Abstract
The enigmatic Bøggild intergrowth in iridescent labradorite crystals was revisited in light of recent work on the incommensurately modulated structures in the intermediated plagioclase. Five igneous samples and one metamorphic labradorite sample with various compositions and lamellar thicknesses were studied in this paper. The lamellar textures were characterized with conventional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The compositions of individual lamellae were analyzed with high-resolution energy-dispersive X-ray spectroscopy (EDS) mapping and atom probe tomography (APT). The average structure states of the studied samples were also compared with single-crystal X-ray diffraction data (SC-XRD). The Na-rich lamellae have a composition of An44–48, and the Ca-rich lamellae range from An56 to An63. Significant differences between the lamellar compositions of different samples were observed. The compositions of the Bøggild intergrowth do not only depend on the bulk compositions, but also on the thermal history of the host rock. The implications on the subsolidus phase relationships of the plagioclase feldspar solid solution are discussed. The results cannot be explained by a regular symmetrical solvus such as the Bøggild gap, but they support an inclined two-phase region that closes at low temperature.
Highlights
Labradorite feldspar is the intermediate to calcic member of the plagioclase feldspar series, the solid solution between albite (Ab: NaAlSi3 O8 ) and anorthite (An: CaAl2 Si2 O8 )
Structure and chemical analyses of labradorites with Bøggild intergrowth in this paper showed that the subsolidus phase relations of intermediate plagioclase may be more complicated than previously expected
The compositions of Ca-rich and Na-rich lamellae depend on the bulk composition of the sample, and vary significantly with different thermal histories, which cannot be explained by the symmetrical miscibility gap proposed by Carpenter [13]
Summary
Labradorite feldspar is the intermediate to calcic member of the plagioclase feldspar series, the solid solution between albite (Ab: NaAlSi3 O8 ) and anorthite (An: CaAl2 Si2 O8 ). The name labradorite was originally from its type locality, Paul’s Island in Labrador, Canada, where the labradorite crystals are characterized by the iridescent optical effect when reflecting light [1]. This interference color, variously known as the “labradorescence” or “schiller” effect [2], has attracted the interest of mineralogists and physicists for centuries. Ove Balthasar Bøggild [4] first measured the orientations of the reflection planes relative to the crystallographic planes in the labradorite crystals, and coined the word labradorescence. The exsolution texture in labradorite that produces the labradorescence has been named after him as the Bøggild intergrowth
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