1,721,191 research outputs found
SULFUR SOLUBILITY IN SILICATE MELTS: A THERMOCHEMICAL MODEL
No full textA termochemical model for calculating sulfur solubility of simple and complex
silicate melts has been developed in the framework of the Toop-Samis polymeric
approach combined with a Flood - Grjotheim theoretical treatment of silicate slags
[1,2]. The model allows one to compute sulfide and sulfate content of silicate melts
whenever fugacity of gaseous sulphur is provided.
"Electrically equivalent ion fractions" are needed to weigh the contribution of the
various disproportion reactions of the type:
MOmelt + 1/2S2,gas ,MSmelt + 1/2O2,gas (1)
MOmelt + 1/2S2,gas + 3/2O2,gas ,MSO4,melt (2)
Eqs. 1 and 2 account for the oxide-sulfide and the oxide-sulfate disproportionation
in silicate melt.
Electrically equivalent ion fractions are computed, in a fused salt Temkin notation,
over the appropriate matrixes (anionic and cationic). The extension of such matrixes
is calculated in the framework of a polymeric model previously developed [1,2,3]
and based on a parameterization of acid-base properties of melts. No adjustable
parameters are used and model activities follow the raoultian behavior implicit in the
ion matrix solution of the Temkin notation. The model is based on a huge amount of
data available in literature and displays a high heuristic capability with virtually no
compositional limits, as long as the structural role assigned to each oxide holds.
REFERENCES:
[1] Ottonello G., Moretti R., Marini L. and Vetuschi Zuccolini M. (2001), Chem.
Geol., 174, 157-179.
[2] Moretti R. (2002) PhD Thesis, University of Pisa.
[3] Ottonello G. (2001) J. Non-Cryst. Solids, 282, 72-85.
An appraisal of endmember energy and mixing properties of rare earth garnets
No full textEnergetics of rare Earth aluminum (REE3Al5O12), iron
(REE3Fe5O12) and gallium (REE3Ga5O12) garnets are assessed
through a critical evaluation of all the existing experimental
data and a thermodynamic treatment of vibrational,
static and volumetric properties of the various substances.
Application of the developed thermodynamic data-base,
coupled with the interionic static potential model previsouly
developed for major silicate garnet end members [1] leads to
establish the mixing properties of the various substances
with the major isostructural silicate components and to determine
the limits of Henry’s law behavior for REE in natural
garnets. Based on calculations, mixing of REE granet
components at trace level (i.e., below about 102 ppm) with
major silicate components is virtually ideal and deviations
from ideality become sensible at trace concentrations exceeding
103 ppm. Corresponding deviations from Nernst’s
law behavior in garnet/fluid REE equilibria follows exponential
trends whose nature is analogous to what has been
experimentally observed in silicate/fluid equilibria involving
other solid phases and other trace elements.
It is finally stressed that the light REE–heavy REE
(LREE/HREE) fractionation observed in natural garnet
specimens is due to the intrinsic energy properties of the
various REE-garnet end members, and not to a structural
effect dictated by the carrier, as commonly assumed in literature.
References: [1] Ottonello G. et al. (1996) Amer. Mineral,
81, 429–447. [2
A NEW METHOD TO COMPUTE FLUIDS SATURATION IN C-H-O-S-SILICATE MELT SYSTEMS
No full textWe developed a method to calculate equilibrium between a
C-O-H-S fluid phase and a silicate melt based on a previous
model for the saturation of H2O-CO2 fluids (Papale, 1999) and
on a thermochemical approach for calculating sulfide and
sulfate solubilities of simple and complex melts. In particular,
this second approach combines the Toop-Samis polymeric
model with the Flood - Grjotheim theoretical treatment of silicate
melts (Ottonello et al., 2001; Moretti, 2002). Moreover,
fugacities in the gaseous phase are computed through the
SUPERFLUID code (Belonoshko et al., 1992). The C-H-O-S
saturation model allows determining the partition of H2O, CO2,
and S between silicate melt and coexisting fluid, and the
composition of the fluid phase in terms of H2O, CO2, SO2, and
H2S, as a function of pressure, temperature, volatile-free liquid
composition, oxygen fugacity, and total amount of volatile
components in the system. For the sake of simplicity, we
assumed that no reduced or oxidized sulfur-saturated solid or
liquid phases nucleate or separate from the liquid-gas system.
Minima in sulfur solubility as a function of oxygen fugacity are
depicted, in good agreement with theory and experiments.
Applications are given for rhyolitic and basaltic melts with
various oxygen fugacities in the range NNO±2, and pressure
from a few hundred MPa to atmospheric. The developed model
accounts for the reciprocal effects of volatiles on their saturation
contents, and the complex relationships between the saturation
surface of a multicomponent fluid and the liquid
composition, volatile abundance, P-T conditions and oxidation
state.
Belonoshko A, Shi PF & Saxena S, Comp. Geosci, 18, 1267-
1269, (1992).
Moretti R, PhD Thesis, University of Pisa
Ottonello G, Moretti R, Marini L& Vetuschi Zuccolini M, Chem.
Geol, 174, 157-179, (2001).
Papale P, Amer. Mineral, 84, 477-492, (1999)
A model for multicomponent fluid saturation in C-O-H-S-silicate melt systems
No full textThe dissolution behavior of volatile components in magmas is essential to model the
volcanic process from the deep regions of magma generation and storage to the shallow
regions of magma eruption and emplacement.
