Publication Date:
2019
Description:
Alkali feldspars with diffuse reflections that violate C-centering symmetry were reported in Na-bearing adularia, orthoclase and microcline. TEM results indicate elongated nano-precipitates with intermediate composition of KNa(Si6Al2)O16 cause the diffuse reflections. Density functional theory (DFT) calculation indicates ordered distribution of K and Na atoms in the nano-phase with Pa symmetry. K and Na atoms are slightly off special positions for K atoms in the orthoclase structure. Formation of the intermediate nano-phase may lower the interface energy between the nano-phase and the host orthoclase. Previously reported P21/a symmetry was resulted from an artifact of overlapped diffraction spots from the nano-precipitates (Pa) and host orthoclase (C2/m). Adularia, orthoclase and microcline with the Pa nano-precipitates indicate very slow cooling of their host rocks at low temperature.
Electronic ISSN:
2075-163X
Topics:
Geosciences
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