Near‐infrared spectra of clinopyroxenes: Effects of calcium content and crystal structure

Abstract

Abstract– Pyroxenes are among the most common minerals in the solar system and are ideally suited for remote geochemical analysis because of the sensitivity of their distinctive spectra to mineral composition. Fe2+ is responsible for the dominant pyroxene absorptions in the visible and near‐infrared, but substitutions of other cations such as Ca2+ change the crystal structure and site geometries and thus the crystal field splitting energies of the Fe cations. To define spectral systematics resulting from major pyroxene cations (Ca2+, Mg2+, and Fe2+), we focus on a suite of pyroxenes synthesized with only Ca2+, Mg2+, and Fe2+ in the two octahedral sites, specifically examining the effect of Ca2+ on pyroxene absorption bands. The modified Gaussian model is used to deconvolve pyroxene spectra into component bands that can then be linked directly to crystal field absorptions. In orthopyroxenes and low‐Ca clinopyroxenes, Ca2+‐content has a strong and predictable effect on the positions of the absorption bands. At a threshold of Wo30, the crystal field environment stagnates and the M2 bands cease to change significantly as more Ca2+ is added. At Wo50, when most of the M2 sites are filled by Ca2+, band positions do not change drastically, although the presence and strengths of the 1 and 2 μm bands are affected by even trace amounts of Fe2+ in the M2 site. It is thus apparent that next‐nearest neighbors and the distortions they impose on the pyroxene lattice affect the electronic states around the Fe2+ cations and control absorption band properties.

Lunar cryptomaria: Mineralogy and composition of ancient volcanic deposits

“…4. Comparison of the model results from this study to the Klima et al (2007Klima et al ( , 2011 synthetic pyroxene suite on a plot of the position of the 1 μm and 2 μm absorption features (McCord and Adams, 1973). Red squares are the modeled orthopyroxene results from this study, which agree well with the band center values from Klima et al (2007).…”

“…The discrepancy between the modeled band center values from this study and those of Klima et al (2007Klima et al ( , 2011 can be explained by differences in the assigned continuum. Variation in absorption band center due to the continuum removal process has been previously noted (Clenet et al, 2011).…”

“…This comparison between Klima et al (2007Klima et al ( , 2011 band center values and those values derived from our automated MGM indicates that the model developed in this study can be used to differentiate between different pyroxene types and compositions. There are limitations to our model; the automated MGM calculates a short wavelength 2 μm absorption feature (Fig.…”

“…All collected spectra are dominated by a pyroxene absorption signature with strong 1 μm and 2 μm features. Orthopyroxenes (low-Ca pyroxene), associated with noritic lunar rocks, have short wavelength 1 μm and 2 μm absorption features near 0.9 μm and 1.9 μm (Adams, 1974;Cloutis and Gaffey, 1991;Klima et al, 2007); clinopyroxenes (high-Ca pyroxene) have 1 μm and 2 μm absorptions features at wavelengths longer than 0.95 μm and 2.05 μm (Adams, 1974;Cloutis and Gaffey, 1991;Klima et al, 2011), which dominate the spectral signature of mare basalts. In order to determine the dominant pyroxene signature in the cryptomaria, the M 3 spectra (0.541-2.976 μm) were modeled assuming that only a single pyroxene composition is present.…”

“…In order to test whether our model produces accurate results, the synthetic pyroxene spectra of Klima et al (2007Klima et al ( , 2011 were processed using the same continuum removal process and MGM input parameters used for the M 3 spectra. Orthopyroxene spectra are accurately modeled using the methods outlined above (Section 2) ( Of the clinopyroxenes, the augite spectra (Fig.…”