High-pressure equation of state and phase transition in PbAl₂Si₂O₈feldspar

Abstract

In situ high-pressure X-ray diffraction study was performed on synthetic lead feldspar with composition PbAl 2 Si 2 O 8 (PbFsp). The crystals were synthesized from the melt and thermally treated at T = 1150 °C for 12 h and at T = 1000 °C for 70 h. At room condition the unit-cell parameters are a = 8.3936(4), b = 13.0498 (7), c = 14.3258(8) Å, b = 115.281(6)°, V = 1418.9(1) Å 3 ; space group: I2/c; Q od = 0.7.A single-crystal of lead feldspar was loaded in an ETH-type diamond-anvil cell and unit-cell parameters were measured at 26 different pressures up to 8.4 GPa at room T. The evolution with P of the unit-cell parameters and volume shows a strong discontinuity between 7.7 and 8.2 GPa indicating a first-order phase transition. The discontinuous character of the transition is especially noticeable in the behavior of the b angle, which decreases from 114.83° to 114.03°, and in the b parameter, which reduces from 12.746 to 12.567 Å.In the P range 0.0001-7.72 GPa, the trend shown by the axial compressibilityis similar to that observed in the previous HP powder diffraction study, performed on lead feldspar using high-brilliance synchrotron radiation up to 7.1 GPa.In the P range 0.0001-4.27 GPa at room T, the P-V data of the I2/c lead feldspar were fitted with a second-order Birch-Murnaghan EoS. The parameters obtained are: V 0 = 1422.2(1) Å 3 and K T0 = 76.4(9) GPa. At P > 4.27 GPa, the volume values deflect from the BM2 curve and show a volume softening, precursor of the reported HP phase transition. A volume softening was recently observed in strontium feldspar (SrFsp) above 4.2 GPa.A second crystal of PbFsp was loaded in the DAC cell and in situ high-pressure X-ray diffraction intensities were measured at P = 0.0001, 2.4, 3.1, 5.4, 6.0, 7.2, 8.4, and 9.7 GPa. The appearance of c and d-type reflections at 8.4 GPa, the analysis of the systematic absence and the structural refinements indicate the HP first-order transformation as an I2/c-P2 1 /c phase transition. Structural results show that the main variations with compression in lead feldspar are in Pb-O bond lengths and in T-O-T bond angles, while T-O distances and O-T-O angles do not change meaningfully, indicating that the Si,Al tetrahedra behave with pressure as a rigid body. Changes observed in the compressional behavior of the structure between 3 and 5 GPa could explain the softening observed at P > 4 GPa in the volume compressibility.The results obtained in the present work allow comparing the pressures of the HP I2/c-P2 1 /c phase transition occurring in lead feldspar with those observed in alkaline-earth feldspars.

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.…”

“…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. (Miletich et al 2000).…”

“…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).…”

“…The discontinuities observed in celsian indicate the occurrence of an analogous first-order phase transition between 5.6 and 5.9 GPa. The appearance of c-and d-type reflections in the data collection at P = 5.9 GPa, the analysis of the systematic absence and the structural refinements at 5.9, 6.5 and 7.8 GPa (Table 2) feldspar (Benna et al 2007) and P2 1 /c lead feldspar (Curetti et al 2015).…”