High-temperature29Si MAS NMR spectroscopy of anorthite (CaAl2Si2O8) and its P(ar 1)-I(ar 1)structural phase transition

✍ Scribed by Brian L. Phillips; R. James Kirkpatrick

• Publisher: Springer-Verlag
• Year: 1995
• Tongue: English
• Weight: 930 KB
• Volume: 22
• Category: Article
• ISSN: 0342-1791
• DOI: 10.1007/bf00202260

✦ Synopsis

We present 29Si MAS NMR data for a well-ordered natural anorthite, obtained in situ at temperatures of from 25 to 500 ~ C, which follow the changes in the aluminosilicate framework through the PI-I1 structural phase transition. Pairs of peaks due to sites offset by ap-I [111] converge through the Pi phase and proximately only four peaks are present above about 241 ~ C. The variation of the peak positions with temperature and correlations based on structural data for the P1 and Ii phases allow assignment of all the MAS-NMR peaks to crystallographic sites. A Landau-type analysis gives an expression that relates the separation of pairs of converging peaks to the local order parameter for the Pl-I1 transition, from which we determine its temperature dependence. Data for the best-constrained set of peak positions give for the order parameter critical exponent [3 = 0.27 _+ 0.04, consistent with previous results indicating that the PI-I1 transition in pure anorthite is tricritical. No significant change in the 29Si spin-lattice relaxation rate occurs across the PI-I1 transition.