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Al-Si ordering in Sr-feldspar SrAl2Si2O8: IR,TEM and single-crystal XRD evidences
Authors:P Benna  M Tribaudino  E Bruno
Institution:1. Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, 1-10125, Torino, Italy
2. Centro di Studi sulla Geodinamica delle Catene Collisionali (C.N.R.), Via Accademia delle Scienze 5, I-10123, Torino, Italy
Abstract: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° C, while, on the contrary, significant changes in IR spectra are observed. A single crystal obtained from melt (Q odap 0) has been progressively heated up to 678 h at T=1350° C and the relevant structural refinements enabled to monitor changes in degree of Al-Si order up to Qod = 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, lsquobrsquo type reflections, while in anorthite, in the same conditions, lsquoersquo type reflections have been observed (Carpenter 1991a). In the first stages of ordering lsquobrsquo APDs sized ap 100 Å (at T=1350° 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° C).
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