RELATIONSHIPS BETWEEN CRYSTAL STRUCTURE AND NIR SPECTROSCOPY OF SYNTHETIC PYROXENES

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. Spectral features arising from pyroxene have long been recognized in remote telescopic and orbital data. Fe²⁺ is responsible for the dominant pyroxene absorption bands in the visible and near-infrared. However, other cations such as Ca²⁺ have drastic effects on a pyroxene spectrum by changing the lattice parameters of its crystal structure and thus the crystal field splitting energies of the Fe cations. We here investigate the relationships between steric parameters of the pyroxene structure and their impact on the intensity and position of bands at 1.0, 1.2, and 2 μm in the near-IR region. Spectra of synthetic pyroxenes covering the Fe-Mg-Ca quadrilateral in 5-10 mol% increments were used. The Modified Gaussian Model was used to deconvolve their spectra into component absorption bands, which were then compared against results of single crystal structure refinements (SREF) of pyroxenes with the same chemical formulas, acquired from the American Mineralogist Crystal Structure Database. For each structure, space group, site occupancies, mean octahedral quadratic elongation (λ), angular variance (σ_Θ), polyhedral volume, and relevant bond distances and angles were tabulated for the M1 and M2 sites. These were then compared to the positions and intensities of the spectral features for samples where both SREF and spectral data were available. Changes in peak positions of 1 and 2 μm bands with space group are conspicuous. Values for λ_M1 vary with 1 μm band position, while λ_M2 varies with both 1 and 2 μm band position. The σ_Θ of the M1 site correlates positively with 1 and 2 μm band positions, and not with 1.2 μm. The O1-M1-O1, O1-M2-O1, and O3-M1-O3 angles all show correlations with 1 and 2 μm band positions. Results predict that cation ordering could affect both 1 and 2 μm band positions by as much as ~0.07-0.1 μm in samples with the same bulk composition. These results show that variations in individual polyhedral shapes cause structural changes throughout the pyroxene structure. Data suggest that the approach of using crystal structure parameters to predict pyroxene band positions is promising.