I(I)over-bar-I2/c ferroelastic phase transition in the Ca0.2Pb0.8Al2Si2O8 feldspar as a function of temperature

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 degrees C, and a displacive continuous ferroelastic transition to a I2/c monoclinic phase was observed. An analysis of the symmetry-required components of 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 spontaneous strain. The transition is analogous to those observed, with increasing temperature, along the join An-SrF and in disordered Na-rich alkali feldspars. A comparison with Ca0.2Sr0.8Al2Si2O8 frldspar (T-C = 680 degrees C, McGuinn and Redfem, 1997) shows that PbF(80)An(20) has a higher spontaneous strain (epsilon(8) = 0.028 in PbF(80)An(20) vs 0.020 in Ca0.2Sr0.8Al2Si2O8) and a higher e(4) component, possibly related to the higher distortion of the non-tetrahedral polyhedron in lead feldspar.