Dean W. Matson

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

Abstract 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.

Raman study of the structure of glasses along the join SiO2GeO2

Abstract In order to better understand the distribution of tetrahedra in multicomponent tetrahedral network structures of melts and glasses, we have investigated the Raman spectra of binary SiO 2 GeO 2 glasses. We compare the Raman spectral features of the end-member glasses and discuss their vibrational origins. The mixing of GeO 2 and SiO 2 melts results in a continuous random network structure of TO 4 tetrahedra (T  Si, Ge) in the glass. Raman bands corresponding to the asymmetric stretch ( v as ) of oxygen in GeOGe, SiOSi and SiOGe bonds are observed in the glasses having intermediate compositions along the SiO 2 GeO 2 join. The presence of three distinct v as (TOT) bands in the spectrum of a glass having Si/Ge one reveals that a considerable degree of SiGe disorder exists in the glass. The presence of a single symmetric oxygen stretching band in the spectra of binary SiO 2 GeO 2 glasses indicates that the symmetric stretch modes ( v s ) of oxygen in SiOSi, SiOGe and GeOGe bonds are strongly coupled. An observed decrease in the halfwidth of the v s (TOT) band in the spectra of SiO 2 GeO 2 glasses with increasing concentration of GeO 2 may be attributed to a decrease in the average TOT bond angle and a predominance of six-membered ring structures. Results of the present study support the assignment of the bands in the 900–1200 cm −1 region of the alumino-silicate glasses, spectra to the v as (AlOSi) and v as (SiOSi) modes. In contrast to the alumino-silicate glasses, however, the SiO 2 GeO 2 glasses have a much higher degree of disorder of the network-forming cations.

Ring distributions in alkali- and alkaline-earth aluminosilicate framework glasses- a raman spectroscopic study☆

Raman spectra of crystalline polymorphs of a number of tectosilicate minerals having various sizes of smallest rings of TO4 tetrahedra (T = Si, Al) have been investigated to identify the bands that are sensitive indicators of the smallest rings in the network. The information obtained from the Raman spectra of tectosilicate minerals (e.g., SiO2 polymorphs, NaAlSi3O8 (Ab), NaAlSiO4 (Ne), KAlSi3O8 (Or), and KAlSi2O6 (Lc)) is used to interpret the Raman spectra of the isochemical glasses. It is shown that the frequency of the dominant νs (TOT) band in the spectra of both crystals and glasses is related to the dominant size of TO4 rings in the structure. In agreement with previous X-ray RDF work, it is found that in the glasses of Ab and Jd (NaAlSi2O6) compositions, six-membered rings of TO4 tetrahedra predominate. The Raman spectrum of Or glass, however, indicates that clusters of intermixed four- and six-membered rings of TO4 tetrahedra, similar to those existing in crystalline leucite, are also present in the glass. Raman evidence indicates that four-membered rings of TO4 tetrahedra predominate in the glass of An composition. Similarly, the higher frequency of the νs (TOT) band in the spectrum of Ne glass as compared with the frequency the νs (TOT) band in the spectra of crystalline cargenieite and nephelite indicates either an admixture of the four- and six-membered rings or the puckering of six-membered rings in the glass structure.

Raman study of some melilites in crystalline and glassy states

Abstract Raman spectra are reported for crystalline akermanite (Ak, Ca2MgSi2O7), hardystonite (Har, Ca2ZnSi2O7), gehlenite [Geh, Ca2Al(AlSi)O7], sodium melilite (SM, CaNaAlSi2O7) and for glasses of corresponding compositions. The spectra of melilites are dominated by the vibrational modes of pyrosilicate (T2O7) units (T = Si or Al) and not by the sheet-like structure formed by interconnected TMO4 tetrahedra (M = Mg, Zn and Al). The frequency of vs(T-O-T), t 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 vs(T-O−) appear in the spectral range characteristic of T-O− stretching in pyrosilicate units. The intensity of the vs(T-O−) band is, however, affected by th presence of A13+ 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− bonds in the pyrosilicate unit resulting from substitution of A13+ for Mg2+ in the adjacent tetrahedral sites. Comparison of the spectra of crystals with glasses indicates that most Al3+ ions act as network-forming cations, whereas Mg2+, Zn2+ and Ca2+ retain their role as network modifiers. Spectra of glasses of Ak and Har composition show SiO4−4 and Si2O6−7 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 Al3+ as network-forming cations in the glass structures.

Temperature dependence of CO2 solubility in high pressure quenched glasses of diopside composition

Abstract The temperature dependence of carbon dioxide solubility in glasses of diopside composition, quenched from 20 kbar, has been investigated using a combination of high-temperature mass spectrometry and Raman spectroscopy. CO 2 -charged diopside glasses were synthesized in a piston-cylinder apparatus. Because of diffusion of hydrogen through the platinum capsules, significant amounts of H 2 O, CH 4 and CO were detected along with CO: in the diopside glasses. All three carbon species show a bimodal release pattern in the mass pyrograms. The CO 2 solubility shows a linear and negative temperature dependence. We do not observe any maxima in the solubility curve as was reported previously ( Mysen and virgo, 1980a). None of the additional bands observed in Raman spectra of CO 2 -charged diopside glasses compared to those in the spectrum of diopside glass can be assigned to molecular CO 2 . These bands are caused by CO −2 3 ions and indicate that the physical solubility of molecular carbon dioxide is negligible. The bimodal release pattern observed for CO 2 in the mass pyrograms, is consistent with the Raman data which strongly suggests that CO −2 3 ions are present in at least two distinct sites in the glass.

