No abstract

Plagioclase feldspars are the most abundant mineral in the Earth's crust. Intermediate plagioclase feldspars commonly display incommensurately modulated or aperiodic structures. Z-contrast images show both Ca-Na ordering and density modulation. The local structure of lamellae domains has I1-like symmetry. The neighboring lamellae domains are in an inversion twinning relationship. With a state-of-the-art X-ray diffraction unit, second-order satellite reflections (f-reflections) are observed for the first time in andesine (An), a Na-rich e-plagioclase. The f-reflections indicate a structure with a density modulation which is close to a Ca-rich e-plagioclase. The similarity between this e-andesine structure and previously solved e-labradorite structure is confirmed. Refinement of the structure shows density modulation of ∼ 7 mol % in compositional variation of the anorthite (An) component. The results from Z-contrast imaging and low-temperature single X-ray diffraction (XRD) provide a structure consistent with density modulation. The discovery of f-reflections in Na-rich e-plagioclase extends the composition range of e1 structure with density modulation to as low as at least An, which is the lower end of the composition range of Bøggild intergrowth. The new result supports the loop-shaped solvus for Bøggild intergrowth, below which is a homogenous stable area for e1 structure in the phase diagram. The phase transition between e2 structure without density modulation and e1 structure with density modulation should happen at low temperature. There is a change in modulation period accompanying the phase transition, as well as higher occupancy of Al in the To site. The andesine with density modulation also indicates extremely slow cooling of its host rock.

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 An to An (An is anorthite, CaAlSiO), 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 An-An, 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.

Four basaltic phenocryst samples of plagioclase, with compositions ranging from An 48 (andesine) to An 64 (labradorite), have been studied with single-crystal X-ray and neutron diffraction techniques. The samples were also subjected to a heating experiment at 1100 C for two weeks in an effort to minimize the Al-Si ordering in their structures. The average and the modulated structures of the samples (before and after the heating experiment) were compared, in order to understand the mechanism of the phase transition from the disordered C " 1 1 structure to the e-plagioclase structure. A comparison between the structures from neutron and X-ray diffraction data shows that the hT-Oi distance does not solely depend on the Al occupancy as previously thought. A dramatic decrease of the Al-Si ordering is observed after heating at 1100 C for two weeks for all four samples, with an obvious change in the intensities of the satellite reflections (e-reflections) in the diffraction pattern. Evident changes in the modulation period were also observed for the more calcic samples. No obvious change in the Ca-Na ordering was observed after the heating experiment. An in situ heating X-ray diffraction experiment was carried out on the andesine sample (An 48 ) to study the change in the satellite intensity at high temperature.A dramatic weakening of the satellite peaks was observed between 477 C and 537 C, which strongly supports the displacive nature of the initiation of e2 ordering. Rigid-Unit Mode (RUM) analysis of the plagioclase structure suggests the initial position of the e-reflections is determined by the anti-RUMs in the framework.