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 tr ansition were observed by transmission electron microscopy (TEM) in sy nthetic samples from the Sr-rich region of the CaAl2Si2O8-SrAl2Si2O8 j oin. Albite and, more rarely, pericline transformation twins are forme d in samples with preexisting polysynthetic Carlsbad twins; they also cut preexisting b-type antiphase domains. Tweed textures associated wi th albite twin lamellae were observed in triclinic samples and, more r arely, in monoclinic samples close to the transition point. Progressiv e annealing of a monoclinic disordered precursor was performed to foll ow 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 t riclinic phase after longer annealing. In situ high-temperature TEM ex periments performed on a grain with pericline twins showed that the pe ricline 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 modulat ions induced by the displacive transition rather than modulation in Al -Si order, and they form close to the transition point as precursors o f the albite and pericline twinning.