A spectral study of the origin of colour and pleochroism of a of a titanaugite from Kaiserstuhl and of a riebeckite from Saint Peter's Dome, Colorado

Polarized spectra of a-b sections of titanaugite (from Germany) show a pleochroic band at about 14000 cm (super -1) assigned to Fe (super 2+) Fe (super 3+) charge-transfer. Intra-cationic d-d bands of octahedrally-bonded Fe (super 2+) , Fe (super 3+) , and Ti (super 3+) were observed at 10000 cm (super -1) , 22000 cm (super -1) , and 19000 cm (super -1) respectively. Polarized spectra of a-c sections of riebeckite from St. Peter's Dome, Colorado, show pleochroic band at 15700 cm (super -1) polarized along metal-metal directions; it is assigned to Fe (super 2+) Fe (super 3+) charge-transfer. The field-independent transition ⁶ A ₁⁴ A ₁⁴ E(G) in Fe (super 3+) is observed at 23000 cm (super -1) . Intra-cation d-d bands of Fe (super 2+) appear as weak shoulders on the limb of the 15700 cm (super -1) band.