The P21/m? C2/m phase transition in synthetic amphibole Na NaMg Mg5 Si8 O22 (OH)2: thermodynamic and crystal-chemical evaluation

The P21/m ↔ C2/m phase transition in amphiboles: new data on synthetic Na(NaMg)Mg5Si8O22F2 and the role of differential polyhedral expansion

“…(1) to the data measured in the T range 100-395 K gave T c ¼ 395 AE 5 K, and b D /a D ¼ 1022 AE 121 K. Errors are mainly due to the limited range of T spanned and to the low intensity of b-type reflections. Because b D /a D is positive, the results are compatible with a second-order character for the P2 1 /m $ C2/m phase-transition (see [23] (OH) 2 composition [12] gave T c ¼ 530 AE 3 K and b D /a D ¼ 943 AE 14 K. Thus the transitions are similar in character, but the fluorine analogue has a definitely lower T c .…”

“…In the O(3) OH amphiboles with the same charge arrangement, signal of structure instability were observed beyond 673 K, and prevented a reliable modelling of the high-T C2/m structure [12]. This problem is overcome by the lower T c characteristic of the sample of this work and its increased thermal stability due to the presence of O(3) F.…”

“…Transition temperature, T c In amphiboles, the square of the displacive order parameter (Q D 2 ) is proportional to the aggregate intensity ratio, and I a those of a-type reflections (see [12] and references therein for detail). Therefore, both the character of the phase transition and the T c value can be obtained following the evolution of P I b / P I a as a function of temperature.…”

“…In order to analyse the effects of fluorine both on the structure and on the phase-transition behaviour, the evolution with T of the structure of the synthetic amphibole A Na B (NaMg) C Mg 5 T Si 8 O 22 O3 F 2 is discussed in this section with reference to that of its OH counterpart [12].…”

“…As a rule of thumb, it was suggested that the shorter is hr M(4) i, the higher is the transition temperature, T c . More recently, further information on phase transitions in amphiboles was obtained from the occurrence of non-standard symmetries in peculiar synthetic amphibole compositions (P2 1 /m in solid-solution terms around A Na B (NaMg) C Mg 5 T Si 8 O 22 O3 (OH) 2 , [10]; C1 in A Na B Na 2 C Mg 5 T Si 8 O 21 O3 (OH) 3 , [11]), and on the study of their high-T phase transition to the standard C2/m symmetry ( [12] and [13] for the same samples, respectively). In particular, [12] concluded that the main factor affecting the stability of the P2 1 /m phase in amphiboles is the relative dimensions of the aggregate ionic radii at the B-and C-group sites.…”