Single-crystal infrared reflectivity of pure Mg2SiO2 forsterite and (Mg0.86,Fe0.14)2SiO4 olivine

Abstract

Abstract. New polarized infrared reflectance spectra of pure synthetic forsterite and natural Fo86-olivine have been recorded from 5000 to 100 cm -1. Out of the 35 expected infrared active modes, 33 have been observed (8 BI,, 12 B2., 13 B3u). The observed frequency shift from pure forsterite to Fo86-olivine is consistent with the higher mass of the substituted iron. The substitution of only 14% of iron also reduces the overal far-infrared reflectivity of olivine as compared to pure forsterite. Several discrepancies associated with previous studies of forsterite are explained by our investigation. We suggest that some of the previous investigations were complicated by polarization mixing. Experimental Procedure Sample PreparationTwo samples were used:-a single-crystal of synthetic pure Mg2SiO4 forsterite grown by the Czochralski method (sample kindly provided by Y. Gueguen, IPG Strasbourg); a single-crystal of natural (Mgo.s6,Feo.14)2SiO4 olivine from the Red Sea.Samples were oriented by X-ray diffraction, cut and polished along the (100) (010) and (001) planes. Typical sample areas were between 50 and 100 mm 2 for the synthetic sample and natural olivine. The accuracy of sample orientation during spectra collection was better than 3 ~ .

Infrared reflection spectra of forsterite crystal

“…1. They closely resemble the spectra shown in the investigation of Reynard (1991), which includes the latest reflectance measurements of single-crystalline forsterite with a resolution of 1 cm −1 in 450−5000 cm −1 and a resolution of 4 cm −1 in 100−700 cm −1 . However, our highresolution spectra with a higher S/N ratio at low frequency show the ∼150 cm −1 feature in B 2u and the ∼200 cm −1 feature in B 3u more clearly.…”

“…Identification of the IR vibration modes in previous works are summarized in Reynard (1991). Most vibration modes identified by Reynard are observed in our measurements; nondetected bands are the mode related to the feature at ω = 420 cm −1 in B 3u and the mode at ω = 370 cm −1 in B 1u .…”

“…Forsterite (Mg 2 SiO 4 ) is a magnesium-rich end-member of the olivine group. Numerous studies have investigated lattice vibrations of forsterite (Servoin & Piriou 1973;Iishi 1978;Hofmeister 1987;Reynard 1991;Fabian et al 2001). The oscillator parameters for the lattice vibrations, from which the optical constants can be derived, have also been obtained (Servoin & Piriou 1973;Iishi 1978;Fabian et al 2001).…”

Revisiting astronomical crystalline forsterite in the UV to near-IR

“…The San Carlos sample is a better comparison than the Sri Lanka sample for this purpose because the latter is metamorphic, higher in Fe content, and contains a large amount of water. The San Carlos sample contains a little water, based on peaks near 3000 cm −1 (3.3 μm; Miller et al, 1987;Rossman, 1996;Yang and Keppler, 2011), and is slightly oxidized, which influences the UV.…”

“…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).…”