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Volatile solubility in Silicate Melts with particular regard to sulphur species: Theoretical Aspects and Application to Etnean Volcanics. PhD Thesis (University of Pisa)
No full textMODELLING THE SULPHIDE CAPACITY OF SILICATE MELTS: IMPLICATIONS FOR VOLCANIC DEGASSING
No full textThe assessment of sulphide capacity of silicate melts over the wide range of natural
compositions represents an important task for geochemists and petrologists dealing
with the study of volcanic degassing in order to allow a better comprehension of the
deep system originating sulphur emission. It has been yet recognised that sulphur
dissolves in silicate melts both as sulphide and sulphate depending on oxygen
fugacity, giving rise to a V-shaped function on a log[S wt%] Vs. logfO2 plot. The
solubility of sulphide in the low oxygen fugacity side –the most important to the
purposes of gas-magma interactions, i.e. logfO2 less than FMQ - is practically given
by the Sulphide Capacity. Here we present our results on about 300 data of
literature, including both synthetic systems of compositions relevant to geosciences
and “simple” metallurgical slags. Given the equilibrium O2- + 1⁄2S2 ó S2- + 1⁄2O2 (1),
sulphide capacity is normally defined as Cs = (S wt%)·(fO2/fS2)1/2. We can estimate
the molar sulphide capacity of silicate melts through a modified Flood – Grjotheim
approach [1] which considers the quantity log K’1 = SNilogK’i - where Ni is the
fraction of the ith ion over the appropriate matrix and logK’i is related to partial
equilibria of the type MeO + 1⁄2S2 ó MeS + 1⁄2O2 and is defined as logKi·nO
2- = log
[nS2-·(fO2/fS2)1/2]. It is important to remark that each K’i term embodies equilibrium
constants related to dissociation equilibria of the corresponding Me-oxide and Mesulphide
in the melt phase. The addition of Margules-type interaction terms over the
cationic and anionic matrix is required and shows a dependence of sulphide
solubility on melt composition more complex than that currently assumed. Further
improvements may be achieved taking into account for iron speciation and
combining the equations developed with a polymeric approach of silicate melts in
order to give us oxide ion O2- activity in silicate melts. Applications and case studies
concerning sulphur degassing of different magmas are discussed as well as previous
results given in literature. BIBLIOGRAPHY: [1]Flood and Grjotheim (1952) J. Iron
Steel Inst., 171, 64-70
On the behavior of redox pairs in anhydrous and hydrous silicate melts: from the oxygen electrode to the mutual interactions of Fe and S (INVITED)
No full textThe need of techniques for determining the oxidation state of magmas from iron and/or sulfur redox ratios has pushed scientists to yield composition-based semi-empirical equations, without much interest for the understanding of how electron transfer takes place, thus disregarding true, or at least most plausible, redox exchanges occurring in melts. Not secondary, it has generated notations (i.e., chemical equilibria) in which standard states, species and components are mixed. Let us then go back to basics, by taking the most geologically important element having multiple oxidation states: iron. In order to model redox exchanges we need i) a formalism for acid-base reactions in silicate melts, ii) a reference electrode, iii) a model for computing proportions and activities of species intervening in acid-base exchanges and redox electrodes, including that of reference. Briefly, the above requirements converge in the adoption of the normal oxygen electrode plus the Toop-Samis polymeric approach, which is based on the ionic Temkin notation (Ottonello et al., 2001). Therefore, in silicate melts redox processes and polymerization are intimately related. Under certain conditions, some unexpected features can be explored, such as the oxidation of iron in closed system with decreasing temperature. Let us now complicate the things by introducing the most geologically important volatile: water. Processing of data on the iron redox ratio in hydrous glasses allows one to model the role of composition, temperature, pressure and oxygen fugacity i) by assessing the acid-base properties of the water component in a notation consistent with the above and ii) by introducing volume terms of interest. The central role of water speciation can be then discussed in terms of its amphoteric behavior, in line with the earlier prediction of Fraser (1977) and the recent NMR findings of Xue and Kanzaki (2004). Finally, let us add another multiple valence state element such as S and investigate how mutual interactions between iron and sulfur take place under various conditions. The adoption of an internal consistent model for sulfur solubility and speciation (Moretti and Ottonello, 2005), based on the same premises adopted so far, permits a full parameterization of how Fe and S interacts. A volcanological application is also given. References: Fraser (1997) Thermodynamics in Geology, D. Reidel Pub. Co.; Moretti and Ottonello (2005) Geochim. Cosmochim. Acta 69, 801-823; Ottonello et al. (2001) Chem. Geol. 174, 157-179; Xue and Kanzaki (2004) Geochim. Cosmochim. Acta 68, 5027-505
Water speciation in melts: searching the consistency between physico-chemical arguments and experimental data
No full textThe dissolution mechanism of water in melts/glasses is somehow questioned in literature.
If the existence of T-OH groups is well ascertained in hydrated alkali-silicate
glasses, things may be different for aluminosilicate melts/glasses. In particular, there
is a great debate whether water dissolution in alkali aluminosilicate melts occurs either
via protonation of bridging oxygens without disruption of the melt network, or through
hydrolysis of T-O-T linkages and consequent depolymerization of the melt structure.
Does this ambiguous structural picture, mainly arising from glasses, correspond necessarily
to an intricate physico-chemical description of how water dissolves in melts?
