The incommensurately modulated structures of volcanic plagioclase: displacement, ordering and phase transition

Four basaltic phenocryst samples of plagioclase, with compositions ranging from An₄₈ (andesine) to An₆₄ (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\bar 1 structure to the e-plagioclase structure. A comparison between the structures from neutron and X-ray diffraction data shows that the 〈T-O〉 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 (An₄₈) 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.