I(1)over-bar-i2/c Phase-transition In Alkaline-earth Feldspars - Evidence From Tem Observations of Sr-rich Feldspars Along the Caal2si2o8-sral2si2o8 Join

Domain textures related to the <I(1)over bar>-I2/c displacive phase transition were observed by transmission electron microscopy (TEM) in synthetic samples from the Sr-rich region of the CaAl2Si2O8-SrAl2Si2O8 join. Albite and, more rarely, pericline transformation twins are formed in samples with preexisting polysynthetic Carlsbad twins; they also cut preexisting b-type antiphase domains. Tweed textures associated with albite twin lamellae were observed in triclinic samples and, more rarely, in monoclinic samples close to the transition point. Progressive annealing of a monoclinic disordered precursor was performed to follow the monoclinic-triclinic transition at room temperature. Triclinic and monoclinic phases coexisted close to the transition, whereas tweed textures, rarely present in monoclinic grains, were retained in the triclinic phase after longer annealing. In situ high-temperature TEM experiments performed on a grain with pericline twins showed that the pericline twins disappeared at T-c, but, during cooling, tweed textures appeared and then were replaced by albite twins at lower temperatures. The tweed textures are interpreted as originating from strain modulations induced by the displacive transition rather than modulation in Al-Si order, and they form close to the transition point as precursors of the albite and pericline twinning.