Structure and lattice vibrations of Mg-Al spinel solid solution

Vibrational spectroscopy of aluminate spinels at 1 atm and of MgAl2O4 to over 200 kbar

“…Although IR powder data are unreliable due to particle size and preparation effects, the powder data on the natural sample (Preudhomme and Tarte 1971) agree reasonably well with single-crystal reflection spectra on a synthetic nonstoichiometric spinel, MgO. 3AlzO3, that has TO modes at 685, 569, 494, and 308 cm -1 (Ishii et al 1982). In the Raman spectrum, only 4 modes at 312 (Tag), 407 (Eg), 666 (Tlg), and 767 cm -1 (Alg) were found for a natural spinel (Fraas et al 1973) but additional peaks at 225, 492 (Tlg), and 727 cm-1 (Alg) were found for synthetic samples (Fraas et al 1973 ;O'Horo et al 1973 ;Ishii et al 1982).…”

“…Chips from this sample have been chemically analyzed by electron microprobe and studied by X-ray crystallography at pressure by Finger et al (1986). The u parameter of 0.2633(3) for the natural sample is significantly larger than u of 02623(1) measured for synthetic spinels (Ishii et al 1982;Yamanaka et al, 1984), indicating that the synthetic samples have a small amount of Mg/A1 disorder. It is likely that the natural sample is fully ordered, owing to the much slower cooling rates and possibly high pressure in geological settings relative to those in the laboratory.…”

“…No modes were found at 225 and 727 cm-1 as previously on synthetic spinel, see Table 2 ( O'Horo et al 1973;Fraas et al 1973; see also Ishii et al 1982). Broad weak features present in the baseline of the spectra at 445, 525, 610, and 718 cm -1 were confirmed as being intrinsic to the crystal by examining the anti-Stokes spectrum and/or using the 457.9-nm laser line for excitation.…”

“…This hypothesis will be tested in the next section by comparison of the spectra from our well-ordered sample to those of MgO'3A1203 spinel (Ishii et al 1982), for which the tetrahedral sites contain both Mg and A1 but the octahedral sites contain only A1, and to those of the synthetic spinel which is presumably partially disordered (O'Horo et al 1973; Fraas et al 1973). a Acoustic modes subtracted to give total optic modes b For the internal modes, C combination of v3 and v4.…”

“…3AlzO3, that has TO modes at 685, 569, 494, and 308 cm -1 (Ishii et al 1982). In the Raman spectrum, only 4 modes at 312 (Tag), 407 (Eg), 666 (Tlg), and 767 cm -1 (Alg) were found for a natural spinel (Fraas et al 1973) but additional peaks at 225, 492 (Tlg), and 727 cm-1 (Alg) were found for synthetic samples (Fraas et al 1973 ;O'Horo et al 1973 ;Ishii et al 1982). One possible cause of the discrepancies could be the facility with which the MgAI204 spinel structure disorders at relatively low temperatures (~ 670 ~ C), by exchanging cations between the octahedral and tetrahedral sites (Suzuki and Kumazawa 1980;Wood et al 1986).…”