High pressure behaviour of lead feldspar (PbAl2Si2O8)

An in situ high pressure powder diffraction study, using high-brilliance sy nchrotron radiation, on lead feldspar (PbAl2Si2O8) was performed. Two sampl es, with Q(od)=0.68 and 0.76, were loaded in a diamond anvil cell and were compressed up to 11 GPa. Up to P=7.1 GPa the only phase present is lead fel dspar. In the range 7.1-9.3 GPa sudden changes in the position of the refle ctions suggest the transformation of lead feldspar to a new phase (probably feldspar-like). The absence of split that would be compatible with triclin ic symmetry rules out the monoclinic-triclinic transition, that was reporte d for the structurally similar strontium feldspar. At P>9.4 GPa some new ex tra reflections not indexable in the feldspar cell ape present as well. Dur ing decompression the lead feldspar was the only phase present at P<6 GPa. Peak enlargement was observed with pressure, probably preliminary to amorph ization. However almost complete amorphization was observed only after fort uitous shock compression at similar to 18 GPa; the crystallinity was recove red at room pressure after decompression. The bulk modulus for lead feldspa r was K=71.0(9) and 67.6(1.2) GPa for the two samples, in the range reporte d for feldspars. The cell parameters show a compression pattern which is si milar to that observed in anorthite, with Delta a/a(0)>Delta c/c(0)>Delta b /b(0); comparison with the high temperature behaviour shows that for lead f eldspar the strain tensor with pressure is more isotropic and the deformati on along a is less prominent. A turnover in the behaviour of the beta angle with pressure suggests a change in the compression behaviour at P similar to 2 cpa. Rietveld refinement of the Pb coordinates was performed in a seri es of spectra with pressure ranging from 0.6 to 6.5 GPa. The combined analy sis of cell parameters and Pb coordinates with pressure showed that the com pression of the structure is mainly achieved by an approach of Pb atoms alo ng a*.