To answer this question, we shall consider alternative ways to gain insights about water
speciation. Epel’Baum (1973) argued that granite melts formed at different water
pressures may have markedly differing basicities and therefore markedly differing degrees
of oxidation. The interrelationship between oxidation and acid-base properties
has been already treated in the framework of polymeric models based on the Toop-
Samis approach and on the ionic notation of Temkin. A major insight of these studies
is the recognition that the depolymerizing role of water has been much overrated with
respect to its actual acid-based properties in melts. Although among the molten oxides
forming silicate melts water is commonly perceived as the most basic one, its optical
basicity has a value (0.40) very close to that of silica and alumina (0.48). Fraser (1975)
stated that “like for other oxides, water can be expected to show amphoteric properties
depending on the nature of the other components present”. Amphoteric behavior
of water in the Temkin hypothesis (complete dissociation of components) means considering
protons and free hydroxyls. The dependence of this behavior on the nature
of the other components, i.e. the bulk basicity of the medium, expressed by free oxygen
concentration, is accounted through the solution of the polymerization equation
between free oxygens, BO’s and NBO’s. At parity of water content, high free hydroxyl
concentration is expected in more basic melts, in agreement with ab-initio MO
calculations of Xue and Kanzaki (2004) and with experimental results of Behrens et
al. (2004) on water diffusivities. Reconciliation with data from InfraRed spectroscopy
(the most commonly adopted tool for ‘water speciation’) is possible and, although uncertainties,
allows to compute proportions of i) molecular water, ii) free hydroxyls, iii)
protons (and T-OH groups)
Ionic-polymeric models and the amphoteric behavior of water in silicate melts
No full textIn silicate melts it is almost impossible to readily distinguish solute and solvent like in aqueous solutions. The anionic
framework of silicate melts, in fact, makes solute and solvents so intimately related that one cannot identify
a solvation shell and identify directly, from structural studies, the complexes needed to define acid-base reactions.
Therefore, the distinction between solute and solvent becomes blurred in systems such as silicate melts, because
speciation is not only complex but changes with the marked depolymerization of the silicate framework that obtains
from pure SiO2 to metal-oxide rich compositions. These features do not allow proper understanding of the actual
physico-chemical role of many species detected by conventional techniques, a fact which can lead to confusing
notation.
However, these may not be serious limits to account correctly for the acid-base reactions that take place in every
kind of magmatic setting, provided a ‘syntax’ describing the effective interactions among significative cationic and
anionic entities. In particular, the syntax for acid-base exchanges is needed such that constituting oxides (i.e. chemical
components) can be treated independently of (but not necessarily extraneous to) structural features in defining
such entities. So-called ionic-polymeric models highlight the mutual correspondence between polymerization and
acid-base properties of dissolved oxides through the Lux-Flood formalism for molten oxides. They thus provide
the syntax to write chemical exchanges, but have no pretension to structural description. In fact the concept of melt
polymerization is used to identify basic anions and cations that can be used, along with their formal charge, to
describe effectively acid-base interactions taking place in melts.
In this respect, an example is given by the description of the amphoteric behavior of water dissolved on melts,
hence water autoprotolysis. Although it exerts a profound influence on properties of magmas, this autoprotolysis
reaction has been hitherto neglected for water dissolved in silicate melts. By mixing cations and anions on distinct
sublattices and quantifying the disproportionation of water dissolved in silicate melts into its ionic products, H+
and OH-, we reconcile conflicting spectroscopic determinations of water speciation, and explain the contrasting
rheology of hydrous basaltic and rhyolitic melts. In fact basalts show much less depression of viscosity by water
addition because of a relative predominance of OH-, such that water increase tends immediately to limit depolymerization
rather than enhance it. This opens new perspectives to the understanding of the chemical control leading
to either effusive or explosive eruptions
RECONCILING DATA ON THE IRON OXIDATION STATE OF ANHYDROUS AND HYDROUS ALUMINOSILICATE GLASSES AND MELTS: A POLYMERIC APPROACH
No full textThe oxidation state of iron has been the object of attentive investigations during the
last four decades.
A first class of investigation involves glasses synthesized under nominally anhydrous
conditions at atmospheric pressure: interpretation of iron oxidation state Vs. composition
has given rise to some contradictions in literature about the structural role
played by this element in silicate melts. Controversies are particularly relevant in the
case of divalent iron, as testified by many spectroscopic determinations.
The second class of investigation concerns hydrous aluminosilicate glasses synthesized
under different T-P conditions. Again, no unique redox pattern has been found
so far in literature, the ferric to ferrous iron ratio depending in a complex fashion on
composition, temperature, pressure and oxygen fugacity of synthesis.
The present study aims at showing that it is possible to reconcile such data by
accounting for the acid-base properties of studied melts/glasses in the framework of
a polymeric approach based on the concept of optical basicity and considering water
speciation. Useful insights may thus be given about the dissociation equilibria of
water in aluminosilicate melts/glasses.
It is concluded that the developed model may be usefully employed for studying the
evolution of the oxidation state of degassing and erupting silicate melts, showing that
redox variations may be more reasonably ascribed to the melt compositional control
rather than to changes in oxygen fugacity during magma migration from depth to
surface
Volatile solubility in silicate melts with particular regard to sulphur species: theoretical aspects and application to Etnean volcanics
No full text
On 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
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