Infrared spectra of pyroxenes (crystalline chain silicates) at room temperature

Abstract

Crystals of pyroxene are common in meteorites but few compositions have been recognized in astronomical environments due to the limited chemistries included in laboratory studies. We present quantitative room-temperature spectra of 17 Mg– Fe– and Ca–bearing ortho- and clinopyroxenes, and a Ca-pyroxenoid in order to discern trends indicative of crystal structure and a wide range of composition. Data are produced using a Diamond Anvil Cell: our band strengths are up to 6 times higher than those measured in KBr or polyethylene dispersions, which include variations in path length (from grain size) and surface reflections that are not addressed in data processing. Pyroxenes have varied spectra: only two bands, at 10.22 μm and 15.34 μm in enstatite (En99), are common to all. Peak-wavelengths generally increase as Mg is replaced by Ca or Fe. However, two bands in MgFe-pyroxenes shift to shorter wavelengths as the Fe component increases from 0 to 60 per cent. A high-intensity band shifts from 11.6 μm to 11.2 μm and remains at 11.2 μm as Fe increases to 100 per cent; it resembles an astronomical feature normally identified with olivine or forsterite. The distinctive pyroxene bands between 13 and 16 μm show promise for their identification in Mid-Infrared-Instrument (MIRI) spectra obtained with the James Webb Space Telescope (JWST). The many pyroxene bands between 40 and 80 μm could be diagnositic of silicate mineralogy if data were obtained with the proposed Space Infrared Telescope for Cosmology and Astrophysics (SPICA). Our data indicate that comparison between room-temperature laboratory bands for enstatite and cold ∼10 − K astronomical dust features at wavelengths ≳ 28 μm can result in the identification of (Mg,Fe)- pyroxenes that contain 7–15 per cent less Fe– than their true values because some temperature shifts mimic some compositional shifts. Therefore some astronomical silicates may contain more Fe, and less Mg, than previously thought.

Comparison of space weathering spectral changes induced by solar wind and micrometeoroid impacts using ion- and femtosecond-laser-irradiated olivine and pyroxene

“…For example, Marchi et al (2005) irradiated Bamble orthopyroxene using Ar + ions. Bamble orthopyroxene has an ≈20% higher enstatite number than our samples (Bowey et al 2020;Cloutis et al 1986), which could have influenced the inclination of the sample to the space weathering related changes. Based on the SRIM simulation of Marchi's experiment the electronic and nuclear stopping powers are comparable, whereas in our case the nuclear stopping power was greater than the electronic stopping power (see Table 2).…”

Vibrational spectroscopic, crystallographic, and microscopic investigation on the Aioun El Atrouss diogenite

“…It is also known that translations occur as a result of the movements of metal cations in the region of about 450 cm −1 . However, chain silicates have been shown to exhibit weak bands in the range of 600–800 cm −1 due to Si‐O‐Si bending modes in the linked oxygen atoms of silicate tetrahedra chains 22 . Likewise, Fe 2+ is responsible for dominant pyroxene absorptions in the visible and near infrared spectra, but substitutions with other cations such as Ca 2+ alter the crystal field separation energies of Fe cations 23 …”

Sakurai's Object revisited: new laboratory data for carbonates and melilites suggest the carrier of 6.9 $μ$m excess absorption is a carbonate

“…Films of known thickness were measured by compressing powder within apertures between the KBr discs. Apertures were made in the ∼ 1.9 𝜇m-thick microphone foils measured by (Bowey, Hofmeister, & Keppel 2020) and with with 6 𝜇m-thick tin gaskets used for IR cells (these were measured with a micrometer). The very thin films required to obtain unsaturated carbonate peaks were obtained by gently squeezing the discs together and rotating them to make a uniform film.…”