http://www.cnr.it/ontology/cnr/individuo/prodotto/ID210792
INFLUENCE OF UNCERTAINTIES AFFECTING ELASTIC PARAMETERS ON THE GIBBS ENERGY CALCULATION: THE CASE OF 3T - 2M1 PHENGITE (Abstract/Poster in convegno)
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- INFLUENCE OF UNCERTAINTIES AFFECTING ELASTIC PARAMETERS ON THE GIBBS ENERGY CALCULATION: THE CASE OF 3T - 2M1 PHENGITE (Abstract/Poster in convegno) (literal)
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PAVESE Alessandro1, DIELLA Valeria2 (2011)
INFLUENCE OF UNCERTAINTIES AFFECTING ELASTIC PARAMETERS ON THE GIBBS ENERGY CALCULATION: THE CASE OF 3T - 2M1 PHENGITE
in Geoitalia 2011, VIII forum italiano di Scienze della Terra, Torino, 19-23 settembre
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- PAVESE Alessandro1, DIELLA Valeria2 (literal)
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- Geoitalia 2011, VIII forum italiano di Scienze della Terra (literal)
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- 1 - Dipartimento di Scienze della Terra, Università degli Studi di Milano,
Italy
2 - Consiglio Nazionale delle Ricerche (CNR)-Istituto per la Dinamica dei
Processi Ambientali (IDPA), Milano, Italy (literal)
- Titolo
- INFLUENCE OF UNCERTAINTIES AFFECTING ELASTIC PARAMETERS ON THE GIBBS ENERGY CALCULATION: THE CASE OF 3T - 2M1 PHENGITE (literal)
- Abstract
- The stability of 2M1 (S.G. C>2/c) versus 3T (S.G. P3112) polytype is
dependent on the P/T ratio of formation: large P/T values are supposed to
promote crystallisation of 3T phengite, presumably because of 3T having a
more flexible structure than 2M1 to arrange cations in the octahedral
sheet. In fact, the trigonal polytype provides two symmetry-independent
octahedral sites (M2 and M3), against only one in 2M1 (Ferraris and Ivaldi
2002 and references therein). All this is related to the occurrence of cation
order, which Pavese et al. (2003 and references therein) have extensively
investigated by neutron powder diffraction. However, the stability of 3T
vs. 2M1 phengites is still an open question. On the basis of experimental
works (Curetti et al. 2006, Gatta et al. 2010) we infer that the stability of
3T increases as a function of P, though the Gibbs energy difference
between the two polytypes still keeps small. Such a behaviour is due to
that K'(3T)< K'(2M1), and K0 (3T) is very similar to K0(2M1): at sufficiently
high pressures 2M1 becomes less compliant to P than 3T, and therefore
the deformation energy contribution to the Gibbs energy is supposed to
favour the latter. The relative stability between trigonal and monoclinic
polytypes remains a matter of thermodynamic inference. In this view,
hence, the ability to fix the uncertainty on the elastic properties, and to
determine how it propagates onto the calculated deformation energy, is a
fundamental step to support any prediction of stability of a given polytype
at high pressure. In a study about the accuracy of the calculated Gibbs
energy, depending on the uncertainties on elastic parameters (bulk
modulus, its first derivative versus pressure and molar volume) we revised
the relative stability, controlled by pressure, between 3T-2M1 phengite.
Above a few GPa the uncertainty due to the deformation energy and
dependent on the parameters governing the equation of state grows the
dominant contribution to the total uncertainty on G(T,P), and it is
imperative to make efforts to abate the uncertainties on K0 and K' so that
one can achieve fully distinguishable deformation energy contributions.
Using another approach to discuss on how to manage the role of the
elastic properties in the relationship between polytypism and stability for
phengite 2M1 and 3T, we developed the general concept of
\"distinguishibility\" between sets of elastic parameters used to calculate the
contribution of deformation to the Gibbs energy. This notion bases on that
different contributions of deformation are actually observed if the related
elastic parameters sets lie in the K0 -K' spaces sufficiently apart, in terms
of a variational principle formulated using the experimental uncertainties.
The results of the comparison between the two polymorphs of mica
showed that a P increase does favour their distinction and promote the
stabilization of 2M1 even if this behaviour is appreciable only in the high
pressure, a regime very different from the actual field of stability of micas
but that stresses the role of pressure in differentiating the two polytypes.
Once more the large errors that affect the elastic observables and the
relevant proximity between the elastic parameters are responsible of the
difficulties in distinguishing the behaviours of the considered polytypes off
ambient conditions.
Curetti N, Levy D, Pavese A, Ivaldi G 2006. Phys Chem Min, 32:670-678
Ferraris G, Ivaldi G 2002. Rev Miner Geochem 46:117-153
Gatta GD, Rotiroti N, Lotti P, Pavese A, Curetti N 2010. Phys Chem Min,
37:581-591
Pavese A, Curetti N, Ferraris G, Ivaldi G, Russo U, Ibberson R 2003. Eur J
Min, 15:357-363 (literal)
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