Water, carbon dioxide, and sulfur compounds are the main volatile components in natural
magmas, constituting in most cases more than 99% of the volcanic gases released
before, during, and after eruption.We have developed a method to calculate the chemical
equilibrium between a fluid phase in the C-O-H-S system and a silicate melt with
composition defined by ten major oxides. The method is based on a previous model
for the saturation of H2O-CO2 fluids [1] and on a sulfur solubility model [2] in silicate
liquids. For the computation of the fugacities of components in fluids with complex
composition we used the SUPERFLUID code [3]. The model allows determining the
partition of H2O, CO2, and S between the silicate liquid and the coexisting fluid, and
the composition of the fluid phase in terms of H2O, CO2, SO2, and H2S, as a function
of pressure, temperature, volatile-free liquid composition, oxygen fugacity, and
total amount of each volatile component in the system. App lications are presented
to several silicate liquids with rhyolitic and basaltic composition, oxygen fugacities
in the range NNO ± 2, and pressure from a few hundred MPa to atmospheric, with
the simplifying assumption that no reduced or oxidized sulfur-saturated solid or liquid
phases nucleate or separate from the liquid-gas system. Results show the well-known
minima in sulfur saturation contents as a function of oxygen fugacity, the reciprocal
effects of volatiles on their saturation contents, and the complex relationships between
saturation surface of a multicomponent fluid, liquid composition, volatile abundance,
P-T conditions, and oxidation state. The method represents therefore a new powerful
tool for the prediction of multicomponent gas-melt equilibria in magmas.
REFERENCES
[1] Papale P. (1999) Am. Mineral., 84, 477-492
[2] Moretti R. and Ottonello G. This issue
[3] Belonoshko A.B., Shi P. and Saxena S,K, (1992) Comp. Geosci., 18, 1267-1269.
Misure dei flussi di 222Rn e 220Rn dai suoli con camera di accumulo e rivelatore allo stato solido di radiazioni alfa. Interferenza della CO2 in aree ad alto flusso
No full textAttività di 238U, 232Th, 226Ra, 40K, 137 Cs e 210Pb in 10 profili di suolo delle aree di Baccu Loci e S’Acqua Callenti
No full textOn the Lux-Flood basicity of melts in solving their chemical reactivity
No full textReactivity of silicate melts is due to charged functional groups (cations, free anions
and polymeric units or structons), which govern mutual interactions between
constituting oxides. The main problem arises when defining thermodynamic oxide
ion activities. This difficulty is overcome if we adopt the Fincham and Richardson
(1954) formalism coupled with a Toop and Samis (1962a,b) polymeric description of
the anion matrix, based on the principle of equal reactivity of co-condensing groups
and involving singly bonded, doubly bonded and free oxygen, that is oxygen species
in three different polarization states. In a chemically complex melt the capability
of transferring fractional electronic charges from the ligand to the central cation
depends in a complex fashion on the melt structure, affecting the polarization state
itself. Therefore, the mean polarization state of the various ligands (mainly oxide
ions in silicate melts) and their ability to transfer fractional electronic charges to the
central cation are conveniently represented by the optical basicity of the medium. On
this basis, Ottonello et al. (2001) related linearly the optical basicity to the extent of
the anionic matrix and hence to the polymerization constant. Although the adopted
functional form is rather brutal, not accounting for temperature effects, it allows
an accurate description of the anionic matrix which enable us to study the iron
oxidation state in both anhydrous and hydrous melts, then solving some controversies
present in literature, to define a consistent model for water speciation which accounts
for the amphoteric behavior of this component and to assess sulfur speciation and
solubility. The latter, in particular, was possible only through the adoption of the
Flood and Grjotheim (1952) thermochemical cycle, which accounts for the standard
state transposition between the Temkin standard state of completely dissociated
components and that of pure component at T and P of interest.
Recently, we applied the Hybrid Polymeric model (Ottonello, 2001) to assess silicate
melt energetics, distinguishing chemical interaction terms from strain energy contributions
(Ottonello and Moretti, 2004). Lux-Flood basic oxides give rise to purely
endothermic effects when admixed to silica along simple bianry joins, whereas acidic
Lux-Flood oxides originate thermal admixtures and amphoteric oxides promote both
enthalpic and entropic (non configurational) chemical interactions.
This makes the Lux-Flood acid-base character of the various oxides consistent
with experimental determinations of nephelauxetic properties of the limiting oxide
components in the mean donor ligand field, represented in terms of optical basicity.
A linear proportionality is observed between endothermic heat of mixing and optical
basicity which allows us to predict the polymerization extent in molten MO-SiO2
binaries.
The extension of this proportionality to complex systems requires the application of
the Flood-Grjotheim (1952) approach and allows us to shift the previously developed
models for iron, water and sulfur to a more rigorous treatment of the anionic matrix
of silicate melts.
REFERENCES
Fincham and Richardson (1954) Proc. Roy. Soc. London, 223A, 40.
Flood and Grjotheim (1952), J. Iron Steel Inst., 171, 64.
Toop and Samis (1962a) Can. Met. Quart., 1, 129.
Toop and Samis (1962b) Trans. Met. Soc. AIME, 224, 878.
Moretti (2003) Ann. Geophys. (submitted).
Moretti and Ottonello (2003a) Metall. Mat. Trans. B, 34B, 399.
Moretti and Ottonello (2003b) J. Non Cryst. Sol., 323, 111.
Ottonello (2001), J. Non Cryst. Sol., 282, 72.
Ottonello and Moretti (2004) J. Phys. Chem. Sol. (submitted).
Ottonello et al. (2001), Chem. Geol., 174, 157
The solubility and speciation of sulfur in silicate melts: development of the conjugated Toop-Samis-Flood-Grjotheim (CTSFG) model
No full textPolymerization and disproportionation of iron and sulfur in silicate melts: insights from an optical basicity-based approach
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