High-pressure I2/c-P21/c phase transformation in SrAl2Si2O8 feldspar

High-pressure structural configuration and phase transition in celsian, BaAl2Si2O8

“…2). A similar discontinuity was observed in An 20 SrFsp 80 feldspar at P ~ 7.3 GPa (Nestola et al 2004), in strontium feldspar at P ~ 6.6 GPa (Pandolfo et al 2011) and in lead feldspar at P ~ 8 GPa (Curetti et al 2015), indicating the occurrence of a phase transition strongly first order in character. The discontinuities observed in celsian indicate the occurrence of an analogous first-order phase transition between 5.6 and 5.9 GPa.…”

“…The value of a H , significantly different from zero, supports the pre-transition softening. As just seen, in celsian the volume softening occurs at P > 2.55 GPa, while in strontium feldspar it was observed at P > 3.67 GPa (Pandolfo et al 2011) and in lead feldspar at P > 4.27 GPa (Curetti et al 2015). Probably, the softening takes place in celsian at lower pressures than in strontium and lead feldspars because the pressure of the first-order transition is lower.…”

“…1) and then slightly decreases. In the other Al 2 Si 2 feldspars, the β angle decreases with pressure Tribaudino et al 1999;Benna et al 2007;Pandolfo et al 2011;Curetti et al 2015) while it increases in alkali feldspars with a plateau effect Nestola et al 2008).…”

“…With increasing disorder (Q od = 0.78), the transition becomes continuous in character and there is an increase in the pressure of the transition (P T = 4.8 GPa) (Angel 1992 Nestola et al (2004) and Benna et al (2007) observed at P ~ 4.3 GPa a first-order I 1 -I2/c transformation, and a further transition at P ~ 7.3 GPa, strongly first order, to the P2 1 /c space group, never observed in feldspars until then. Recently, Pandolfo et al (2011) found in synthetic strontium feldspar (SrAl-2 Si 2 O 8 ) the same first-order I2/c to P2 1 /c transition at P = 6.6 GPa. Curetti et al (2015) observe the I2/c-P2 1 /c transition in synthetic lead feldspar (PbAl 2 Si 2 O 8 ) at P ~ 8 GPa.…”

High-pressure equation of state and phase transition in PbAl2Si2O8feldspar

“…The volume bulk modulus of P1 anorthite is 83.3 GPa (Hackwell and Angel 1992) and a value of around 82.5 GPa has been extrapolated for I1 anorthite from the compression behavior of plagioclases (Angel 2004). Since the volume bulk modulus of the triclinic An 20 SrFsp 80 is 88.7 GPa, the substitution of 80% of SrAl 2 Si 2 O 8 component into the anorthite structure causes a slight increase in the bulk modulus (Pandolfo et al 2011). In recent studies, Pandolfo et al (2011) and Mrosko et al (2011), with completely different analytical techniques (in situ single-crystal XRD and mid/far micro-FTIR spectroscopy), found in strontium feldspar a similar first-order I2/c to P2 1 /c transition at P = 6.6 and 6.5 GPa, respectively.…”