Springer Online Journal Archives 1860-2000
Abstract Measurements have been made of the polarized absorption spectra (360-2200 nm.) of compositionally zoned pyroxene minerals in rocks 10045, 10047 and 10058 and olivines in rocks 10020 and 10022. Specimens in the form of petrographic thin sections were mounted on polarizing microscopes equipped with three-axis universal stage attachments and inserted into a Cary 17 spectrophotometer. The Apollo 11 pyroxenes with relatively high Ti/Fe ratios were chosen initially to investigate the presence of crystal field spectra of Fe2+ and Ti3+ ions in the minerals. Broad intense bands at about 1000 and 2100 nm. arise from spin-allowed, polarization-dependent transitions in Fe2+ ions in pyroxenes. Several weak sharp peaks occur in the visible region. Peaks at 402, 425, 505, 550 and 585 nm. represent spin-forbidden transitions in Fe2+ ions, while broader bands at 460–470 nm. and 650–660 nm. are attributed to Ti3+ ions. Charge transfer bands, which in terrestrial pyroxenes often extend into the visible region, are displaced to shorter wavelengths in lunar pyroxenes. This feature correlates with the absence of Fe3+ ions in these minerals. The magnitudes of the intensity ratios: band 465 nm. (Ti3+) to band 1000 nm. (Fe2+) are similar to Ti/Fe ratios from lunar pyroxene bulk chemical analyses, suggesting that an appreciable amount of titanium occurs as Ti3+ ions in the lunar pyroxenes. The 505 nm. spin-forbidden peak in Fe2+, together with absorption at 465 nm. by Ti3+, contribute to the pink or pale reddish-brown colors of lunar pyroxenes in transmitted lights. The absorption spectral measurements not only provide information on the redox behavior and crystal chemistry of lunar pyroxenes, but also form a basis for interpreting spectral reflectivity properties of lunar rocks and the Moon's surface.
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