The color of cordierite comes from its Fe2+ and Fe3+.
Fe2+ in the octahedral site causes the pair of absorption bands
near 1000 and 1300 nm which have almost no effect on the color. The strong
absorption band in the gamma direction is also from Fe2+, but
in a different site. The color is predominantly due to the Fe2+
- Fe3+ intervalance charge transfer band near 600 nm. Sharp
bands near 1400 and 1900 nm are from water molecules in channels which
run parallel to the c-axis of the crystal.
Visible Spectra: polarizations: (a = alpha direction; b = beta direction;
g = gamma direction)