Quantitative Spectra and Optics of Some Meteoritic and Terrestrial Titanian Clinopyroxenes

Abstract

Of all mineral spectra, those of titanian clinopyroxenes are among the most difficult to measure, interpret, and assign. Many overlapping (and often unresolvable) single-ion d-d (Fe²⁺, Fe^{3 +}, and Ti³⁺) and intermetallic charge-transfer (Fe²⁺→Fe^{3 +}, Fe²⁺→Ti⁴⁺, Ti³⁺→Ti⁴⁺) bands contribute to a broad absorption envelope stretching from 420 to beyond 800 nm, which is superimposed on the tail of a strong ligand-metal charge-transfer band centered in the near ultraviolet (Manning and Nickel, 1969; Burns et al, 1976). The measurement and interpretation of these spectra is further complicated by large differences between the orientations of the principal indices and principal absorbances in the (010) plane (Dowty, 1978) and by the occurrence of both spin-allowed (Smith, 1978a) and spin-forbidden (Ferguson et al, 1966) d-d transitions of exchange-coupled ion-pairs. These pair transitions have the energy, width, width/energy ratio, and pressure dependence of single-ion d-d bands, but the polarization-, composition-, and temperature-dependence of charge-transfer absorptions. The result is that such bands have often been assigned by different authors to both charge-transfer and d-d origins, depending on the type of measurement made. Examples are the 725- and 1100-nm bands in tourmaline (Smith, 1978b) and the 620-nm band in the Allende fassaite, assigned to Ti³⁺→Ti⁴⁺ charge transfer by Dowty and Clark (1973) and as a d-d transition of Ti³⁺ by Burns and Huggins (1973) and Mao and Bell (1974).

Deceased.