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Investigations of the phase relations among e1, e2 and C{\bar {\bf 1}} structures of Na-rich plagioclase feldspars: a single-crystal X-ray diffraction study.

The subsolidus phase relations of plagioclase feldspar solid solution have been puzzling mineralogists and petrologists for decades, mainly due to the complicated structures of intermediate plagioclase at low temperature. The crystal structures of 12 Na-rich plagioclase samples are investigated by single-crystal X-ray diffraction analyses. The samples studied cover a compositional range from An21 to An49 (An is anorthite, CaAl2Si2O8), as well as a wide variety of origins, from extremely slow-cooled gabbroic rocks to pegmatite and metamorphic rocks. The structures fall into three different types: C{\bar 1}, e2 and e1, with an obviously increasing trend in the ordering states of the structures. The phase transitions from C{\bar 1} to e2 and e2 to e1 are both continuous in nature, as no abrupt structure change is required for the transformation. However, the structural difference between C\bar 1 and e1 is large enough to create a miscibility gap causing the Bøggild intergrowth. As the plagioclase structure becomes more and more ordered, Al-Si reorganization in the framework would occur before the ordering of Ca and Na in M sites. Dramatic variations of Na occupancy would only appear in e1 structure with density modulation. This result confirms that Al-Si ordering is the major driving force of the formation of e-plagioclase structure. The composition of the lower end of the Bøggild intergrowth is precisely constrained to An44-An45, based on the structural differences between two samples from the same pegmatite crystal. The modulation periods and directions of e-plagioclase are dependent on the conditions at which e-ordering starts to happen, other than the composition of the plagioclase. However, the three components (δh, δk and δl) of the q vector show strong linear correlations among one another, indicating some crystallographic constraint on the modulation direction which might be independent from the composition. The detailed subsolidus phase relations among e1, e2 and C{\bar 1} are illustrated with a local phase diagram, and schematic free energy curves at different temperatures are provided.