Volatile contents of phlogopite micas from South African kimberlite

Phlogopite micas from nodules in South African kimberlites were analyzed for major elements with the electron microprobe and for volatile contents by high temperature mass spectrometry. The micas are from primary- (deformed) and secondary- (undeformed) textured grains in perodotite xenoliths, glimmerites, MARID (mica-amphibole-rutile-ilmenite-diopside) suite nodules and a mica megacryst. The major element and volatile contents of micas exhibiting these modes of occurrence overlap to a greater extent than indicated in previous studies. Concentrations of volatile species occupying structurally defined crystallographic sites (H2O, F, Cl) are greater for many of the micas than predicted on the basis of the mica formula, particularly for the glimmerite and MARID suite samples. A correlation exists between micas with tetrahedral and octahedral cation deficiencies and those with excess H2O, F and Cl. Substitution of H+ for tetrahedral and possibly octahedral cations may be responsible for the excess H2O in these micas. Except for one sample, the major element and volatile data for the peridotite, glimmerite and MARID suite micas indicate that they crystallized at oxygen fugacities below the quartz-fayalite-magnetite buffer. F and K2O are in the correct proportion in the micas to provide the source for these elements in alkali basalts, but not in mid-ocean ridge basalts. Kaersutite amphibole is a more likely source of potassium and fluorine in mid-ocean ridge basalts.

Structures of sodium alumino- and gallosilicate glasses and their germanium analogs

Abstract Polarized Raman spectra of SiO 2 and GeO 2 glasses are related on the basis of frequencies, band-shapes, and polarization characteristics. The spectra are consistent with structures composed of continuous networks of SiO 4 and GeO 4 tetrahedra. The presence of depolarized TO and LO components of the antisymmetric stretching mode in the high frequency region of both spectra indicates that these single component glasses retain some degree of intermediate range order. Features in the polarized spectra of NaAlSi 3 O 8 , NaAlSi 2 O 6 , and NaAlSiO 4 glasses and their Ga- (for Al) and/or Ge- (for Si) substituted counterparts are also consistent with the assignment of three-dimensional tetrahedral framework (TDTF) structures to these glasses. The gallosilicate and alumino- and gallogermanate glasses provide structural analogs for the framework aluminosilicate glasses of geological significance. The high-frequency antisymmetric stretching features in the Raman spectra of the TDTF aluminosilicate glasses and their Ga- and/or Ge-substituted analogs show an overlapping doublet of bands whose frequencies vary continuously with glass composition. Both the intensity and position of the higher frequency component are more strongly dependent on Al(Ga) content than is the lower frequency component. Some coupling of the antisymmetric stretching modes of Si(Ge)O 4 tetrahedra and Al(Ga)O 4 tetrahedra is indicated. The higher frequency component of the doublet is more strongly polarized than the lower frequency component, which results from the splitting of the antisymmetric stretching (F) mode of Si(Ge)O 4 tetrahedra. This splitting results from the lowering of the average of Si(Ge)O 4 tetrahedra site symmetries because of the presence of Al(Ga)O 4 units in the glass network.

Volatiles in amphiboles from xenoliths, Vulcan's Throne, Grand Canyon, Arizona, USA

Abstract Analyses of major element and volatile components of amphiboles from Vulcans Throne, a Recent volcano on the north rim of the Grand Canyon, Arizona, USA, have been performed by using the electron microprobe and high temperature mass spectrometry. The amphiboles occur as megacrysts, as oikocrysts in peridotite and pyroxenite xenoliths, in amphibole-rich selvages on lherzolite xenoliths, and as grains in hornblendite xenoliths. Total volatiles range from 1.27 to 1.75 wt.%. In all samples, H 2 O is the principal volatile species. Lesser amounts of structurally bound fluorine, chlorine, and oxygen were also released. The amphiboles studied are hydroxyl-deficient. The O(3) site is probably partially occupied by O 2− , which was detected as O 2 during degassing of the amphibole. Ti shows a strong positive correlation with the amount of hydroxyl deficiency in the amphiboles except for one oxidized sample. Thus, Ti probably is significant in charge balancing the substitution of O 2− for OH − and the substitution probably occurred during crystallization rather than by dehydrogenation. Small amounts of both oxidized and reduced carbon and sulfur-bearing volatile species ( e.g ., CO 2 , CO, CH 4 , SO 2 , H 2 S) were detected in all samples. The observation of reduced carbon species supports the hypothesis that the oxygen fugacity of at least portions of the upper mantle is probably less than the quartz-fayalite-magnetite buffer.

Effect of high pressures on the structure of anhydrous and hydrated GeO2 glasses

Abstract The effect of high pressures on the structures of anhydrous and hydrated GeO 2 glasses, formed by quenching GeO 2 melts above the liquidus temperature and at P ⩽ 18 kbar, is investigated using normal and differential Raman spectroscopy. It is shown that at least up to 18 kbar Ge 4+ ions remain fourfold coordinated in GeO 2 melt, and both low and high pressure glasses contain predominantly six-membered rings of GeO 4 tetrahedra. Normal and differential Raman spectra of hydrated GeO 2 glasses clearly show formation of GeOH bonds in the glass by the appearance of a polarized band at 760 cm −1 . It is found that water preferentially attacks the defect sites responsible for the ∼ 520 cm −1 band. The observed densification of high-pressure quenched GeO 2 glasses is attributed to a decrease in the volume of voids in the melt at high pressures. It is suggested that the observed anomalous decrease in the viscosity of the GeO 2 melt at high pressure results from the increase in the diffusivities of germanium and oxygen ions in the melt at high pressure.