Si-Al order and the I1-I2/c structural phase transition in synthetic CaAl₂Si₂O₈-SrAl₂Si₂O₈feldspar; a²⁹Si MAS-NMR spectroscopic study

Cation ordering and phase transitions in feldspars along the join CaAl2Si2O8-SrAl2Si2O8: a TEM, IR and XRD investigation

“…It is suggested therefore that the equilibrium degree of order has not, or has only just, been attained even in the longest runs. This is in agreement with the observation by Phillips et al (1997), that Al-Si ordering is greater close to the monoclinic-triclinic transition boundary in respect of the end-members anorthite and Sr-feldspar.…”

“…It is observed that the e 4 and e 6 values have a linear coupling in An 30 SrF 70 , whereas in An 60 SrF 40 , e 4 changes with time, but not e 6 . With e 6 being mostly related to order-disorder processes, the most obvious interpretation is that in the Sr-richer sample, a greater degree of order can be achieved, in agreement with the NMR results of Phillips et al (1997) and Rocquefelte et al (2007), and/or that in the Ca-richer feldspar equilibrium ordering can be more readily achieved.…”

“…Due to the steepness of the monoclinic-triclinic phase boundary, the differences between the T c in samples with different annealing history and different degree of order, may be as high as several hundred degrees. The transition with temperature is second order (Tribaudino et al, 1993;McGuinn and Redfern, 1997;Phillips et al, 1997), with suggestions of a first-order behaviour in feldspars with no or intermediate Al-Si order (Tribaudino et al, 1993). These experimental results were interpreted by the lattice simulation studies by Dove and Redfern (1997).…”

Ferroelastic phase transitions: structure and microstructure

“…A common application of this concept in minerals is to study the effect of some (generally rather slowly equilibrating) cationordering process on a (phonon time scale) displacive change in a mineral's structural framework. Coupling models have been used to describe the interaction between the displacive mode and (Al, Si) ordering in albite (Salje et al, 1985; and anorthite (Phillips et al, 1997), and between the displacive mode and (Fe, Mg) ordering in pyroxene (Ca  mara et al, 2003). The same concept has also been applied to the interaction between the antiferroelectric and ZrO 6 tilting modes in PbZrO 3 (Fujishita et al, 2003), to the two tilt modes (the M 3 and R 25 soft modes) in NaMgF 3 perovskite (Zhao et al, 1993), and to the interaction between ferroelectricity and Li-ion conduction in LiNbO 3 (Lehnert et al, 1997).…”

TEM observations on theP1̅-I1̅ phase transition in feldspars along the join CaAl2Si2O8-SrAl2Si2O8

“…The monoclinic I2/c field increases with temperature (Tribaudino et al 1993;McGuinn and Redfern 1997) and with increasing Al-Si disorder (Tribaudino et al 1993;Dove and Redfern 1997). The NMR results (Phillips et al 1997) on feldspars along the join An-SrF have shown that anorthite is P1 on a local scale and that triclinic Srricher feldspars are I1 -, but the onset of the P1 --I1 transition was not investigated. Previous TEM investigations on pure Sr feldspar (Benna et al 1995) and on Sr-rich samples synthesized along the join An-SrF (Tribaudino et al 1995) have shown the absence of reflections violating the I lattice in the range An 40 SrF 60 -SrF 100 .…”