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1

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Deformation of foamed rhyolites under internal and external stresses: an experimental investigation (1993)

Bagdassarov, N Sh ; Dingwell, D B

Springer

Bulletin of volcanology 55 (1993), S. 147-154

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ISSN: 1432-0819

Keywords: Rhyolite ; rheology ; vesiculation ; bubble growth ; shear stresses ; foaming ; two-phase suspension

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract The style of magma eruption depends strongly on the character of melt degassing and foaming. Depending on the kinetics of these processes the result can be either explosive or effusive volcanism. In this study the kinetics of foaming due to the internal stresses of gas expansion of two types of obsidian have been investigated in time series experiments (2 min-24 h) followed by quenching the samples. The volumetric gas-melt ratio has been estimated through the density measurements of foamed samples. The variation of gas volume (per unit or rhyolite melt volume) with time may be described by superposition of two exponentials responsible for gas generation and gas release processes respectively. An observed difference in foaming style in this study is interpreted as the result of variations in initial contents of microlites that serve as bubble nucleation centers during devolatilization of the melts. Quantitatively the values of the gas generation rate constants (k g) are more than an order of magnitude higher in microlite-rich obsidian than in microlite-free obsidian. Possible origins of differences in the degassing style of natural magmas are discussed in the light of bubble nucleation kinetics in melts during foaming. In a complementary set of experiments the mechanical response of vesicular melt to external shear stress has been determined in a concentric cylinder viscometer. The response of vesicular melt to the pulse of shear deformation depends on the volume fraction of bubbles. The obtained response function can be qualitatively described by a Burgers body model. The experimental shear stress response function for bubble-bearing melt has an overshoot due to the strain-dependent rheology of a twophase liquid with viscously deformable inclusions.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00301512

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2

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Pressure-induced coordination change of Ti in silicate glass: a XANES study (1994)

Paris, E. ; Dingwell, D. B. ; Seifert, F. A. ; [et al.]

Springer

Physics and chemistry of minerals 21 (1994), S. 510-515

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ISSN: 1432-2021

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology , Geosciences , Physics

Notes: Abstract The effect of pressure on titanium coordination in glasses, with composition K2TiSi4O11, quenched isobarically from liquids equilibrated at high pressure (5, 10, 15, 20, 25, 30 kbar respectively) and T=1600° C has been investigated by X-ray absorption spectroscopy (XAS). The XANES spectra collected at the Ti K-edge clearly show a variation with pressure that is related to changes in the geometrical environment around the Ti atoms. By comparison with spectra of standard materials, the XANES spectra of the glasses suggest a relatively low average coordination number (near 5) in samples quenched at low pressure and a higher coordination number (near 6) in samples quenched from the highest pressure. The combination of XANES data with density and compressibility measurements supports the idea that a mixture of 6- and lower coordinated (4- and/ or 5-coordinated) Ti geometries are present in the 1 bar glass, and an increasing proportion of 6-coordinated Ti occurs in the glasses synthesized at progressively higher pressures.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00203925

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3

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A volume temperature relationship for liquid GeO2 and some geophysically relevant derived parameters for network liquids (1993)

Dingwell, D. B. ; Knoche, R. ; Webb, S. L.

Springer

Physics and chemistry of minerals 19 (1993), S. 445-453

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ISSN: 1432-2021

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology , Geosciences , Physics

Notes: Abstract The thermal expansivity of liquid GeO2 at temperatures just above the glass transition has been obtained using a combination of scanning calorimetry and dilatometry. The calorimetric and dilatometric curves of c p and dV/dT are normalized to the temperature derivative of fictive temperature versus temperature using the method of Webb et al. (1992). This normalization, based on the equivalence of relaxation parameters for volume and enthalpy, allows the completion of the dilatometric trace across the glass transition to yield liquid expansivity and volume. The values of liquid volume and expansivity obtained in this study are combined with high temperature densitometry determinations of the liquid volume of GeO2 by Sekiya et al. (1980) to yield a temperature-volume relation for GeO2 melt from 660 to 1400 °C. Liquid GeO2 shows a strongly temperature-dependent liquid molar expansivity, decreasing from 20.27 × 10−4 cm3 mol−1°C−1 to 1.97 × 10−4cm3 mol−1 °C−1 with increasing temperature. The coefficient of volume thermal expansion (α v ) decreases from 76.33 × 10−6 °C−1 to 2.46 × 10−6 °C−1 with increasing temperature. A qualitatively similar volume-temperature relationship, with α v decreasing from 335 × 10−6 °C−1 to 33 × 10−6 °C−1 with increasing temperature, has been observed previously in liquid B2O3. The determination of the glass transition temperature, liquid volume, liquid and glassy expansivities and heat capacities in this study, combined with compressibility data for glassy and liquid GeO2 from the literature (Soga 1969; Kurkjian et al. 1972; Scarfe et al. 1987) allows the calculation of the Prigogine-Defay ratio (Π), c p -c v and the thermal Grüneisen parameter (γ th) for GeO2. From available data on liquid SiO2 it is concluded that liquid GeO2 is not a good analog for the low pressure properties of liquid SiO2.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00203184

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4

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X-ray absorption study of Ti-bearing silicate glasses (1994)

Dingwell, D. B. ; Paris, E. ; Seifert, F. ; [et al.]

