Si-Al order and the I{bar 1}-I2/c structural phase transition in synthetic CaAl{sub 2}Si{sub 2}O{sub 8}-SrAl{sub 2}Si{sub 2}O{sub 8} feldspar: A {sup 29}Si MAS-NMR spectroscopic study

We present {sup 29}Si MAS-NMR spectroscopic data for a series of synthetic feldspar samples along the join CaAl{sub 2}Si{sub 2}O{sub 8}-SrAl{sub 2}Si{sub 2}O{sub 8}, from which the composition dependence and coupling of order parameters describing Si-Al order and the triclinic-monoclinic displacive transition were determined. Spectra of SrAl{sub 2}Si{sub 2}O{sub 8} contain narrow peaks for the two crystallographic 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 short-range order parameter. For the triclinic feldspar samples, short-range Si-Al order increases continuously with Sr content from {sigma} = 0.89(3) for CaAl{sub 2}Si{sub 2}O{sub 8} to 0.97(1) for Sr{sub 0.80}Ca{sub 0.20}Al{sub 2}Si{sub 2}O{sub 8} but then decreases discontinuously to 0.93(2) for the monoclinic samples, Sr{sub 0.85}Ca{sub 015}Al{sub 2}Si{sub 2}O{sub 8} to SrAl{sub 2}Si{sub 2}O{sub 8}. The variation of peak positions with composition is consistent with a structural phase transition near Sr{sub 0.85}Ca{sub 0.15}Al{sub 2}Si{sub 2}O{sub 8} from I1 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-parameter critical exponent of {beta}more » - 0.49(2), indicating classical second-order behavior. 11 refs., 4 figs., 1 tab.« less