SI-AL ORDER AND THE I(1)OVER-BAR-I2 C STRUCTURAL PHASE-TRANSITION IN SYNTHETIC CAAL2SI2O8-SRAL2SI2O8 FELDSPAR - A SI-29 MAS-NMR SPECTROSCOPIC STUDY/

We present Si-29 MAS-NMR spectroscopic data for a series of synthetic feldspar samples along the join CaAl2Si2O8-SrAl2Si2O8, from which the composition dependence and coupling of order parameters describing Si- Al order and the triclinic-monoclinic displacive transition were deter mined. Spectra of SrAl2Si2O8, contain narrow peaks for the two crystal lographic Si sites, plus additional peaks for Si having three and two Al nearest neighbors, indicating the presence of approximately 0.14 Al -O-Al linkages per formula unit and a value of sigma = 0.93 for the sh ort-range order parameter. For the triclinic feldspar samples, short-r ange Si-Al order increases continuously with Sr content from sigma = 0 .89(3) for CaAl2Si2O8 to 0.97(1) for Sr0.80Ca0.20Al2Si2O8 but then dec reases discontinuously to 0.93(2) for the monoclinic samples, Sr0.85Ca 0.15Al2Si2O8 to SrAl2Si2O8. The variation of peak positions with compo sition is consistent with a structural phase transition near Sr0.85Ca0 .15 Al2Si2O8 from I (1) over bar to I2/c. The order parameter for this displacive transition is reflected by the chemical shift of the Tlmz crystallographic site, and its composition dependence gives an order-p arameter critical exponent of beta = 0.49(2), indicating classical sec ond-order behavior.