Raman spectroscopy studies of alkali and alkaline earth silicate glasses and melts are reviewed, and the major Raman bands observed are summarized. To realize the full potential of Raman spectroscopy will require a detailed understanding of their vibrational properties in relation to structure. A number of vibrational calculations have been carried out to address this problem. These are briefly summarized, and some limitations of the method are noted. Systematic variations in bulk properties are examined for viscosity and the immiscibility behavior.

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Structural studies of silicate glasses and melts—applications and limitations of Raman spectroscopy

Viscosity of liquid silica, silicates and alumino-silicates

Normal Raman spectra of xM2O · (100 − x)SiO2 glasses (M = Li, Na, K, Rb, Cs and x = 0, 5, 10, 15, 20, 25, 30) and differential Raman spectra of the 5, 10, and 15% M2O glasses are presented. The Raman spectra reflect distinct structural differences in the silicate networks of these glasses caused by the presence of different alkali cation types. The structural origin and localization of vibrational modes producing characteristic spectral bands of these glasses are discussed. Bands in the 900–1200 cm−1 region of the spectra of the alkali-silicated glasses result from highly localized Si-nonbridging oxygen stretching modes and relative intensities of these bands may be used to determine alkali distributions around SiO4 tetrahedra. Comparison of the Raman Spectra of the alkali-silicate glasses has shown that smaller alkali cations exhibit a greater tendency than their larger counterparts to cluster in pairs around SiO4 tetrahedra in these glasses, even at low alkali contents. The high frequency spectral features at 1100 and 1150 cm−1 also indicate the presence of two distinct structural environments in which an SiO4 tetrahedron contains one nonbridging oxygen in all alkali-silicate glasses containing ⩽ 25% M2M except those containing lithium. Intensity of the low frequency spectral band at 440 cm−1 indicates that regional alkali clustering such as that required for phase separation is also more prevalent in glasses containing smaller alkali cations. It is proposed that modeling of the silicate structure in alkali-silicate glasses must account for both long-range and localized alkali cation distribution.

The structure of high-silica alkali-silicate glasses. A Raman spectroscopic investigation

The compressibility of basic melt at 1 atmosphere is about an order of magnitude higher than that of mantle minerals. Consequently, the density contrast between melt and the principal residual crystals in mantle source regions is expected to decrease with increasing source region depth. The increasingly olivine-normative character of primary melts produced at greater depths is also expected to result in a decrease in this density contrast with increasing source region depth. Once vertical permeability is established by melt generated during partial melting, buoyancy-driven melt percolation can under some circumstances segregate melt from the residual crystals in its source region on a geologically rapid time scale. Limits to this process are provided by cooling of the source region (freezing melt in) and rigidity of the crystalline matrix (mechanically trapping melt). Source region size influences these limits strongly: consequently, small, partially molten diapirs (∼km in diameter) may be able to trap large melt fractions (≳30%), but larger source regions would be unable to do so. The reduction in density contrast with pressure reduces the buoyant force driving melt percolation and provides another limit to melt segregation. Diapirs at depth may thus stably contain large fractions of melt but may decompress and unload their melt during ascent; this effect would be enhanced in small diapirs and may be relevant to the genesis of komatiitic magma. Melt compression may also be a factor in explaining why the very different maximum depths inferred for typical basic melt segregation from source regions on different planets—∼500 km on the moon, ∼250 km on Mars, ∼100 km on earth—correspond to similar pressures (25–35 kbar); at greater pressures, melt may no longer be capable under ordinary conditions of segregating upwards by buoyancy. This may also help to explain why depleted peridotites overlie more fertile peridotites and how deep regions of the mantle are able to remain fertile over geologic time.

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Melt segregation from partially molten source regions: The importance of melt density and source region size

Infrared spectra demonstrate that at pressures above 20 gigapascals and room temperature the regular tetrahedral coordination of oxygen around both silicon and aluminum ions is severely disrupted in SiO2, CaMgSi2O6, and CaAlSi2O8 composition glasses. The spectra are consistent with gradual, pressure-induced increases in the coordination numbers of silicon and aluminum. A variety of coordination environments, from sixfold to fourfold, appears to be present at pressures as high as about 40 gigapascals. This apparent change in coordination is not quenchable at room temperature: on decompression, the glasses return to tetrahedral coordination. This continuous and reversible coordination change in amorphous silicates explains the lack of observation of coordination changes in silicate glasses quenched from high pressure, the shallow melting slopes observed for mantle silicates at high pressures, and the possible presence of neutrally buoyant magmas deep within the terrestrial planets.

https://science.sciencemag.org/content/sci/239/4842/902.full.pdf

Spectroscopic evidence for pressure-induced coordination changes in silicate glasses and melts.

The coordination of Al3* in glasses of jadeite composition, synthesized at 1450" and 1550"C and over the pressure range 0.fi)l-40 kbar, has been determined using laser Raman spectroscopy. The Raman spectrum of glass ofjadeite composition at I atm resembles that of SiO2 glass, except that the Si-O stretching bands occur at lower frequencies than in SiO, glass and are weakly polarized in contrast to the depolarized bands in SiO, glass. The shift of the Si-O bands in the jadeite glass to lower frequencies is due to Si(Al)-O coupled modes. The weakly polarized stretching bands indicate that the local ordering in jadeite glass may resemble that of c-cristobalite. The spectra ofjadeite glasses formed from liquids quenched at high pressure are similar to that of glass formed from liquid quenched at I atm, except that the S(AI)-O stretching bands become strongly polarized with increasing pressure and are shifted to lower frequencies. There is also a decrease in the intensity of the Rayleigh tail. The major spectral differences betweon the l-atm and high-pressure glasses are explained in terms of differences in the symmetry of the local ordering of the network structure and a systematic decrease in the bond angle at the oxygen atom shared by two adjacent silicon atoms. Overall, the effect ofpressure on the structure of NaAlSirOu melts is to increase the degree of local ordering and to change the local network structure to a coesite-type structure. Al3* remains tetrahedrally coordinated over the pressure range investigated; there is no support for the hypothesis that a pressure-induced coordination change ofAl3* from fourfold to sixfold is responsible for the observed decrease in viscosity of melts ofjadeite composition at elevated pressures.

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B. O. Mysen

Papers

Raman study of the coordination of aluminum in jadeite melts as a function of pressure