Vibrational spectroscopic studies of olivines, pyroxenes, and amphiboles at high temperatures and pressures

Abstract

Structural properties of olivines, pyroxenes, and amphibole crystals are investigated using Raman and infrared spectroscopy. Raman spectra of olivine crystals are measured along the forsterite-monticellite solid solution series in the 100-1200 cm-1 region. In the spectrum of Fo, strong Si-O- symmetric stretching bands appear at 824 and 855 cm^-1, and a medium intensity anti-symmetric Si-O- stretching band appears at 964 cm^-1 These bands show a systematic frequency variation with composition and are attributed to decreased distortion of SiO4 tetrahedra. The mixing characteristics of ν1 and ν3 modes in orthosilicates were found to strongly depend on the ratio of average inter-(O-O) to (Si-O) bond lengths. In the low-frequency region of the spectra, Fo-rich compositions show progressive broadening of the bands with the addition of up to 16 wt% monticellite and is attributed to increasing positional disorder caused by the substitution of large calcium ions into the M1 octahedral sites. Structural properties of several pyroxene crystals have been investigated using Raman spectroscopy at various temperatures. The strong band around 700 cm^-1appears in all pyroxenes, is the characteristic feature of single chain pyroxenes and it increases with the addition of smaller cations. Intensity ratio of Si-O- to stretching modes to Si-O- Si symmetric stretching mode suggests that the Si-O--M (M=cation) bonds in diopside and hendenbergite are more in ionic character than spodumene and jadeite. The high-temperature spectra of spodumene indicates that above 700 C, it's structure becomes metastable. Ortho-to-proto phase transition in enstatite takes place around 1320 K as indicated by the appearance of a new band at 98 cm^-1 , High temperature results indicate that the force constants of the silicate bonds in pyroxene decrease with temperature. Raman and infrared spectra of cummingtonite, actinolite, and several Ti-rich amphibole crystals from various locations in the United States have been measured in the 100-4000 cm-1, region. Raman spectra of cummingtonite and actinolite shows a strong symmetric stretching band at 662 and 671 cm^-1, respectively. The Raman bands above 1000 cm-1 are assigned to the anti-symmetric stretching of the Si-O-Si bonds and in the hydroxyl region up to four bands appear at about 3675 cm-1 indicating different cation environment of hydroxyl ions in these two minerals. In Ti-rich amphibole crystals the symmetric and anti-symmetric bands appear broad and weak and they vary in their frequencies and intensities with MgO and TiO content respectively. In the hydroxyl stretching region both Raman and IR spectra have only one broad band and its intensity varies linearly with the total H20 and shifts toward high-frequency with increasing MgO content. With the present experimental setup up to 0.5 wt% water in minerals can be detected. The presence of broad band indicates random distribution of cation in M1, M2, M3, M4, and A sites resulting in increased positional disorder.