A reassessment of the role of iron in the 5,000–30,000 cm⁻¹ region of the electronic absorption spectra of tourmaline

Abstract

The E∥c and E ⊥ c polarized optical absorption spectra of a variety of blue/green tourmalines and a schorl were measured from room temperature down to helium temperatures. Heat treatments at 750–800° C in air and hydrogen were carried out on several green tourmalines.

From the results obtained, absorptions at 7,900 and 13,800 cm⁻¹ in the E∥c spectra of tourmalines are assigned to Fe²⁺ in the b-site. In the same polarization, bands detected at 9,000 and 13,400 cm⁻¹ are attributed to Fe²⁺ in the smaller c position. In contrast to previous interpretations, the E ⊥ c polarized bands at 9,000 and 13,800 cm⁻¹ are not assigned to single ion transitions, but are largely associated with nearest neighbour Fe²⁺-Fe³⁺ pairs. Correlations between near-infrared band absorption coefficients and FeO concentration reinforce these assignments.

The temperature dependence and the reaction to heat treatment of the strongly polarized (E⊥c≫E∥c) band near 18,000 cm⁻¹ in blue and green tourmaline spectra are shown to be consistent with previous assignments of the band to Fe²⁺+Fe³⁺→Fe³⁺+Fe²⁺ charge transfer.

Similar results are discussed for broad absorptions (also E⊥c≫E∥c) found in the 22,000–25,000 cm⁻¹ region of the spectra of certain green and brown tourmalines. It is concluded that these absorptions are due to Fe²⁺+Ti⁴⁺→Fe³⁺+Ti³⁺ charge transfer.

The proposal is made that the initial effect of heating green tourmalines in air and hydrogen is to reduce Fe³⁺ cations located in both b- and c-sites. Further heat treatment in air and hydrogen results in the oxidation of Fe²⁺→Fe³⁺ and leads to the generation of bands near 19,100 and 21,600 cm⁻¹. The newly formed bands are assigned to Fe³⁺-Fe³⁺ pairs.