Al-Si ordering in Sr-feldspar SrAl2Si2O8: IR,TEM and single-crystal XRD evidences |
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Authors: | P Benna M Tribaudino E Bruno |
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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
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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
od 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, 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 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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