A technique for the fabrication of fully dense Ca-rich plagioclase (An(70)-An(100)) samples suitable for studying the plastic rheology of bytownite (An(80))

Two different types of solid samples of bytownite (Ca0.7Na0.3Al2Si2O8 to Ca 0.9Na0.1Al2Si2O8) composition were fabricated from synthetic crystal and gl ass powders. The crystal and the glass powders were produced by crystallisa tion or melting of a gel of bytownite composition. Cold pressing under vacu um followed by hot isostatic pressing (hip) of the powders produced fully d ense samples composed either of 100% bytownite crystals or of 90% bytownite crystals and 10% bytownite glass. The cold-pressed samples were composed o f a matrix of nanometer sized bytownite needles and larger crystals of up t o 3 mu m in size. During hot pressing the grain sizes in the matrix increas ed slightly while larger crystals increased to close to 4 mu m. The rheolog ical behaviour of the hot isostaticly pressed samples for cases of tri axia l compression and torsion was tested in a gas confining deformation apparat us at high temperature and confining pressure. Grain growth was observed du ring the deformation experiments. The maximum flow stress was typically les s than 200 MPa and was attained by sample strain of 10% during axial compre ssion and a shear strain of 1.0 during torsion. The resulting microstructur es were dominated by fibrous grains for the compressive deformation and by more round-shaped grain boundaries during torsion. Both type of experiments induced a preferred shape and crystallographic orientation. (C) 1999 Kluwe r Academic Publishers.