Al-si Ordering In Sr-feldspar Sral2si2o8 - Ir, Tem and Single-crystal Xrd Evidences

Al-Si ordering in Sr-feldspar has been followed by isothermal annealing, starting from a disordered metastable configuration. Ordering could not be followed by changes in the spontaneous strain as cell parameters did not show significant changes with thermal treatment from 0.016 h to 452 h at T=1350 degrees C, while, on the contrary, significant changes in IR spectra are observed. A single crystal obtained from melt (Q(od)approximate to 0) has been progressively heated up to 678 h at T=1350 degrees C and the relevant structural refinements enabled to monitor changes in degree of Al-Si order up to Q(od)=0.86. In isothermal treatment for Sr-feldspar it is observed a significantly lower Q(od) than in anorthite after the same annealing time. TEM observation has shown in Sr-feldspar, also for shortest annealing, 'b' type reflections, while in anorthite, in the same conditions, 'e' type reflections have been observed (Carpenter 1991a). In the first stages of ordering 'b' APDs sized approximate to 100 Angstrom (at T=1350 degrees C, 0.33 h) have been observed in Sr-feldspar; APD coarsening occurs with an activation energy of 120+/-7 kcal mol(-1), not significantly different from anorthite. The ordering process seems to be a slower process in Sr-feldspar than in anorthite, even though data from longer annealing suggest that the Q(od) close to the equilibrium is the same in Sr and Ca-feldspar (Q(od)=0.86 at T=1350 degrees C).