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)
Anno
  • 2011-01-01T00:00:00+01:00 (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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Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#autori
  • PAVESE Alessandro1, DIELLA Valeria2 (literal)
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  • Geoitalia 2011, VIII forum italiano di Scienze della Terra (literal)
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  • 4 (literal)
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  • Poster (literal)
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  • 1 - Dipartimento di Scienze della Terra, Universit√† 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. C2/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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