe in cordierite is generally small, the color is
mainly visible in thick section.
The small narrow peaks between 1400 and 1500 nm
come from harmonic vibrations of the water molecule present in the