The P21/m <--> C2/m phase transition in the synthetic amphibole Na (NaMg) Mg5 Si8 O22 (OH)2. Thermodynamic and crystal-chemical evaluation. (Abstract/Comunicazione in atti di convegno)

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Label
  • The P21/m <--> C2/m phase transition in the synthetic amphibole Na (NaMg) Mg5 Si8 O22 (OH)2. Thermodynamic and crystal-chemical evaluation. (Abstract/Comunicazione in atti di convegno) (literal)
Anno
  • 2003-01-01T00:00:00+01:00 (literal)
Alternative label
  • F. Cámara1, R. Oberti1, G. Iezzi2, G. Della Ventura3 (2003)
    The P21/m <--> C2/m phase transition in the synthetic amphibole Na (NaMg) Mg5 Si8 O22 (OH)2. Thermodynamic and crystal-chemical evaluation.
    in GEOITALIA 2003. Forum Italiano di Scienze della Terra ., Bellaria, 16-18 settembre 2003
    (literal)
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  • F. Cámara1, R. Oberti1, G. Iezzi2, G. Della Ventura3 (literal)
Pagina inizio
  • 614 (literal)
Pagina fine
  • 615 (literal)
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  • GEOITALIA 2003, Abstracts (literal)
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  • 2 (literal)
Note
  • Abstract (literal)
  • Comunicazione (literal)
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  • 1CNR-Istituto di Geoscienze e Georisorse, Sezione di Pavia, 2Bayerisches Geoinstitut, Universität Bayreuth 3Dipartimento di Scienze Geologiche, Università di Roma Tre, Roma (literal)
Titolo
  • The P21/m <--> C2/m phase transition in the synthetic amphibole Na (NaMg) Mg5 Si8 O22 (OH)2. Thermodynamic and crystal-chemical evaluation. (literal)
Abstract
  • The recent finding that the synthetic amphibole Na (NaMg) Mg5 Si8 O22(OH)2 has P21/m symmetry (Iezzi et al., this meeting) allowed investigation of the effects of composition on the thermodynamic behaviour of the P21/m->C2/m phase transition, which has been so far detected and characterised in cummingtonites (Boffa Ballaran et al., 2000, and references therein). In particular, this sample is particularly critical to model the effect of the presence of anomalously large cations at the B-group site and of the occupancy of the A-group sites. All these features are expected to significantly affect both the elastic and the thermodynamic behaviour of the transition. The transition was followed by in situ annealing of a crystal with unit formula Na0.8 Na0.8Mg1.2 Mg5 Si8 O22 (OH)2 in the T range 25-400 °C, using a micro-furnace mounted on a Philips PW1100 diffractometer. Changes in the unit-cell parameters and in the intensities of a set of superlattice reflections were monitored at steps of 15 °C. Polynomial fitting of a (246) Landau potential to the evolution of the order parameter with T yielded a Tc of 252±3 °C, and Landau coefficients compatible with a second order transition, coherently with what observed in cummingtonite. Four complete data collections and structure refinements were done at 25, 140, 270 and 370 °C in order to follow structural changes as a function of temperature and of change in symmetry; the differences with cummingtonites are compatible with the differences in chemistry. Strain components were calculated and compared to those given by Boffa Ballaran et al. (2000) for the HP phase transition in cummingtonite. Similarly to cummingtonite, the principal component ?2 is constrained by symmetry to be parallel to the y-axis, whereas ?1 and ?3 are approximately parallel to the x- and the crystallographic c*-axes, respectively. However, ?1 is three times smaller for the sample of this study. This is coherent with previous observation that the presence of small but significant (Ca + Al + Na) contents decreased the calculated strain. This work ascribes these effects to the presence of A-group cations. Boffa Ballaran T., Angel R.J., Carpenter M.A. Eur. J. Mineral., 12, 1195 1213 (2000). (literal)
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