P. Benna et al., I(I)over-bar-I2/c ferroelastic phase transition in the Ca0.2Pb0.8Al2Si2O8 feldspar as a function of temperature, MINERAL MAG, 64(2), 2000, pp. 285-290
Feldspar of composition Ca0.2Pb0.8Al2Si2O8 (PbF(80)An(20)) was synthesized
from melt and subsequently isothermally annealed at T = 960 degrees C for 4
days. hi situ HT X-ray powder spectra of PbF(80)An(20) feldspar, triclinic
IT at room temperature, were collected in the temperature range 20-800 deg
rees C, and a displacive continuous ferroelastic transition to a I2/c monoc
linic phase was observed. An analysis of the symmetry-required components o
f the spontaneous strain tensor reveals the second order character (beta =
0.46 +/- 0.02) of the transition with T-C = 680 +/- 15 degrees C. A linear
coupling was observed between the e(4) and c(6), components of the spontane
ous strain. The transition is analogous to those observed, with increasing
temperature, along the join An-SrF and in disordered Na-rich alkali feldspa
rs. A comparison with Ca0.2Sr0.8Al2Si2O8 frldspar (T-C = 680 degrees C, McG
uinn and Redfem, 1997) shows that PbF(80)An(20) has a higher spontaneous st
rain (epsilon(8) = 0.028 in PbF(80)An(20) vs 0.020 in Ca0.2Sr0.8Al2Si2O8) a
nd a higher e(4) component, possibly related to the higher distortion of th
e non-tetrahedral polyhedron in lead feldspar.