American Mineralogist 82(1-2):1-7

Si-Al order and the I1-I2/c structural phase transition in synthetic CaAl₂Si₂O₈-SrAl₂Si₂O₈ feldspar: A ²⁹Si MAS-NMR spectroscopic study

Brian L. Phillips,¹ Martin D. McGuinn,² and Simon A.T. Redfern²,*

¹Division of Materials Science and Engineering, University of California, Davis, California 95616, U.S.A.

²Department of Geology, University of Manchester, Manchester M13 9PL, U.K.

Abstract

We present ²⁹Si 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 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 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 CaAl₂Si₂O₈ to 0.97(1) for Sr_0.80Ca_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_0.15Al₂Si₂O₈ from I1 to I2/c. The order parameter for this displacive transition is reflected by the chemical shift of the T1mz crystallographic site, and its composition dependence gives an order-parameter critical exponent of beta = 0.49(2), indicating classical second-order behavior.