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We present  29 Si MAS-NMR spectroscopic data for a series of synthetic feldspar samples along the join CaAl 2 Si 2 O 8 -SrAl 2 Si 2 O 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 2 Si 2 O 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 σ = 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 CaAl 2 Si 2 O 8  to 0.97(1) for Sr 0.85 Ca 0.20 Al 2 Si 2 O 8  but then decreases discontinuously to 0.93(2) for the monoclinic samples, Sr 0.85 Ca 0.15 Al 2 Si 2 O 8  to SrAl 2 Si 2 O 8 . The variation of peak posi­tions with composition is consistent with a structural phase transition near Sr 0.85 Ca 0.15 Al 2 Si 2 O 8  from I1̄ 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 β = 0.49(2), indicating classical second-order behavior.

Si-Al order and the I1̄-I2/c structural phase transition in synthetic CaAl 2 Si 2 O 8 -SrAl 2 Si 2 O 8 feldspar: A 29 SiMAS-NMR spectroscopic study