Raman study of some melilites in crystalline and glassy states

Journal Article · Mon Aug 01 00:00:00 EDT 1988 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(88)90177-9· OSTI ID:7266110

Sharma, S K; Matson, D W ^[1]; Yoder, Jr, H S ^[2]

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1. Hawaii Institute of Geophysics, Honolulu (USA)
2. Geophysical Lab., Washington, DC (USA)

Raman spectra are reported for crystalline akermanite, hardystonite, gehlenite, sodium melilite and for glasses of corresponding compositions. The spectra of melilites are dominated by the vibrational modes of pyrosilicate units and not by the sheet-like structure formed by interconnected TMO{sub 4} tetrahedra. The frequency of {nu}{sub s}(T-O-T), the symmetric stretching mode of the bridging oxygen in the pyrosilicate unit, is directly related to the angle of the T-O-T linkage. The symmetric stretching bands of nonbridging oxygens {nu}{sub s}(T-O{sup {minus}}) appear in the spectral range characteristic of T-O{sup {minus}} stretching in pyrosilicate units. The intensity of the {nu}{sub s}(T-O{sup {minus}}) band is, however, affected by the presence of Al{sup 3+} in tetrahedral sites adjacent to the pyrosilicate units. The lowering of the intensities of nonbridging oxygen stretching bands in the spectra of Geh and SM is attributed to a change in the degree of covalency of T-O{sup {minus}} bonds in the pyrosilicate unit resulting from substitution of Al{sup 3+} for Mg{sup 2+} in the adjacent tetrahedral sites. Comparison of the spectra of crystals with glasses indicates that most Al{sup 3+} ions act as network-forming cations, whereas Mg{sup 2+}, Zn{sup 2+} and Ca{sup 2+} retain their role as network modifiers. Spectra of glasses of Ak and Har composition show SiO{sub 4}{sup 4{minus}} and Si{sub 2}O{sub 7}{sup 6{minus}} bands, indicating a redistribution of silicate species among monomer, dimer, trimer and tetramer chains. Glasses of Geh and SM compositions are more highly polymerized than their respective crystalline counterparts because of the role of Al{sup 3+} as network-forming cations in the glass structures.

• OSTI ID: 7266110
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Vol. 52:8; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
SILICATE MINERALS
CRYSTALLOGRAPHY
RAMAN SPECTRA
AMORPHOUS STATE
CHEMICAL COMPOSITION
CRYSTAL STRUCTURE
CRYSTALLIZATION
GEOCHEMISTRY
QUANTITATIVE CHEMICAL ANALYSIS
RAMAN SPECTROSCOPY
CHEMICAL ANALYSIS
CHEMISTRY
LASER SPECTROSCOPY
MINERALS
PHASE TRANSFORMATIONS
SPECTRA
SPECTROSCOPY
580000* - Geosciences