Infrared laboratory absorbance spectra of olivine: using classical dispersion analysis to extract peak parameters

Abstract

Laboratory measurements quantifying the effect of Fe substituting for Mg in olivine are needed to distinguish compositional from temperature, grain size and grain shape effects in observational data. To address this need, we study room temperature absorption spectra of a large suite of olivines evenly spaced across Mg and Fe compositions. We apply the principle that classical dispersion theory may be used to determine peak positions as well as peak widths, strengths and possibly optical function (n(λ) and k(λ)) estimates from absorption spectra of thin film samples of these olivines and two additional isotropic and anisotropic minerals with varying hardness and numbers of spectral bands. For olivine, we find that this method provides good estimates of peak position and that accounting for asymmetric peak shapes in this way increases the error on full width at half‐maximum and oscillator strengths. Values from classical dispersion fits better match published n and k derived from reflectivity of single crystals when the dust proxy is soft and the thickness of the sample is independently constrained. Electronic data and peak parameter trends for the laboratory olivine absorption spectra and the viability of the extracted n and k are discussed with regard to astronomy.

Midinfrared spectroscopy of synthetic olivines: Thermal emission, specular and diffuse reflectance, and attenuated total reflectance studies of forsterite to fayalite

“…All 10 of these omnipresent bands, as defined by the position of the band emissivity minimum, shift to smaller wave numbers (longer wavelengths) with decreasing Mg content (increasing Fe content) as shown in Figure 1. Numerous previous works show this relationship as well for the Mg‐Fe‐rich olivines and have correlated the spectral feature positions with the mass and ionic radius of the octahedral cation in the olivine [e.g., Tarte , 1963; Duke and Stephens , 1964; Burns and Huggins , 1972; Farmer , 1974; Jeanloz , 1980; Reynard , 1991; Hofmeister , 1987, 1997; Chopelas , 1991; Fabian et al , 2001; Koike et al , 2003; Kolesov and Geiger , 2004; Hofmeister and Pitman , 2007; Koeppen and Hamilton , 2008; Pitman et al , 2010; Hamilton , 2010]. The variation in position of each of the 10 bands that appear throughout the Mg‐Fe solid solution is graphically shown in Figures 6–9.…”

“…We applied linear regressions to the band position data (Tables 1–5) to determine the goodness of fit ( R 2 ) of our data to the olivine composition (Fo # ). Hofmeister and Pitman [2007] and Pitman et al [2010] noted that many of the trends for olivine bands (measured in transmission) were better fit by two linear segments with different slopes at ∼Fo 70 . Hofmeister and Pitman [2007] suggest that the change in slope results from changes in bond length associated with the larger Fe ion substituting for the smaller Mg cation and distorting the “oxygen sublattice about the impurity site.” Koike et al [2003] found a linear relationship for most of the midinfrared, but noted that for the longer‐wavelength region of the data into the far‐infrared (∼145–300 cm −1 ) linearity occurs near the end‐members, but not in the middle compositional range (∼Fo 40–60 ).…”

“…The midinfrared spectral behavior of natural olivine (commonly contaminated by Mn 2+ or other divalent cations) has been studied by numerous researchers prior to this work [e.g., Lehmann et al , 1961; Duke and Stephens , 1964; Burns and Huggins , 1972; Runciman et al 1973; Salisbury et al , 1991b; Tarantino et al , 2003; Koeppen and Hamilton , 2008; Hamilton , 2010]; however, this work introduces a large Mg‐Fe suite of synthetic olivine samples. Although some synthetic olivine spectra have been published before in transmission [e.g., Tarantino et al , 2003; Koike et al , 2003; Hofmeister and Pitman , 2007; Pitman et al , 2010] and reflectance [ Sogawa et al , 2006], this work represents the first time emissivity and attenuated total reflectivity spectra of a full Mg‐Fe synthetic ‐olivine suite have been published, and extends the spectral range for reflectance data acquired of more numerous chemical subdivisions than most previous (dominantly transmission) works (with the exception of Hofmeister and Pitman [2007] and Koike et al [2003]), and presents the spectra of a single set of synthesized samples both analyzed and cross‐compared using an unprecedented four different midinfrared spectroscopic techniques.…”

Revisiting astronomical crystalline forsterite in the UV to near-IR

“…Whereas there is a general lack of optical functions (real and imaginary indices of refraction n and k) for many cosmic dust species, there are in fact several choices of laboratory and optical function data over the electromagnetic spectrum and different temperatures for important species, e.g., crystalline and disordered olivine (Mg 2x Fe 2−2x SiO 4 ) and its Mg-endmember forsterite (e.g., Huffman and Stapp, 1973;Weeks, 1974;Reynard, 1991;Jäger et al, 1994;Pollack et al, 1994;Colangeli et al, 1995;Dorschner et al, 1995;Henning et al, 1999 and references therein;Fabian et al, 2001;Suto et al, 2006;Koike et al, 2006;Sogawa et al, 2006;Pitman et al, 2010 and references therein). Modelerconstructed "cosmic," "circumstellar," or "astronomical" silicates, based in part on real mineral data, also exist (e.g., Ossenkopf et al, 1992;David and Pegourie, 1995;Draine, 2003a, b).…”

Visible to near-infrared optical properties of pure synthetic olivine across the olivine solid solution

“…Relying on the constrained results would, in effect, mean that the results for the Fe-rich samples would be determined by the results of the more Mg-rich samples, and would carry an implicit a priori assumption that the Fe-rich samples should have similar trends to the Mg-rich samples. While this is a reasonable assumption, some research (using mid-infrared instead of VNIR data) suggests that Mg-rich olivines may behave slightly differently from more intermediate and Fe-rich olivines (Hofmeister and Pitman 2007;Pitman et al 2010). However, other mid-infrared studies of olivine [including work by Lane et al (2011) on some of the same samples studied in this work] did not find a need to define distinct trends for Mg-rich samples (e.g., Koike et al 2003;Lane et al 2011).…”