High-temperature 29Si MAS NMR spectroscopy of anorthite (CaAl2Si2O8) and its P $$\bar 1$$ -I $$\bar 1$$ structural phase transition

Abstract

Abstract. 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.

Medium-range order in disordered K-feldspars by multinuclear NMR

“…The 29 Si spectra of natural specimens and synthetic samples are also compatible with t 1 O ≠ t 1 m and t 2 O ≠ t 2 m, with a t 1 O value higher than the Al occupancy of the other three sites, involving triclinic-like local distortions as noted from the chemical shifts. Loewenstein's rule of Al-O-Al avoidance seems to be respected, as in other framework aluminosilicates (Klinowski et al 1982;Phillips and Kirkpatrick 1995). However, the (4Si,0Al) and (0Si,4Al) environments are less developed in experimental than in the simulated spectra, which are closer to experimental ones than in the case of valencianite.…”

“…Additional difficulties come from the lack of information about chemical shifts for the Al ↔ Si substitutions in the second-coordination sphere, as well as the effect of K and Na atoms. Finally, we chose 4.85 ppm as a shift between Si(nAl) and Si([n+1] Al) peaks for all the simulations, a value compatible with other NMR studies of aluminosilicates with tectosilicate structures (e.g., Klinowski et al 1982;Phillips and Kirkpatrick 1995;Xiao et al 1995); its use allows the reproduction of the main spectral features of most K-feldspars.…”

Understanding the Formation of CaAl2Si2O8 in Melilite-Based Glass-Ceramics: Combined Diffraction and Spectroscopic Studies

“…As shown previously, the 29 Si peaks can shift with higher temperature, resulting in only three overlapping peaks with a relative ratio of 3:3:2. 49 Therefore, the peaks in the range of −93 to −81 ppm (Figure 6) are assigned to the Q 4 (4Al) units, which are characteristics of anorthite. 49 On the other hand, the broad peak appearing at a more negative chemical shift can be assigned to a disordered aluminosilicate framework, which commonly contains 0–4 IV Al units as NNNs for Si.…”

TEM observations on theP1̅-I1̅ phase transition in feldspars along the join CaAl2Si2O8-SrAl2Si2O8

“…Since the pioneering work of Brown et al (1963), several efforts have been made to clarify the thermodynamic behavior of the transition, the local and average structure at the transition, and the behavior of the transition with respect to varying pressure (P), temperature (T), composition, and the macroscopic order parameter (Q od ) (Salje 1987;Redfern and Salje 1987;Hatch and Ghose 1989;Redfern and Salje 1992;Phillips and Kirkpatrick 1995). Since the pioneering work of Brown et al (1963), several efforts have been made to clarify the thermodynamic behavior of the transition, the local and average structure at the transition, and the behavior of the transition with respect to varying pressure (P), temperature (T), composition, and the macroscopic order parameter (Q od ) (Salje 1987;Redfern and Salje 1987;Hatch and Ghose 1989;Redfern and Salje 1992;Phillips and Kirkpatrick 1995).…”

“…High-temperature (HT) X-ray and transmission electron microscopy (TEM) investigations have shown the persistence of a diffuse component in c-and d-type (c: h + k = 2n, l = 2n + 1; d: h + k = 2n +1, l = 2n, as defined by Bown and Gay 1958) superstructure reflections up to temperatures 100-200 °C higher than the transition temperature (T c ). HT spectroscopic IR and NMR data (Staehli and Brinkmann 1974;Redfern and Salje 1992;Phillips and Kirkpatrick 1995) have shown a body-centered configuration for the framework at T > T c , whereas structural refinements indicate that the Ca atoms occupy split positions that can be related to the Ca positions in the primitive structure (Foit and Peacor 1973;Ghose et al 1993). HT spectroscopic IR and NMR data (Staehli and Brinkmann 1974;Redfern and Salje 1992;Phillips and Kirkpatrick 1995) have shown a body-centered configuration for the framework at T > T c , whereas structural refinements indicate that the Ca atoms occupy split positions that can be related to the Ca positions in the primitive structure (Foit and Peacor 1973;Ghose et al 1993).…”