Springer

Physics and chemistry of minerals 21 (1994), S. 501-509

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ISSN: 1432-2021

Source: Springer Online Journal Archives 1860-2000

Topics: Chemistry and Pharmacology , Geosciences , Physics

Notes: Abstract Ti K-edge XANES spectra have been collected on a series of Ti-bearing silicate glasses with metasilicate and tetrasilicate compositions. The intensity of the preedge feature in these spectra has been found to change with glass composition and varies from 29 to 58% (normalized intensity) suggesting a variation in structural environent around the absorbing atom. The pre-edge peak intensity increases for the alkali titanium tetrasilicate glasses from 35% to 58% in the order Li 〈 Na 〈 K 〈 Rb, Cs whereas for the metasilicate compositions there is a maximum for the K-bearing glass. The pre-edge peak intensity remains constant for the alkaline earth titanium metasilicate glasses, Ca and Sr (34%) but increases slightly for Ba (41%). As the intensity of this feature is inversely correlated with coordination number, a comparison of the pre-edge intensity data for the investigated glasses with those of materials of known coordination number leads us to establish a regression equation and to infer that the average coordination number of Ti in these glasses ranges from 4.8 to 5.8. Large alkali cations appear to stabilize a relatively low average coordination number for Ti in silicate melts. The Ti structural environment results appear also to vary as a function of SiO2 content within the K2O-TiO2-SiO2 system. A number of physical properties of the melts from which these glasses were quenched and of other Ti-bearing silicate melts, have been determined in recent years. Clear evidence of a variable coordination number of Ti, consistent with the interpretation of the present XANES data is available from density measurements. These and other property determinations are compared with the present spectroscopic observations in an attempt to relate structure and properties in these melts which contain a major component with variable coordination number.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00203924

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5

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The effect of phosphorus on the iron redox ratio, viscosity, and density of an evolved ferro-basalt (1994)

Toplis, M. J. ; Dingwell, D. B. ; Libourel, G.

Springer

Contributions to mineralogy and petrology 117 (1994), S. 293-304

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Despite the abundant evidence for the enrichment of phosphorus during the petrogenesis of natural ferro-basalts, the effect of phosphorus on the physical properties of these melts is poorly understood. The effects of phosphorus on the viscosity, density and redox ratio of a ferro-basaltic melt have been determined experimentally. The viscosity measurements were obtained using the concentric cylinder method on a ferro-basaltic melt above its liquidus, at 1 atm, in equilibrium with air and with CO2. The density measurements were performed using the double Pt-bob Archimedean method at superliquidus conditions under 1 atm of air. The redox ratio was obtained by wet chemical analysis of samples collected during physical property measurements. Phosphorus pentoxide reduces ferric iron in ferro-basaltic melt. The reduction due to P2O5 is much larger than that for most other oxide components in basaltic melts. A coefficient for the reduction of ferric iron has been generated for inclusion in calculation schemes. The effect of P2O5 on the viscosity is shown to be complex. The initial reduction of ferric iron with the addition of P2O5 results in a relatively small change in viscosity, while further addition of P2O5 results in a strong increase. The addition of phosphorus to a ferro-basaltic melt also reduces the density. A partial molar volume of 64.5±0.7 cm3/mol for P2O5 in this melt has been obtained at 1300° C, yielding a volume of 12.9 cm3/mol per oxygen, consistent with a tetrahedral coordination for this high field strength cation. The effects of P2O5 on redox state, density and viscosity provide constraints on the structural role of phosphorus in these melts. The results suggest a complex interaction of phosphorus with the aluminosilicate network, and tetrahedral ferric iron. In light of the significant effects of phosphorus on the physical and chemical properties of ferro-basaltic liquids, and the extreme enrichments possible in these liquids in nature, the role of phosphorus in these melts should, in future, be considered more carefully.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00310870

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6

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Non-linear temperature dependence of liquid volumes in the system albite-anorthite-diopside (1992)

Knoche, R. ; Dingwell, D. B. ; Webb, S. L.

Springer

Contributions to mineralogy and petrology 111 (1992), S. 61-73

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract The temperature-dependent thermal expansivities of glasses and liquids in the ternary albite-anorthite-diopside have been determined using a combination of calorimetry, dilatometry and Pt and Ir double bob Archimedean densitometry. Supercooled liquid volumes and molar thermal expansivities were determined across the glass transition using a combination of scanning calorimetry and dilatometry, based upon the equivalence of relaxation of volume and enthalpy in the vicinity of the glass transition. Superliquidus volumes were determined using double Pt bob Archimedean densitometry at temperatures up to 1,650°C and double Ir bob densitometry at 1,800°C. Experimental access to liquid volumes near the glass transition temperatures (680–920°C) and at superliquidus temperatures (1,400–1,800°C) for these compositions results in the observation of a nonlinear temperature dependence of molar volume, i.e., temperature-dependent thermal expansivities. The diopside composition wxhibits the largest temperature dependence of thermal expansivity, decreasing by ∼50% between 800 and 1,500°C. Linear extrapolation of the high-temperature volume data of diopside to 810°C would result in a 3% overestimation of the molar voltime. The temperature dependence of the molar volume of anorthite is approximately linear. The thermal expansivities of the liquids in the albite-anorthite-diopside system appear to converge at high temperature. This study uses a combination of methods that allows interpolation rather than extrapolation of the extant melt-volume data into the petrologically meaningful (subliquidus) temperature range.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00296578

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