Local strain heterogeneity associated with Al/Si ordering in anorthite, CaAl2Si2O8, with implications for thermodynamic mixing behavior and trace element partitioning in plagioclase feldspars

Abstract

Hard Mode IR powder absorption spectroscopy has been used to characterise local strain relaxation associated with Al/Si ordering in a suite of synthetic anorthite samples with structural states that vary from a high degree of Al/Si order through a metastable incommensurate structure at intermediate states of order to long range order with I1 ̅ symmetry. The dominant feature accompanying the changing structural states is line broadening, which has been quantified by autocorrelation analysis and is attributed to local heterogeneous strain variations on a length scale of at least 1-5 unit cells. The autocorrelation results are consistent with contributions to the line broadening as being due to order parameters for both the C1 ̅  I1 ̅ and I1 ̅  P1 ̅ transitions which couple biquadratically, 𝜆𝑄 od 2 𝑄 displ 2 .Close correlation with enthalpy variations from previously published calorimetric data indicates that the driving force for ordering can be understood in terms of elimination of strain fields arising from accommodating more or less rigid AlO 4 and SiO 4 tetrahedra in the feldspar framework. The metastable This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.