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Incommensurate density modulation in a Na‐rich plagioclase feldspar: Z ‐contrast imaging and single‐crystal X‐ray diffraction study
Author(s) -
Xu Huifang,
Jin Shiyun,
Noll Bruce C.
Publication year - 2016
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s205252061601578x
Subject(s) - plagioclase , diffraction , anorthite , materials science , crystal twinning , crystallography , modulation (music) , mineralogy , geology , chemistry , optics , physics , quartz , microstructure , acoustics , composite material
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 I 1‐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 45 ), 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 e 1 structure with density modulation to as low as at least An 45 , 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 e 1 structure in the phase diagram. The phase transition between e 2 structure without density modulation and e 1 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 T 1 o site. The andesine with density modulation also indicates extremely slow cooling of its host rock.