Intermediate Infrared Spectroscopy of Pyroxene: Determination of Ca‐Mg‐Fe Composition in the 4–8 Micron Wavelength Range.

Although pyroxene has been detected remotely across the Solar System, limited information is available from infrared remote sensing about the Mg‐Fe composition of pyroxene, and distinguishing between augite (20 < CaSiO3 < 45) and diopside‐hedenbergite (CaSiO3 > 45) remains challenging. The characteristics of pyroxene in the intermediate infrared range (4–8 μm), meanwhile, have not been documented. Using reflectance spectra of 72 samples ranging across the pyroxene quadrilateral, we investigate the effect of variations in Mg# (Mg/[Mg + Fe] × 100) and Ca‐content on the positions of strong and well‐defined spectral bands at ∼5.1 and ∼5.3 μm in high‐Ca pyroxene and ∼5.2 in low‐Ca pyroxene. We find that the 5.1, 5.2, and 5.3 μm bands move to shorter wavelengths as Mg# increases, whereas Ca‐content does not significantly affect the positions of these bands, enabling the determination of pyroxene Mg# directly from band positions alone. We also find that the ∼5.1 μm band is significantly more distinctive in diopside‐hedenbergite and the ∼5.3 μm band significantly more so in augite. Therefore, the 5.1, 5.2, and 5.3 μm spectral bands enable discrimination among diopside‐hedenbergite, low‐Ca pyroxene, and augite. Additionally, the 5.1, 5.2, and 5.3 μm bands enable direct determination of Mg# of diopside‐hedenbergite, low‐Ca pyroxene, and augite within ±23, ±10, and ±29 mol% Mg‐Fe, respectively. Plain Language Summary: Pyroxene is one of the most common minerals in the solar system. Although infrared remote sensing has allowed spacecraft to remotely detect pyroxene on planetary bodies (including Mars, the Moon, and numerous asteroids), the Mg‐Fe composition of this mineral remains challenging to determine remotely, leaving much unknown about the crustal evolution of these bodies. We describe a new tool for remotely determining the Mg‐Fe composition of pyroxene using spectra measured in the 4–8 μm intermediate infrared range, between the wavelength ranges of the more thoroughly characterized visible‐near infrared (0.5–3 μm) and mid‐infrared (8–15 μm) ranges. Using the positions of spectral bands in the 4–8 μm range, it is possible to determine the Mg‐Fe composition of pyroxene within ±23, ±10, and ±29 mol% for diopside‐hedenbergite, low‐Ca pyroxene, and augite, remaking intermediate infrared spectroscopy an additional valuable remote sensing tool. Key Points: High‐Ca and low‐Ca pyroxene spectra have strong, distinctive bands in the intermediate infrared (4–8 μm) rangeBand positions change systematically with respect to Mg‐Fe composition across a suite of pyroxene spanning the Ca‐Fe‐Mg quadrilateralSpectra in the intermediate infrared range are valuable for remotely determining pyroxene composition [ABSTRACT FROM AUTHOR]