The P2(1)/m<---->C2/m displacive phase transition in the synthetic end-member amphibole Na NaMg Mg-5 Si-8 O-22 (OH)(2) has been studied by monitoring changes in unit-cell parameters and the intensities of superlattice reflections at 25-400degreesC. This amphibole allows investigation of the effects of compositional variations at the A- and B- group sites upon the transition. Polynomial fitting of a 24 Landau potential to the evolution of the order parameter with T yielded a T-c of 257 +/- 3degreesC, and Landau coefficients compatible with a second-order transition. Structure refinement of single-crystal data collected at 25, 140, 270 and 370degreesC allowed modelling of the structural changes as a function of T and symmetry. Crystal-chemical analysis suggests that differences in Tc in cummingtonites and in the crystal of this work depend mainly upon the relations between the aggregate ionic radii of the B- and C-group cations.
Camara, F., Oberti, R., Iezzi, G., DELLA VENTURA, G. (2003). The P21/m – C2/m phase transition in the synthetic amphibole NaNaMgMg5Si8O22(OH)2: thermodynamic and crystal-chemical evaluation. PHYSICS AND CHEMISTRY OF MINERALS, 30, 570-581 [10.1007/s00269-003-0348-9].
The P21/m – C2/m phase transition in the synthetic amphibole NaNaMgMg5Si8O22(OH)2: thermodynamic and crystal-chemical evaluation
DELLA VENTURA, Giancarlo
2003-01-01
Abstract
The P2(1)/m<---->C2/m displacive phase transition in the synthetic end-member amphibole Na NaMg Mg-5 Si-8 O-22 (OH)(2) has been studied by monitoring changes in unit-cell parameters and the intensities of superlattice reflections at 25-400degreesC. This amphibole allows investigation of the effects of compositional variations at the A- and B- group sites upon the transition. Polynomial fitting of a 24 Landau potential to the evolution of the order parameter with T yielded a T-c of 257 +/- 3degreesC, and Landau coefficients compatible with a second-order transition. Structure refinement of single-crystal data collected at 25, 140, 270 and 370degreesC allowed modelling of the structural changes as a function of T and symmetry. Crystal-chemical analysis suggests that differences in Tc in cummingtonites and in the crystal of this work depend mainly upon the relations between the aggregate ionic radii of the B- and C-group cations.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
