Si-Al order and the I1-I2/c structural phase transition in synthetic CaAl₂Si₂O₈-SrAl₂Si₂O ₈ feldspar: A ²⁹Si MAS-NMR spectroscopic study

We present ²⁹Si MAS-NMR spectroscopic data for a series of synthetic feldspar samples along the join CaAl₂Si₂O₈-SrAl₂Si₂O 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 SrAl₂Si₂O₈ 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 a = 0.93 for the short-range order parameter. For the triclinic feldspar samples, short-range Si-Al order increases continuously with Sr content from σ = 0.89(3) for CaAl₂Si₂O₈ to 0.97(1) for Sr_0.30Ca_0.20Al₂Si₂O₈ but then decreases discontinuously to 0.93(2) for the monoclinic samples, Sr_0.85Ca_0.15Al₂Si₂O₈ to SrAl₂Si₂O₈. The variation of peak positions with composition is consistent with a structural phase transition near Sr_0.85Ca_0.15 Al₂Si₂O₈ from I1 to I2/c. The order parameter for this displacive transition is reflected by the chemical shift of the T1mz cristallographic site, and its composition dependence gives an order-parameter critical exponent of β = 0.49(2), indicating classical second-order behavior.