Synthesis of P21/m amphibole crystals close to the nominal composition Na(NaMg)Mg5Si8O22(OH)2 and suitable dimensions allowed investigation of the effects of composition on the thermodynamic behaviour of the P21/m<->C2/m phase transition, which had been so far detected and characterised in cummingtonites, (Vacancy)(Mg,Fe, Mn)(Mg,Fe,Mn)5Si8O22(OH)2. The transition was followed during in situ annealing in the T range 25-400 °C by monitoring changes in unit-cell parameters and in the intensities of a set of superlattice reflections. 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 second order transition. Strain analysis showed that Epsilon1 and Epsilon3 are approximately parallel to the a- and the c*-axes, respectively, and Epsilon2 is constrained by symmetry to be parallel to the b-axis; Epsilon1 is smaller in this sample than in cummingtonites. Single-crystal structure refinements at 25, 140, 270 and 370 °C allowed modelling of the structural changes as a function of T and symmetry. Crystal-chemical analysis suggests that differences in Tc mainly depend on the relations between the aggregate ionic radii of the B- and C-group cations. Short-range ordering according to ANa-M4Na and A(Vacancy)-M4Mg clusters also decreases local heterogeneity and enhance the stability of the P21/m phase.
The P21/m --> C2/m phase transition in the synthetic amphibole Na NaMg Mg5 Si8 O22 (OH)2: Thermodynamic and crystal-chemical evaluation.
OBERTI R;
2003
Abstract
Synthesis of P21/m amphibole crystals close to the nominal composition Na(NaMg)Mg5Si8O22(OH)2 and suitable dimensions allowed investigation of the effects of composition on the thermodynamic behaviour of the P21/m<->C2/m phase transition, which had been so far detected and characterised in cummingtonites, (Vacancy)(Mg,Fe, Mn)(Mg,Fe,Mn)5Si8O22(OH)2. The transition was followed during in situ annealing in the T range 25-400 °C by monitoring changes in unit-cell parameters and in the intensities of a set of superlattice reflections. 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 second order transition. Strain analysis showed that Epsilon1 and Epsilon3 are approximately parallel to the a- and the c*-axes, respectively, and Epsilon2 is constrained by symmetry to be parallel to the b-axis; Epsilon1 is smaller in this sample than in cummingtonites. Single-crystal structure refinements at 25, 140, 270 and 370 °C allowed modelling of the structural changes as a function of T and symmetry. Crystal-chemical analysis suggests that differences in Tc mainly depend on the relations between the aggregate ionic radii of the B- and C-group cations. Short-range ordering according to ANa-M4Na and A(Vacancy)-M4Mg clusters also decreases local heterogeneity and enhance the stability of the P21/m phase.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
