Investigations of the phase relations among e1, e2 and C1 structures of Na-rich plagioclase feldspars: a single-crystal X-ray diffraction study

摘要

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 singlecrystal X-ray diffraction analyses. The samples studied cover a compositional range from An_(21) to An49 (An is anorthite, CaAl_2Si_2O_8), 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: C1, e2 and e1, with an obviously increasing trend in the ordering states of the structures. The phase transitions from C1 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 C1 and e1 is large enough to create a miscibility gap causing the B?ggild 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 B?ggild intergrowth is precisely constrained to An_(44)-An_(45), 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